For water to change from one state to another, heat energy must be added to it or released form it. The amount of energy absorbed or released must be sufficient to affect the hydrogen bonds between molecules. This relation between water and heat energy is important to atmospheric processes. In fact, the heat exchanged between physical states of water provides more than 30% of the energy that powers general circulation of the atmosphere.
Figure 5.2: Three Physical States of Water and Phase Changes Between Them. GRAPH IS IMPORTANT!
Melting and Freezing describe the familiar phase changes between solid (ice) and liquid (water).
Condensation is the process through which water vapor in the air becomes liquid water- this is the process that forms clouds. The process through which liquid water becomes water vapor is Evaporation. This phase change is called Vaporization when water is at boiling temperature.
The phase changes between solid ice and gaseous water vapor may be less familiar. Deposition is the process through which water vapor attaches directly to an ice crystal, leading to the formation of Frost. You may have seen this on your windows or car windshield on a cold morning. It also occurs inside your freezer. Sublimation is the process by which ice changes directly to water vapor. A classic sublimation example is the water vapor clouds associated with the evaporation of dry ice (frozen carbon dioxide) when it is exposed to air. Sublimation is an important contributor to the shrinking of snowpacks in dry, windy environments.
ICE, The Solid Phase:
As water cools from room temperature, it behaves like most compounds and contracts in volume. At the same time, it increases in density, as the same number of molecules now occupies a smaller space.
When water has cooled to the point of its greatest density (39F), it is still in a liquid state. Below this temperature, water behaves differently from other compounds. Continued cooling makes it expand as more hydrogen bonds form among the slowing molecules, creating the hexagonal (six-sided) crystalline structure characteristic of ice.
As temperatures descend further below freezing, ice continues to expand in volume and decrease in density.
Without this unusual pattern of density change, much of earth's freshwater would b be bound in masses of ice on the ocean floor (the water would freeze, sink, and remain in place forever).
At the same time, the expansion process just described is to blame for highway and pavement damage and burst water pipes as well as the physical breakdown of rocks known as weathering and the freeze-thaw processes that affect soils in cold regions.
In nature, the density of ice varies slightly with age and the air contained within it. As a result, the amount of water that is displaced within an iceberg varies, with an average of about one-seventh (14%) of the mass exposed and about six-sevenths (86%) submerged between the ocean's surface. With water temperatures higher than air temperatures, underwater portions melt faster than those above water. Therefore, icebergs are inherently unstable and will overturn.
WATER, The Liquid Phase:
Water, as a liquid, is a non-compressible fluid that assumes the shape of its container. For ice to change to water, heat energy must increase the motion of the water molecules enough to break some of the hydrogen bonds. The heat energy of a phase change is Latent Heat and is hidden within the structure of water's physical state. In total, 80 calories of heat energy must be absorbed for the phase change of 1 g of ice melting to 1 g of water- this latent heat transfer occurs despite the fact that the sensible temperature remains the same: Both ice and water measure 32F. When the phase change is reversed and a gram of water freezes, latent heat is Released rather than Absorbed.
The latent heat of melting and the latent heat of freezing are each 80 cal/g.
No phase change is involved in the raising of water temperature from freezing (32F) to boiling (212F).
WATER VAPOR, The Gas Phase:
Water vapor is an invisible and compressible gas in which each molecule moves independent of the others. When the phase change from liquid to vapor is induced by boiling, it requires the addition of 540 cal for each gram, under normal sea-level pressure; this amount of energy is the Latent Heat of Evaporation. When water vapor condenses to a liquid, each gram gives up its hidden 540 cal as the Latent Heat of Condensation. We see water vapor in the atmosphere after condensation has occurred in the form of clouds, fog, and steam.
The Latent Heat of Sublimation that is absorbed as a gram of ice transforms into vapor. Water vapor freezing directly to ice releases a comparable amount of energy.
HEAT EXCHANGE under NATURAL CONDITIONS:
In a lake or stream or in soil water, at 68F, every gram of water that breaks away from the surface through evaporation must absorb from the environment approximately 585 cal as the Latent Heat of Evaporation. This is slightly more energy than would be required if the water were at a higher temperature. You can feel this absorption of latent heat as evaporative cooling on your skin when it is wet. This latent heat exchange is the dominant cooling process in Earth's energy budget.
The process reverses when air cools and water vapor condenses back into the liquid state, forming moisture droplets and thus liberating 585 cal for every gram of water as the Latent Heat of Condensation. When you realize that a small, puffy, fair-weather cumulus cloud holds 500-1000 tons of moisture droplets, think of the tremendous latent heat released when water vapor condenses to droplets.
Satellites using infrared sensors now routinely monitor water vapor in the lower atmosphere. Water vapor absorbs long wavelengths (infrared), making it possible to distinguish areas of relatively high water vapor from areas of low water vapor. This technology is important to weather forecasting because it shows the available moisture in the atmosphere and therefore the available latent heat energy and precipitation potential. Water vapor is also an important greenhouse gas in Earth's atmosphere.
The amount of water vapor in the air is Humidity. The capacity of air for water vapor is primarily a function of the temperatures of both the air and the water vapor, which are usually the same.
Humidity and air temperature determine our sense of comfort. North Americans spend billions of dollars a year to adjust the humidity in buildings, either with air conditioners, which remove water vapor as they cool building interiors, or with air humidifiers, which add water vapor to lessen the drying effects of cold temperatures and dry climates.
