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Geol 1340 Chapter 5
Terms in this set (61)
Water and Atmospheric Moisture
Water on Earth.
Unique Properties of Water.
Clouds and Fog.
Water vapor is the gas phase:
- Each molecule moves independently
- Compressible gas
Water is the liquid phase:
- Water reaches it's greatest density at 4 oC (39o F)
- Density decreases below this temperature
Ice is the solid phase: Solid ice less dense than liquid water (ice floats)
direct change of water vapor to ice or ice to water vapor.
Specific Humidity = Mass of water vapor (in grams) per mass of air (in kilograms) at a given temperature:
= Grams of water vapor / Kg of air
Relative Humidity (RH) expressed in percent
= (Mass of Water in Air x 100) / (Mass Of Water Air Can Hold)
How is maximum specific humidity related to temperature?
Maximum specific humidity increases with temperature. Warmer air can hold more water vapor. More water vapor is required to saturate warmer air.
Temperature at Which Water Vapor Begins to Condense
Tdb, can be measured using a normal thermometer freely exposed to the air
Wet bulb temperature
Twb, is indicated by a moistened thermometer bulb exposed to air flow.
The Amount of Pressure Exerted By Water Vapor in the Atmosphere.
As more water vapor enters the atmosphere, the amount of pressure exerted by that water vapor increases. When the vapor pressure maximum is reached, no more water can enter the atmosphere and the atmosphere is saturated.
Saturation Vapor Pressure
The higher the temperature, the greater the saturation vapor pressure
- Differences in temperature create changes in density within an air parcel:
Warmer air: lower density
Cold air: higher density
- Two opposing forces work on a parcel of air: Upward buoyancy force, Downward gravitational force
- A parcel of lower density air will rise (more buoyant) while denser air will descend (less buoyant)
The warming and cooling rates for a parcel of expanding or compressing air are termed adiabatic: Ascending air parcel cools by expansion in response to reduced pressure at higher altitudes, Descending air heats by compression due to increasing pressure at lower altitudes.
- Dry adiabatic rate (DAR) is the rate at which "dry" air cools by expansion or heats by compression
- Moist adiabatic rate (MAR) is the average rate at which ascending air that is moist (saturated) cools by expansion: Latent heat of condensation in moist air is liberated as sensible heat, reducing the adiabatic rate of cooling. MAR less than DAR
Dry adiabatic rate
- 10 C°/1000 m
- 5.5 F°/1000 ft
Moist adiabatic rate
- 6 C°/1000 m
- 3.3 F°/1000 ft
Normal lapse rate
Average drop in temperature with increasing altitude (6.4 Co per 1000 m) for still, calm air
Environmental lapse rate
Actual lapse rate for air at a particular place and time: Can be lower or higher than the normal lapse rate depending on conditions
Stable Atmospheric Conditions
- Environmental lapse rate less than both the DAR and MAR
- Both moist and dry air parcels have adiabatic rates higher than the environmental lapse rate
- Both parcels remain cooler than surrounding atmosphere and are forced to settle back to original positions
Unstable Atmospheric Conditions
- Rising air parcel has a lower adiabatic rate than the environmental lapse rate of the surrounding air.
- Air parcel always warmer than surrounding atmosphere.
- Parcel continues to rise, leading to saturation and cloud formation.
A cloud is an aggregation (grouping) of moisture droplets and ice crystals suspended in air:
- Rising air parcel cools to dew point
- Further lifting causes active condensation of water vapor around condensation nuclei (dust, soot, ash, etc.)
- Cloud initially composed of microscopic moisture droplets
- A million or more moisture droplets aggregate to form a rain drop
Clouds Defined By Form And Altitude
Three basic forms:
- Stratiform: Develop horizontally as flat and layered clouds
- Cumuliform: Puffy and globular clouds
- Cirroform: Wispy
- Low clouds: Surface up to 2,000 m - Middle: 2,000
- 6,000 m (preface alto-)
- High: Above 6,000 m
- Vertically developed
- Cirrus are high altitude clouds composed of ice crystals
- Exhibit thin, wispy forms
- Often associated with an approaching storm
- Stratus clouds usually low to middle altitude
- Composed of water droplets
- Appear dull, gray, and featureless
- Nimbostratus clouds yield precipitation where showers typically fall as drizzling rain
- Cumulus are low-level puffy clouds composed of water droplets
- Associated with fair weather
- Stratocumulus clouds are lumpy, grayish, low-level clouds that sometimes indicate clearing weather
- Altocumulus clouds are middle-level clouds that appear in patchy rows or wave patterns
- Towering giant, vertical clouds often called thunderheads
- Associated with strong storms, thunder and lightening
- High-altitude winds may shear the top into the characteristic anvil shape
- Fog is a cloud layer on the ground that restricts visibility to less than 1 km
- Several types: Advection fog, Evaporation fog, Upslope fog, Valley fog, Radiation fog
- Advection fog forms when air in one place migrates to another place where conditions cause saturation
- Warm, moist air moves over cooler ocean currents, lake surfaces or snow masses
- Air layer directly above the cooler surface is chilled to the dew point
- Evaporation fog forms when water molecules evaporate from the water surface into cold, overlying air
- Also known as steam fog or sea smoke
- A type of advection fog where moist air is forced to higher elevations along a hill or mountain
- Adiabatic expansion results in cooling and condensation
- Resulting upslope fog forms a stratus cloud at the level of saturation
- Common in winter and spring along the Appalachians and eastern slopes of the Rockies
- Cool, dense air settles in low-lying areas
- Valley fog forms as a chilled, saturated layer near the ground
- Radiation fog forms when radiative cooling of a surface chills the air layer directly above the ground to the dew-point temperature.
