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Extreme Weather Midterm 2
Terms in this set (98)
Cloud near the ground
•2 ways that fog can form:
-Air is cooled to its dew point
-Moisture is added to the air
Also know as ground fog
• Formed by the cooling of land after sunset by thermal (infrared) radiation in calm conditions
• Air is cooled to dew point
Air cools at higher elevations. Cold air moves into valley. Cold air drainage leads to temperature drop to dew/condensation point
Created when relatively humid air moves up a gradual slope.
•Because of the upward movement, air expands and cools.
•If the dew point is reached, an extensive layer of fog is formed
Formed by the slow passage of relatively warm, moist, stable air over a colder (sometimes wet) surface.
•Advection = Horizontal Movement
(i.e. Fog on Golden Gate Bridge)
Evaporation (Stream) Fog
Warm water, cool air
• Water evaporates but then condenses in the cool atmosphere
• Common in early fall
Precipitation falls from air. Evaporative cooling leads to saturation/fog
Cloud Condensation Nuclei "CCN"
small particle, 0.2 microns (0.002 mm)
Collision and Coalescence
Must be a high water content within the clouds
• There must be sufficiently strong and consistent updrafts within the clouds
• A large range of clouds droplets occur within the clouds
• The cloud must be thick enough so that the clouds droplets have enough time to gather smaller droplets
Air converges as it is forced upward
• This process carries smaller cloud droplets up into the cloud while larger droplets stay suspended within the cloud
• But as you might have guessed some of the droplets collide with one another.
A drop must be larger than the other drops to be an efficient collider
•If the drop is too big it will be less efficient, because it create high pressure that pushes smaller drops out of the way
Have temperatures below 0 degrees C throughout
• Consist entirely of ice crystals, supercooled droplets, or a mixture of the two
Have temperatures above 0 degrees C in the lower reaches of the clouds
• Subfreezing conditions above!
Bergeron Ice-Crystal Process
• Clouds consist of both liquid and frozen water
• Supercooled droplets will freeze instantly onto a particle with a crystal structure
• Water molecules continue to freeze directly onto the snowflakes.
• Process will continue until the flake gets large enough to fall
Examples of Liquid Precipitation
Rain, Drizzle, Virga
•Can originate as liquid drops or ice •Ice melts as it falls through warmer air •Droplets can come in multiple shapes •Drizzle: droplets are smaller (less than 0.5 mm)
Any form of precipitation that does not reach the ground
• The precipitation evaporates as it falls through the atmosphere
• Occurs mostly in the summer
- Warmer temperatures, higher evaporation
Examples of Frozen Precipitation
Snow, Flurries, Blizzard
Ice crystals grow in a cloud
•Variety of shapes depending on the temperature and humidity
Examples of Freezing Precipitation
Hail, Sleet, Freezing Rain, Grapple
Consists of ice pellets
•Updrafts carry a particle into the colder reaches of a cloud, and the liquid water coats the particle
Occurs when an ice crystal takes on additional mass by accretion/accumulation of additional layers of supercooled cloud droplets
•Contains air pockets •Has a spongy texture
Small frozen raindrops • Most common along warm fronts • Occurs as rain falling from a cloud, passes through a cold layer,
and freezes into ice pellets (or snow melting in a warm layer only to freeze in a colder layer near the ground)
Ice crystal falls through warm air aloft and melting occurs
•Then liquid water falls through air at or near 0 degrees C
•Droplets refreeze or become supercooled
Process to make rain
•Involves injecting a material into current nonprecipitating clouds
•Common Seeding Materials:
-Dry Ice: promotes freezing and the production of supercooled rain or ice
-Silver Iodide: Initiates the Bergeron process by acting as an ice nucleus
•Fans to mix air if it is a little warmer 10 meters above the ground
In the French wine-growing regions church-bells were traditionally rung in the face of oncoming storms, later replaced by firing rockets or cannons.
•pseudoscientific device and purported shock wave generator claimed to disrupt the formation of hailstones in the atmosphere.
•no scientific evidence for their effectiveness.
caused by the weight of all the air in the atmosphere pressing down on Earth
When air rises, it leaves behind an area of lower pressure, because the upward-moving air is not pressing down so hard on the surface. Associated with warm temperatures
Formed when air is sinking back down, and so pushing down harder on the surface. Associated with cooler temperatures.
Standard Sea Level
How does pressure change with altitude
How does pressure change with temperature
How does pressure change with moisture/saturation
Cold Dry Air
Warm Moist Air
Winds blowing into a region
Winds blowing away from a region
ITCZ (Inter Tropical Convergence Zone)
Around the equator. Air flowing away from high pressure 30 degrees N and S
Pressure Gradient Force
System want to return to equilibrium. Thus air moves from high pressure to low
• Gradient: difference in pressure over a distance.