The most common measure of humidity in weather reports is Relative Humidity, a ratio (expressed as a percentage) of the amount of water vapor that is actually in the air compared to the maximum amount of water vapor that is possible in the air at a given temperature.
Relative humidity varies because of water vapor or temperature changes in the air. The formula to calculate the relative humidity ratio and express it as a percentage places actual water vapor present in the air as the numerator and maximum water vapor possible in the air at that temperature as the denominator.
Warmer air increases the evaporation rate from water surfaces, whereas cooler air tends to increase the condensation rate of water vapor onto water surfaces. Because there is a maximum amount of water vapor that can exist in a volume of air at a given temperature, the rates of evaporation and condensation can reach equilibrium at some point; the air is then saturated, and the balance is saturated equilibrium.
SATURATION AND DEW POINT:
Relative humidity tells us how near the air is to saturation and is an expression of an ongoing process of water molecules moving between air and moist surfaces. At SATURATION, or 100% relative humidity, any further addition of water vapor or any decrease in temperature that reduces the evaporation rate results in active condensation (forming clouds, fog, or precipitation).
The temperature at which a given sample of vapor-containing air becomes saturated and net condensation begins to form water droplets is the DEW-POINT TEMPERATURE. The air is saturated when the dew-point temperature and the air temperature are the same. When temperatures are below freezing, the FROST POINT is the temperature at which the air becomes saturated, leading to the formation of frost (ice) on exposed surfaces.
A cold drink in a glass provides a familiar example of these conditions. The air near the glass chills to the dew point temperature and becomes saturated, causing the water vapor in that cooled air to form water droplets on the outside of the glass. As you walk to classes on some cool mornings, you perhaps notice damp lawns or dew on windshields, an indication of dew point temperature conditions.
Daily Relative Humidity Patterns:
An inverse relationship occurs during a typical day between air temperature and relative humidity- as temperature rises, relative humidity falls. Relative humidity is highest at dawn, when air temperature is lowest. If you park outdoors, you know about the wetness of the dew that condenses on your car or bicycle overnight.
Relative humidity is lowest on the late afternoon, when higher temperatures increase the rate of evaporation. The amount of water vapor actually present in the air may remain the same throughout the day. However, relative humidity changes because the temperature, and therefore the rate of evaporation, varies from morning to afternoon.
There are several ways to express humidity and relative humidity. Each has its own utility and application. Two examples are vapor pressure and specific humidity.
As free water molecules evaporate from surfaces into the atmosphere, they become water vapor. Now part of the air, water-vapor molecules exert a portion of the air pressure along with nitrogen and oxygen molecules. The share of air pressure that is made up of water-vapor molecules is Vapor Pressure.
Air that contains as much water vapor as possible at a given temperature is at saturation vapor pressure. Any temperature increase or decrease will change the saturation vapor pressure.
For every temperature increase of 18F, the saturation vapor pressure in air nearly doubles. This relationship explains why warm tropical air over the ocean contain so much water vapor, thus providing much latent heat to power tropical storms. It also explains why cold air is "dry" and why cold air toward the poles does not produce a lot of precipitation (it contains too little water vapor, even though it is near the dew point temperature).
Saturation vapor pressure is greater above a water surface than over an ice surface- that is, it takes more water-vapor molecules to saturate air above water than it does above ice. This fact is important to condensation processes.
A useful humidity measure is one that remains constant as temperature and pressure change. SPECIFIC HUMIDITY is the mass of water vapor (in grams) per mass of air (kilograms) at any specified temperature. Because it is measured in mass, specific humidity is not affected by changes in temperature or pressure, as occur when air rises to higher elevations. Specific humidity stays constant despite volume changes.
The maximum mass of water vapor possible in a kilogram of air at any specified temperature is the maximum specific humidity. Specific humidity is useful in describing the moisture content of large air masses that are interacting in a weather system and provides information necessary for weather forecasting.
Various instruments measure humidity. Examples are the Hair Hygrometer, which uses the principle that human hair changes as much as 4% in length between 0% and 100% relative humidity, and the Sling Psychrometer, which compares the temperatures recorded by side-by-side dry and wet thermometers to determine relative humidity.
One indicator of weather conditions relates to the vertical movement of air parcels in the atmosphere. Meteorologists use the term parcel to describe a body of air, with specific temperature and humidity characteristics.
Two opposing forces- an upward Buoyant Force and a downward Gravitational Force- decide the vertical position of a parcel of air. A parcel of lower density than the surrounding air is Buoyant, so it rises; a rising parcel expands as external pressure decreases. In contrast, a parcel of higher density descends under the force of gravity because it is not buoyant; a falling parcel compresses as external pressure increases.
The temperature of the volume of air determines the density of the air parcel- warm air has lower density; cold air has higher density. Therefore, buoyancy depends on density, and density depends on temperature.
STABILITY refers to the tendency of an air parcel either to remain in place or to change vertical position by ascending (rising) or descending (falling). An air parcel is Stable if it resists displacement upward or, when disturbed, tends to return to its starting place. An air parcel is Unstable if it continues to rise until it reaches an altitude where the surrounding air has a density and temperature similar to its own. The behavior of a hot air balloon illustrates these concepts.
The stability or instability of an air parcel depends on two temperatures: the temperature inside the parcel and the temperature of the air surrounding the parcel. The difference between these two temperatures determines stability.