- Often occurs on clear nights over moist ground.
- Air Masses
- Atmospheric Lifting Mechanisms
- Mid-latitude Cyclonic Systems
- Violent Weather
- An air mass reflects the characteristics of its source region
- Examples: Cold Canadian air mass; Moist, tropical air mass
- Air masses are generally classified according to the temperature and moisture characteristics of their source regions: Moisture designated as maritime (m) (wetter) or continental (c) (dryer); Temperature designated by latitude as arctic (A), polar (P), tropical (T), equatorial (E) or Antarctic (AA)
Air Mass Modification
- As air masses migrate from their source regions, their temperature and moisture characteristics are modified by the areas over which they pass
Warm, humid maritime air mass passes into cooler continental areas
Dry, cold continental air masses from polar regions move south and east over the Great Lakes
- Air masses eventually lose their initial characteristics due to migration into areas of different moisture and temperature characteristics
- Air flows toward an area of low pressure
- Air flows from different directions into the same low pressure area
- Converging air is forced upward
- Uplifted air undergoes adiabatic cooling, cloud formation, and possibly precipitation
- Example: Trade winds converge along the Intertropical Convergent Zone
- Air lifting stimulated by local surface heating
- Caused by relatively cooler air mass moving over warmer land
- Heating from the warmer land causes lifting and convection in air mass
- Warmer land can include: Urban heat island, Area of darker soil in a plowed field
- Air is forced over a barrier such as a mountain range
- Mountain acts as a topographic barrier to migrating air mass: Air forcibly lifted upslope on the windward side; Lifting air cools adiabatically: Precipitation may result; Descending air mass on leeward side undergoes adiabatic heating and becomes dryer
- Air lifted along the leading edges of contrasting air masses
- Fronts are boundaries between HIGH PRESSURE air masses with differing properties: COLD and DRY (high density). WARM and MOIST (low density).
- The less-dense warm mT air is lifted up and over the denser cold cP air: Frontal boundary is often the site of storm activity and precipitation
- Cold dense air pushing low density warmer air. Cold air forces warm air aloft. 400 km wide (250 mi) - Relatively rapid movement. Convective activity (thunderstorms)
- Cumulonimbus clouds are commonly associated with cold fronts
- Warm low density air pushing cold dense air. Warm air moves up and over cold air. 1000 km wide (600 mi). Relatively slow movement. Slow steady rain or snow.
- Cirrus clouds followed by stratus clouds are commonly associated with an approaching warm front.
Faster moving cold front overtakes a warm front: Two bodies of cold air associated with each front collide. This forces the warm air between them to rise. Results in uplift of warm air that is no longer in contact with ground. This uplift of warm air results in cloud formation even though frontal system has no contact with the ground.
Cold And Warm Air Masses Converge And Are Drawn Into Conflict
Warm Air Moves Northward And Cold Air Advances Southward
Cold Air Mass Overtakes The Warm Front
Lifting Mechanism Of Cyclone Results In Continuous Layer Of Cooler Air Beneath Warmer Air.
- Cyclonic storms and associated air masses move across the continent along storm tracks guided by the jet stream
- Storm tracks shift in latitude with the seasons: Northward shift in spring occurs when cP and mT air masses are in greatest conflict; Strongest frontal activity thus occurs in spring and often associated with thunderstorms and tornadoes.
- Thunderstorms may develop within an air mass, along a cold front, or result from orographic lifting along a mountain slope.
- Large quantities of water vapor in clouds condense, releasing tremendous amount of latent heat: Liberated heat lowers density and increases buoyancy of surrounding air
How Lightening Possibly Forms
- Precipitation formation and turbulence within cumulonimbus cloud causes positive and negative charges to develop on different sized water droplets and/or ice crystals: Lighter positive charges rise while heavier negative charges sink within the cloud; Negative charges at base of cloud repel negative charges on ground surface, leaving a net positive charge on ground; Charge difference is then neutralized by lightning bolt.
A Lightening Stroke Creates Thunder
A lightening stroke can heat air to 30,000 oC, causing rapid expansion of the air and formation of a compression wave that we hear as thunder.
- Hail generally forms within a cumulonimbus cloud
- Hail typically pea or marble sized, but can occasionally achieve the size of golf balls and even baseballs
- Derechos are strong linear winds in excess of 26 m/s (58 mph) associated with thunderstorms and bands of showers
- Capable of overturning boats, hurling flying objects, and breaking tree limbs
- Form when strong downbursts in a thunderstorm system blast strong winds outward: Linear paths fan out along curved-wind fronts over a wide area of land
Formation of a Tornado
- Begins with a layer of unstable warm, moist air trapped beneath a ceiling of cold, dense air
- Warm buoyant air suddenly finds an opening in the overlying ceiling
- Warm air rushes rapidly and violently upward through the opening to create a narrow zone of very intense low pressure
- Surrounding air and associated debris near ground level rushes in from all directions, creating a vortex
Development Of Cyclonic Motion
Cyclonic motion begins with slowmoving easterly wave of low pressure in the trade wind belt and sea-surface temperatures greater than 26 oC (79o F): Cyclone forms on eastern side of migrating trough of low pressure.
Cross Section Of Hurricane
- Counterclockwise rotation in Northern Hemisphere
- Strong winds spiral upward in eye wall
- Lowest pressure within eye where sinking air produces calm conditions
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