The Normal lapse Rate, is the average decrease in temperature with increasing altitude. This rate of temperature change is for still, calm air, and it can vary greatly under different weather conditions. In contrast the Environmental Lapse Rate (ELR) is the actual lapse rate at a particular place and time. It can vary by several degrees per thousand meters.
Two generalizations predict the warming or cooling of an ascending or descending parcel of air. An ascending parcel of air tends to cool by expansion, responding to the reduced pressure at higher altitudes. In contrast, a descending parcel of air tends to heat by compression. These mechanisms of heating and cooling are adiabatic. ADIABATIC means occurring without a loss or gain of heat- that is, without any heat exchange between the surrounding environment and the vertically moving parcel of air. Adiabatic temperature changes are measured with one of two specific rates, depending on moisture conditions in the parcel: Dry Adiabatic Rate (DAR) and Moist Adiabatic Rate (MAR).
DRY ADIABATIC RATE:
The DAR is the rate at which "dry" air cools by expansion as it rises or heats by compression as it falls. "Dry" refers to air that is less than saturated (its relative humidity is less than 100%). The average DAR is 10C/1000m.
MOIST ADIABATIC RATE:
The MAR is the rate at which an ascending air parcel that is moist, or saturated, cools by expansion. The average MAR is 6C/1000m. Note that a descending parcel of saturated air warms at the MAR as well because the evaporation of liquid droplets, absorbing sensible heat, offsets the rate of compressional warming.
The cause of this variability, and the reason that the MAR is lower than the DAR, is the latent heat of condensation. As water vapor condenses in the saturated air, latent heat is liberated, becoming sensible heat and thus decreasing in adiabatic rate. The release of latent heat may vary with temperature and water vapor content. The MAR is much lower than the DAR in warm air, whereas the two rates are more similar in cold air.
Stable and Unstable Atmospheric Conditions:
The relationship of the DAR and MAR to the environmental lapse rate, or ELR, at a given time and place determines the stability of the atmosphere over an area. In turn, atmospheric stability affects cloud formation and precipitation patterns, some of the essential elements of weather.
Temperature relationships in the lower atmosphere produce conditions ranging from stable to unstable. When the ELR is less than the DAR and MAR, the atmosphere is Unstable.
When the ELR is between the DAR and MAR, the atmosphere is neither stable nor unstable, a situation known as CONDITIONAL INSTABILITY. Under these conditions, if an air parcel is less than saturated, it will resist upward movement, unless forced. But if the air parcel becomes saturated, it will now be unstable and continue to rise.
Under unstable atmospheric conditions, as the air parcel continues rising and cooling, it may eventually achieve the dew-point temperature, saturation, and active condensation. This is the Lifting Condensation Level (LCL), an altitude that you sometimes see in the sky as the flat bottoms of clouds.
CLOUDS AND FOG
A Cloud is an aggregation of tiny moisture droplets and ice crystals that are suspended in air and are great enough in volume and concentration to be visible. Fog, is simply a cloud in contact with the ground.
Clouds may contain raindrops, but not initially. At the outset, clouds are a great mass of moisture droplets, each invisible without magnification.
As an air parcel rises, it may cool to the dew point temperature and 100% relative humidity. More lifting of the air parcel cools it further, producing condensation of water vapor into water. Condensation requires Cloud-Condensation Nuclei, microscopic particles that are always present in the atmosphere. Continental air masses average more cloud condensation nuclei than marine air masses.
The lower atmosphere never lacks cloud condensation nuclei.
Given the presence of saturated air, cloud condensation nuclei, and cooling (lifting) mechanisms n the atmosphere, condensation occurs. Clouds are a result of these processes.
CLOUD TYPES AND IDENTIFICATION:
Altitude and Shape are key to cloud classification. The four altitudinal classes are low, middle, high, and clouds vertically developed through the troposphere.
Combinations of shape and altitude result in 10 basic cloud types: Stratus, Stratocumulus, Nimbostratus, Altostratus, Altocumulus, Cirrus, Cirrostratus, Cirrocumulus, Cumulus, and Cumulonimbus.
Low clouds, ranging from the surface up to 2000m in the middle latitudes, are Stratus or Cumulus.
STRATUS clouds appear dull, gray, and featureless. When they yield precipitation, they become NIMBOSTRATUS (nimbo- denotes stormy or rainy), and their showers typically fall as drizzly rain.
CUMULUS clouds appear bright and puffy, like cotton balls. Vertically developed cumulus clouds can extend beyond low altitudes into middle and high altitudes.
Sometimes near the end of the day, patches of lumpy, grayish, low-level STRATOCUMULUS clouds may fill the sky. Near sunset, these spreading, puffy, stratiform remnants may catch and filter the sun's rays, sometimes indicating clearing weather.
The prefix alto-(meaning high) denotes middle-level clouds. They are made with water droplets, mixed, when temperatures are cold enough, with ice crystals. ALTOCUMULUS clouds, in particular, represent a broad category that includes many different styles: patchy rows, wave patterns, a 'mackerel sky', or lens-shaped clouds.
Ice crystals in thin concentrations compose clouds occurring above 6000m. These wispy filaments, usually white except when colored by sunrise or sunset, are CIRRUS clouds (latin for curl of hair). Cirrus clouds look as though an artist took a brush and made delicate feathery strokes high in the sky. Cirrus clouds can indicate an oncoming storms, especially if they thicken and lower in elevation. The prefix Cirro-, as in Cirrostratus or Cirrocumulus, indicates other high clouds that form a thin veil or have a puffy appearance, respectively.
A cumulus cloud can develop into a towering giant called a CUMULONIMBUS cloud. Such clouds are known as thunderheads because of their shape and associated lightning and thunder. Note the surface wind gusts, updrafts and downdrafts, heavy rain, and ice crystals present at the top of the rising cloud column. High altitude winds may then shear the top of the cloud into the characteristic anvil shape of the mature thunderhead.
PROCESSES THAT FORM FOG:
FOG is a cloud layer on the ground.
The presence of fog tells us that the air temperature and the dew-point temperature at ground level are nearly identical, indicating saturated conditions.
A temperature inversion layer generally caps a fog layer (warmer temperatures above and cooler temperatures below the inversion altitude), with as much as a 40F difference in air temperature between the cooler ground under the fog and the warmer, sunny skies above.
When radiative cooling of a surface chills the air layer directly above that surface to the dew point temperature, creating saturated conditions, a Radiation Fog forms. This fog occurs over moist ground, especially on clear nights; it does not occur over water because water does not cool appreciably overnight.
When air in one place migrates to another place where conditions are right for saturation, an Advection Fog forms. For example, when warm, moist air moves over cooler ocean currents, lake surfaces, or snow masses, the layer of migrating air directly above the surface becomes chilled to the dew point, and fog develops. Off all subtropical west coasts in the world, summer fog forms in the manner just described.
One type of advection fog forms when moist air flows to higher elevations along a hill or mountain. This upslope lifting leads to adiabatic cooling by expansion as the air rises. Eventually, the air reaches the lifting condensation level. The resulting Upslope Fog forms a stratus cloud at the altitude where saturation occurs and condensation begins.
Another advection fog associated with topography is Valley Fog. Because cool air is denser than warm air, it settles in low-lying areas, producing a fog in the chilled, saturated layer near the ground in the valley.
A type of fog that is related to both radiation and advection forms when cold air lies over the warm water of a lake, an ocean surface, or even a swimming pool. This wispy Evaporation Fog, or steam fog, may form as water molecules evaporate from the water surface into the cold overlying air, effectively humidifying the air to saturation, followed by condensation to form fog.
Each region of earth's surface imparts its temperature and moisture characteristics to overlying air. The effect of a region's surface on the air creates a homogeneous mix of temperature, humidity, and stability that may extend through the lower half of the atmosphere. Such a distinctive body of air is an AIR MASS, and it initially reflects the characteristics of its source region. Examples include the Cold Canadian air mass and the Moist Tropical air mass often referred to in weather forecasts. The various masses of air over Earth's surface interact to produce weather patterns.
AIR MASSES AFFECTING NORTH AMERICA:
We classify air masses according to the general moisture and temperature characteristics of their source regions: Moisture is designed M for maritime (WET) or C for continental (DRY).
Temperature is directly related to latitude and designated A for Arctic, P for Polar, T for Tropical, E for Equatorial, or AA for Antarctic.
Continental Polar (cP) air masses form only in the northern hemisphere and are most developed in winter and cold weather conditions. These cP air masses are major players in middle- and high-latitude weather, as their cold, dense air displaces moist, warm air in their path, lifting and cooling the warm air and causing its vapor to condense. An area covered by cP air in winter experiences cold, stable air; clear skies; high pressure; and anticyclonic wind flow. The Souther Hemisphere lacks the necessary continental landmasses at high latitudes to create such a cP air mass.
Maritime Polar (mP) air masses in the Northern Hemisphere sit over the norther oceans. Within them, cool, moist, unstable conditions prevail throughout the year. The Aleutian and Icelandic subpolar low-pressure cells reside within these mP air masses, especially in their well-developed winter pattern.
Two maritime tropical (mT) air masses- the mT Gulf/Atlantic and the mT Pacific- influence North America. The humidity experienced in the eastern and midwestern areas of North America is created by the mT Gulf/Atlantic air mass, which is particularly unstable and active from late spring to early fall. In contrast, the mT Pacific is stable to conditionally unstable and generally lower in moisture content and available energy. As a result, the western US, influenced by this weaker Pacific air mass, receives lower average precipitation than the rest of the country.
The longer an air mass remains stationary over a region, the more definite its physical attributes become. As air masses migrate from source regions, their temperature and moisture characteristics slowly change to the characteristics of the land over which they pass.
ATMOSPHERIC LIFTING MECHANISMS:
When an air mass is lifted, it cools adiabatically (by expansion). When the cooling reaches the dew-point temperature, moisture in the saturated air can condense, forming clouds and perhaps precipitation. Four principal lifting mechanisms operate in the atmosphere:
-CONVERGENT LIFTING results when air flows toward an area of low pressure.
-CONVECTIONAL LIFTING happens when air is stimulated by local surface heating.
-OROGRAPHIC LIFTING occurs when air is forced over a barrier such as a mountain range.
-FRONTAL LIFTING occurs as air is displaced upward along the leading edges of contrasting air masses.
Air flowing from different directions into the same low-pressure area is converging, displacing air upward in convergent lifting. All along the equatorial region, the southeast and northeast trade winds converge, forming the intertropical convergence zone (ITCZ) and areas of extensive convergent uplift, towering cumulonimbus cloud development, and high average annual precipitation.
When an air mass passes from a maritime source region to a warmer continental region, heating from the warmer land surface causes lifting and convection in the air mass. Other sources of surface heating include urban heat islands and the dark soil in plowed fields; the warmer surfaces produce Convectional Lifting. If conditions are unstable, initial lifting continues and clouds develop.
Florida's precipitation generally illustrates both convergent and convectional lifting mechanisms. Heating of the land produces a convergence of onshore winds from the Atlantic and Gulf of Mexico.
Because the sun's radiation gradually heats the land throughout the day and warms the air above it, convectional showers tend to form in the afternoon and early evening. Thus, Florida has the highest frequency of days with thunderstorms in the US.
The physical presence of a mountain acts as a topographic barrier to migrating air masses. Orographic Lifting occurs when air is forcibly lifted upslope as it is pushed against a mountain. The lifting air cools adiabatically. Stable air forced upward in this manner may produce stratiform clouds, whereas unstable or conditionally unstable air usually forms a line of cumulus and cumulonimbus clouds. An orographic barrier enhances convectional activity and causes additional lifting during the passage of weather fronts and cyclonic systems, thereby extracting more moisture from passing air masses and resulting in Orographic Precipitation.
Air beginning its ascent up a mountain can be warm and moist, but finishing its descent on the leeward slope, it becomes hot and dry. The term Rain Shadow is applied to this dry, leeward side of mountains.
In North America, the Chinook Winds are the warm, downslope airflows characteristic of the leeward side of mountains. Such winds can bring a 36F jump in temperature and greatly reduce relative humidity.
Frontal Lifting (COLD AND WARM FRONTS):
The leading edge of an advancing air mass is its Front. A front is a place of atmospheric discontinuity, a narrow zone forming a line of conflict between two air masses of different temperature, pressure, humidity, wind direction and speed, and cloud development. The leading edge of a cold air mass is a Cold Front, whereas the leading edge of a warm air mass is a Warm Front.
The steep face of an advancing cold air mass reflects the ground-hugging nature of cold air, caused by its greater density and more uniform characteristics compared to the warmer air mass it displaces. Warm, moist air in advance of the cold front lifts upward abruptly and experiences the same adiabatic rates of cooling and factors of stability and instability that pertain to all lifting air parcels.
A day or two ahead of the cold front's arrival, high cirrus clouds appear. Shifting winds, dropping temperature, and lowering barometric pressure mark the fronts advance due to lifting of the displaced warmer air along the front's leading edge. At the line of the most intense lifting, usually traveling just ahead of the front itself, air pressure drops to a local low. Clouds may build along the cold front into characteristic cumulonimbus form and may appear as an advancing wall of clouds. Precipitation usually is heavy, containing large droplets, and can be accompanied by hail, lightning and thunder. The aftermath of a cold front's passage usually brings northerly winds in the northern hemisphere and southerly winds in the souther hemisphere as anticyclonic high pressure advances. Temperatures drop, and air pressure rises in response to the cooler, denser air; cloud cover breaks and clears.
A fast-advancing cold front can cause violent lifting, creating a zone known as a Squall Line right along or slightly ahead of the front. (A Squall is a sudden episode of high winds that is generally associated with bands of thunderstorms.) Along a squall line, wind patterns are turbulent and wildly changing, and precipitation is intense. The well-defined frontal clouds rise abruptly, feeding the formation of new thunderstorms along the front. Tornadoes may also develop.
Warm air masses can be carried by the jet stream into regions with colder air, such as when warm air flow called the 'pineapple express' carries warm, moist air from Hawaii and the Pacific into California and the Southwest during July and August. The leading edge of an advancing warm air mass is unable to displace cooler, passive air, which is denser along the surface. Instead, the warm air tends to push the cooler, underlying air into a characteristic wedge shape, with the warmer air sliding up over the cooler air. Thus, in the cooler-air region, a temperature inversion is present, sometimes causing poor air drainage and stagnation.
Figure 5.27 illustrates a typical warm front, in which gentle lifting of mT air leads to stratiform cloud development and characteristic nimbostratus clouds as well as drizzly precipitation. A warm front presents a progression of cloud development to an observer: High cirrus and cirrostratus clouds announce the advancing frontal system; then come lower and thicker altostratus clouds; and, finally, still lower and thicker stratus clouds appear within several hundred kilometers of the front. A line with semicircles facing in the direction of frontal movement denotes a warm front on weather maps.
MIDLATITUDE CYCLONIC SYSTEMS
The conflict between contrasting air masses can develop a Midlatitude Cyclone, also known as a Wave Cyclone. These are migrating low pressure weather systems that occur in the middle latitudes, outside the tropics. They have a low pressure center with converging, ascending air spiraling inward counterclockwise in the northern hemisphere and inward clockwise in the southern hemisphere, owing to the combined influences of the pressure gradient force, coriolis force, and surface friction. These systems, which can be 1000 miles wide, dominate weather patterns in the middle and higher latitudes. Because of the undulating nature of frontal boundaries and of the jet streams that steer these cyclones across continents, the term Wave is appropriate.
The intense high speed winds of the jet streams guide cyclonic systems, with their attendant air masses, across the continent along storm tracks that shift in latitude with the sun and the seasons. Typical storm tracks crossing North America are farther northward in summer and farther southward in winter. As the storm track begin to shift northward in the spring, cP and mT air masses are in their clearest conflict. This is the time of the strongest frontal activity, featuring thunderstorms and tornadoes.
LIFE CYCLE OF A MIDLATITUDE CYCLONE
On average, a midlatitude cyclonic system takes 3-10 days to progress through this life cycle from the area where it develops to the area where it finally dissolves.
-The first stage is Cyclogenesis, the atmospheric process in which low pressure wave cyclones develop and strengthen. This process usually begins along the polar front, where cold and warm air masses converge and are drawn into conflict, creating potentially unstable conditions. For a wave cyclone to from along the polar front, a compensating area of divergence aloft must match a surface point of air convergence. Even a slight disturbance along the polar front, perhaps a small change in the path of the jet stream, can initiate the converging, ascending flow of air and thus a surface low pressure system.
-In the Open Stage, warm air to the east of the developing low pressure center of a Northern Hemisphere midlatitude cyclone begins to move northward along an advancing front, while cold air advances southward west of the center. As the midlatitude cyclone matures, the counterclockwise flow draws the cold air mass from the north and west and the warm air mass from the south.
-Next is the Occluded Stage. Remember the relation between air temperature and the density of an air mass. The colder cP air is denser than the warmer mT air mass. This cooler, more unified air mass, acting like a bulldozer blade, moves faster than the warm front. Cold front can travel at an average 25 mph, whereas warm front travel at 10-15 mph. Thus, a cold front often overtakes the cyclonic warm front and wedges beneath it, producing an Occluded Front (occlude means to close).
-Finally, the Dissolving Stage of the midlatitude cyclone occurs when its lifting mechanism is completely cut off from the warm air mass that was its source on energy and moisture. Remnants of the cyclonic system then dissipate in the atmosphere, perhaps after passage across the country.
WEATHER MAPS AND FORECASTING
Synoptic Analysis is the evaluation of weather date collected at a specific time. Building a database of wind, pressure, temperature, and moisture conditions is key to numerical, or computer-based, weather prediction and the development of weather-forecasting models. Development of numerical models is a great challenge because the atmosphere operates as a nonlinear system, tending toward chaotic behavior. Slight variations in the input data or slight changes in the model's basic assumptions can produce widely varying forecasts.
Weather data necessary for the preparation of a synoptic map and forecast include the following:
-Pressure Tendency (steady, rising, falling)
-Surface Air temperature
-Dew Point Temperature
-Wind Speed, Direction, and Character (gusts, squalls)
-Type and Movement of Clouds
-State of the Sky (current sky conditions)
-Visibility; vision obstruction (fog, haze)
-Precipitation since last observation
Weather is a continuous reminder that the flow of energy across the latitudes can at times set into motion destructive, violent events.
By definition, a Thunderstorm is a type of turbulent weather accompanied by lightning and thunder. Such storms are characterized by a buildup of giant cumulonimbus clouds that can be associated with squall lines of heavy rain, including sleet, blustery winds, hail, and tornadoes. Thunderstorms may develop within an air mass, in a line along a front (particularly a cold front), or where mountain slopes cause orographic lifting. Thousands of thunderstorms occur on earth at any given moment. Equatorial regions and the ITCZ experience many of them, especially Kampala, Uganda, which sits right on the equator and averages a record 242 days a year with thunderstorms. In North America, most thunderstorms occur in areas dominated by mT air masses.
A thunderstorm is fueled by the rapid upward movement of warm, moist air. As the air rises, cools, and condenses to form clouds and precipitation, tremendous energy is liberated by the condensation of large quantities of water vapor. This process locally heats the air, causing violent updrafts and downdrafts as rising parcels of air pull surrounding air into the column and as the frictional drag of raindrops pulls air towards the ground.
Turbulence and Wind Shear
A distinguishing characteristic of thunderstorms is turbulence, which is created by the mixing of air at different densities or by the movement of air layers at different speeds and directions in the atmosphere. Thunderstorm activity also depends on wind shear, the variation of wind speed and direction with high altitude- high wind shear (extreme and sudden variation) is needed to produce hail and tornadoes, two by-products of thunderstorm activity.
Thunderstorms can produce severe turbulence in the form of Downbursts, which are strong downdrafts that cause exceptionally strong winds near the ground. Downbursts are classified by size: A Macroburst is at least 2.5 mi wide and has winds in excess of 130 mph; a Microburst is smaller in size and speed. Downbursts are characterized by the dreaded high-wind shear conditions that can bring down an air craft. Such turbulence events are short-lived and hard to detect.
The strongest thunderstorms are known as supercells, and give rise to some of the world's most severe and costly weather events (such as hailstorms and tornadoes). Supercells often contain a deep, persistently rotating updraft called a MesoCyclone, a spinning, cyclonic rising column of air associated with a convective storm and ranging up to 6 miles in diameter. A well-developed mesocyclone will produce heavy rain, large hail, blustery winds, and lightning; some mature mesocyclones will generate tornado activity.
The conditions to conducive to forming thunderstorms and more intense supercells- lots of warm, moist air and strong convective activity- are enhanced by climate change. However, wind shear, another important factor in thunderstorm and supercell formation, will likely lessen in the midlatitudes as Arctic warming reduces overall temperature differences across the globe.
LIGHTNING AND THUNDER
Lightning is the term for flashes of light caused by enormous electrical discharges. A buildup of electrical energy polarity between areas within a cumulonimbus cloud or between the cloud and ground creates lightning. The violent expansion of this abruptly heated air sends shock waves through the atmosphere as the sonic bang of Thunder.
Data from NASA's Lightning Imaging Sensor show that about 90% of all strikes occur over land in response to increased convection over relatively warmer continental surfaces.
Ice pellets that form within a cumulonimbus cloud are known as hail- or hailstones, after they fall to the ground. During hail formation, raindrops circulate repeatedly above and below the freezing level in the cloud, adding layers of ice until the circulation in the cloud can no longer support their weight. Hail may also grow from the addition of moisture on a snow pellet.
Hail is common in the US and Canada, although somewhat infrequent at any given place.
A tornado is a violently rotating column in air in contrast with the ground surface, usually visible as a spinning vortex of clouds and debris.
The updrafts associated with thunderstorm squall lines and supercells are the beginning stages of tornado development (however, fewer than half of supercells produce tornadoes). As moisture-laden air is drawn up into the circulation of a mesocyclone, energy is liberated by condensation, and the rotation of air increases speed. The narrower the mesocyclone, the faster the spin of converging parcels of air being sucked into the rotation; this movement may form a smaller, dark gray Funnel Cloud that pulses from the bottom side of the supercell cloud. This funnel cloud may lower to earth, resulting in a tornado. When tornado circulation occurs over water, surface water is drawn some 10-16ft up into the funnel, forming a Waterspout.
Pressures inside a tornado usually are about 10% less than those in the surrounding air. The inrushing convergence created by such a horizontal pressure gradient causes high wind speeds.
North America faces more tornadoes than anywhere on Earth because its latitudinal position and topography are conducive to the meeting of contrasting air masses and the formation of frontal precipitation and thunderstorms. Tornado occurrence in the US is highest in Texas and Oklahoma, the southern part of the region called tornado alley. May and June are the peak months for tornadoes in the US.
Any actual increases in tornado occurrence may in part relate to rising sea surface temperatures with climate change. Warmer oceans increase evaporation rates, which increase the availability of moisture in the mT air masses, thus producing more intense thunderstorm activity over certain areas in the US. Other factors in tornado development, such as wind shear, are not as well understood, are difficult to model, and cannot yet be definitively linked to climate change.
Originating entirely within tropical air masses, Tropical Cyclones are powerful manifestations of the Earth-Atmosphere energy budget. (Remember that the tropics extend from the Tropic of Cancer at 23.5 N latitude to the Tropic of Capricorn at 23.5 S latitude.
Tropical cyclones are classified according to wind speed; the most powerful are HURRICANES, TYPHOONS, or Cyclones, which are different regional names for the same type of tropical storm. The three names are based on location: Hurricanes occur around North America, Typhoons in Japan and Philippines, and cyclones in Indonesia, Bangladesh, and India.
A Super Typhoon is when wind speeds reach 150 mph.
Cyclonic storms forming in the tropics are quite different from midlatitude cyclones because the air of the tropics is essentially homogeneous, with no fronts or conflicting air masses of differing temperatures. In addition, the warm air and warm seas ensure abundant water vapor and thus the necessary latent heat to fuel these storms. Tropical cyclones convert heat energy from the ocean into mechanical energy in the wind- the warmer the ocean and the atmosphere, the more intense the conversion and the more powerful the storm.
A tropical cyclone begins with the cyclonic motion of a slow moving easterly wave of low pressure in the trade wind belt of the tropics. if the sea surface temperature exceed 79 F, a tropical cyclone may form along the eastern (leeward) side of one of the migrating troughs of low pressure, a place of convergence and rainfall. Surface airflow then converges into the low pressure area, ascends, and flows outward aloft. This important divergence aloft acts as a chimney, pulling more moisture-laden air into the developing system. To maintain and strengthen this vertical convective circulation, there must be little to no wind shear to interrupt or block the vertical airflow.
Tropical cyclones have steep pressure gradients that generate inward-spiraling winds toward the center of low pressure-low central pressure causes stronger pressure gradients, which, in turn, cause stronger winds. However, other factors come into play, so that the storms with the lowest central pressure arent always the strongest or the most damaging.
As winds rush toward the center of a tropical cyclone, they turn upward, forming a wall of dense rain bands called the Eyewall- this is the zone of most intense precipitation. The central area is designated the Eye of the storm, where wind and precipitation subside; this is the warmest area of the storm, and although clear skies can appear here, they may not always be present, as commonly believed.
Vertically, a tropical cyclone dominates the full height of the troposphere. These storms move along over water at about 10-25 mph. The strongest winds are usually recorded in the storms right-front quadrant (relative to its directional path). At Landfall, where the eye moves ashore, dozens of fully developed tornadoes may be located in this high-wind sector.
The relationship of the DAR and MAR to the environmental lapse rate, or ELR, at a given time and place determines the stability of the atmosphere over an area. In turn, atmospheric stability affects affects cloud formation and precipitation patterns, some of the essential elements of weather.
Temperature relationships in the lower atmosphere produce conditions ranging from stable to unstable.
When the ELR is less than the DAR and MAR, the lower atmosphere is STABLE.
When the ELR exceeds the DAR, the atmosphere is UNSTABLE.
When the ELR is between the DAR and MAR, the atmosphere is neither stable or unstable, a situation known as CONDITIONAL INSTABILITY.
Under these conditions if an air parcel is less than saturated, it will resist upward movement, unless forced. But if the air parcel becomes saturated, it will now be unstable and continue to rise.
Low clouds, ranging from surface levels up to 2000m in the middle latitudes, are STRATUS (flat clouds, in layers) or CUMULUS (puffy clouds, in heaps).
When stratus clouds yield precipitation, they are NIMBOSTRATUS.
Sometimes near the end of the day, lumpy, grayish, low-level clouds called STRATOCUMULUS may fill the sky in patches.
Middle-level clouds are denoted by their prefix Alto-.
ALTOCUMULUS clouds, in particular, represent a broad category that includes many different types.
Clouds at high altitude, principally composed of ice crystals, are called CIRRUS.
A Cumulus cloud can develop into a towering giant CUMULONIMBUS cloud. Such clouds are called Thunderheads because of their shape and their associated lightning, thunder, surface wind gusts, updrafts and downdrafts, heavy rain, and hail.
FOG is a cloud that occurs at ground level.
Radiative Cooling of a surface that chills the air layer directly above the surface to the dew-point temperature creates saturated conditions and Radiation Fog.
ADVECTION FOG forms when air in one place migrates to another place where conditions exist that can cause saturation- for example, when warm, moist air moves over cooler ocean currents.
UPSLOPE FOG is produced when moist air is forced to higher elevations along a hill or mountain.
Another fog caused by topography is VALLEY FOG, formed because cool, denser air settles in low-lying areas, producing fog in the chilled, saturated layer near the ground.
Another type of fog resulting from evaporation and advection forms when cold air flows over the warm water of a lake, ocean surface, or swimming pool. This EVAPORATION FOG, or steam fog, may form as the water molecules evaporate from the water surface into the cold overlying air.
An AIR MASS is a regional volume of air that is homogenous in humidity, stability, and cloud coverage and that may extend through the lower half of the troposphere.
Air masses are categorized by their moisture content- m for maritime (wetter) or c for continental (drier)- and their temperature, a function of latitude- designated A (arctic), P (polar), T (tropical), E (equatorial), or AA (antarctic).
The air masses of greatest significance to the US/Canada are:
Continental Polar (cP), Maritime Polar (mP), and Maritime Tropical (mT).
As air masses migrate from source regions, their temperature and moisture characteristics slowly change to the characteristics of the land over which they pass.
For example, an mT air mass may carry humidity north to Chicago and on to Winnipeg, but it gradually loses its initial high humidity and warmth with each day's passes northward.
Conflicting air masses may produce a COLD FRONT (and sometimes a zone of strong wind and rain) or a WARM FRONT.
A zone right along or slightly ahead of the front, called a Squall Line, is characterized by turbulent and wildly changing wind patterns and intense precipitation.
COLD FRONT: The steep face of an advancing cold air mass reflects the ground-hugging nature or cold air, caused by its greater density and more uniform characteristics compared to the warmer air mass it displaces.
Warm, moist air in advance of the cold front lifts upward abruptly and experiences the same adiabatic rates of cooling and factors of stability or instability that pertain to all lifting air parcels.
WARM FRONT: Warm air masses can be carried by the jet stream into regions with colder air, such as when an air-flow called the Pineapple Express carries warm, moist air from Hawaii and the Pacific to California and the Southwest during July and August.
The leading edge of an advancing warm air mass is unable to displace cooler, passive air, which is denser along the surface. Instead, the warm air tends to push the cooler, underlying air into a characteristic wedge shape, with the warmer air sliding up over the cooler air. Thus, in the cooler air region, a temperature inversion is present, sometimes causing poor drainage and stagnation.
Thunderstorms are fueled by rapid upward movement of warm, moist air and are characterized by turbulence and wind shear.
In strong thunderstorms known as Supercells, a cyclonic updraft- a MESOCYCLONE- may form within a cumulonimbus cloud, sometimes rising to the mid-troposphere.
Thunderstorms produce lightning (electrical discharges in the atmosphere), Thunder (sonic bangs produced by the rapid expansion of air after intense heating by lightning), and Hail (ice pellets formed within cumulonimbus clouds).
Strong linear winds in excess of 58mph, known as DERECHOS, are associated with thunderstorms and bands of showers crossing a region. These straight line winds can cause significant damage and crop losses.
A Tornado is a violently rotating column of air in contact with the ground surface, usually visible as a dark gray FUNNEL CLOUD pulsing from the bottom side of the parent cloud. A waterspout forms when a tornado circulation occurs over water.
THE ENHANCED FUJITA SCALE, or EF Scale, which classifies TORNADOES according to wind speed as indicated by related property damage.
To assist with wind estimates, the EF Scale includes damage indicators, representing types of structures and vegetation affected, along with degree of damage ratings.
Tropical Cyclones have steep pressure gradients that generate inward-spiraling winds toward the center of low pressure-lower central pressure causes stronger pressure gradients, which, in turn, cause stronger winds.
As winds rush toward the center of a tropical cyclone, they turn upward, forming a wall of dense rain bands called the Eyewall- this is the zone of most intense precipitation. The central area is designated the Eye of the storm, where wind and precipitation subside; this is the warmest area of the storm, and although clear skies can appear here, they may not always be present, as commonly believed.
Vertically, a tropical cyclone dominates the full height of the troposphere. These storms move along over water at about 10-25 mph. The strongest winds are usually recorded in the storm's right-front quadrant relative to its directional path). At Landfall, where the eye moves ashore, dozens of fully developed tornadoes may be located in this high-wind sector.
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