Winds move perpendicular to isobars (lines connecting points of equal pressure)
The Coriolis Effect
Deflection of Winds caused by the rotation of the Earth! Turning of an object's trajectory due to Earth's curvature. Greater at faster speeds. Causes air to travel parallel to isobars. Causes low pressure systems. to spin counterclockwise in the northern hemisphere and clockwise in the southern hemisphere
Macroscale Weather Events
Includes large planetary flows
Examples: Trade Winds
Synoptic Scale Weather Events
horizontal length scale of the order of 1000 kilometers
Example: Hurricane, mid-latitude depressions
Mesoscale Weather Events
Is associated with local winds that generally last from minutes to hours
Examples: Tornadoes, Thunderstorms, and Local Winds Events
Microscale Weather Events
Events that have a life span from a few seconds to minutes
Examples: Wind Gusts, and Dust Devils
Land heats up, warm air rises. Rising air forms clouds. Cooler air sinks in sea (which is cooler than land). Sinking air spreads along surface, and moves back onto land.
Cool air from land sinks to the ground and travels to water, where the warmer air rises, forms clouds, and travels back to land.
During the day, air along mountain slopes is heated more intensely than air in the bottom of a valley. This leads to warm air rising up along the mountain slope
Rapid radiation heat loss along the mountain slopes cools the air, which drains into the valley below
Warm, dry winds sometimes move down the east slope of the Rockies
Santa Ana Winds
These hot, dry, dust bearing winds invade California most often in autumn and bring temperatures that often exceed 90 degrees F.
Single Cell Circulation Model
Model invented by George Hadley in 1735. Hadley was aware that solar energy drives the winds. He proposed that the large temperature contrast between the poles and the equator creates one large convective cell in each hemisphere. While Hadley's principles are correct, it does not take into consideration the earth rotation on its axis.
Prevailing winds occurring around the equator
Prevailing winds between the subtropical high and subpolar low (in both hemispheres)
Sinking air in the subtropics piles up and must diverge. Subsiding air causes clear skies and warm temperatures. Weak winds. Cause of many major deserts. Some air flow northward, some flows southward to create trade winds.
Low is responsible for much of the stormy weather in the middle latitudes, particularly in the winter.
Super cold, dense air sinks. Air diverges toward lower latitudes.
Narrow Ribbons of high-speed winds that meander for thousands of kilometers.
Blow from west to east but
the flow can shift to north south.
100 kt + wind speed. 10-12 km in altitude. The thin ribbons of fast moving air occur where the warm subtropical and cool polar air masses meet. In this area there is a steep pressure / temperature gradient.
How would you identify a moist cold air mass?
Maritime Polar (mP)
How would you identify a dry cold air mass?
Continental Polar (cP)
How would you identify a moist warm air mass?
Maritime Tropical (mT)
How would you identify a dry warm air mass?
Continental Tropical (cT)
Movement of Air Mass
After an air mass forms, it normally migrates from the area where it acquired its distinctive properties to a region with different surface characteristics. Once movement starts the air mass causes changes to weather patterns. Eventually the characteristics of the mass start to change.
Maritime Polar (mP) Air Mass
Originates in the West Coast. Tends to be unstable. Heavy rains as cool moist air flows over mountains along the west coast.
Maritime Tropical (mT) Air Mass
Largely originates in the Gulf of Mexico. Warm, Moist, Unstable. Important source of moisture feeding storms all year round
Continental Tropical (cT) Air Mass
COriginates in SW. Hot, dry air. Boundary between cT and mT is often called the dry line. The dry line is often seen in surface and satellite data and is favored location form storm initiation
Continental Arctic Air Mass (cA)
Continental arctic air is distinguished from cP by its generally lower temperature. Sometimes these two air masses are considered one! Carries cold dry weather to southern areas
Occurs in the US. Describing the arrival of an extremely cold air mass of Arctic origins. AIR MASS DOES NOT
ORIGINATE IN SIBERIA. Associated with cA and cP masses.
Pushes Jet stream south (spilt) -
picks up tropical moisture, heavy rains over west coast.
Lake Effect Snow
temp contrast between water and
earth, causes heavy snow to the east of Great Lakes.
Boundary between 2 unlike air masses. Different temperature and moisture content
Warm air occupies areas formerly covered by cooler air. Often feature cirrus clouds
When cold, air advances into a region occupied by warmer air. Steeper slope than warm fronts. Associated with heavy precipitation
Air masses do not move. But winds blow parallel to the front. Can produce moderate precipitation. If the front stays
stationary for long periods of time flooding can start to take place
Cold front moves faster, catches up to warm front. Think of a zipping together of fronts. Once the warm air has disappeared (precipitated out) the atmosphere is stable
Cold front "lifts" the warm front up and over the very cold air
• Associated weather is similar to a warm front as the occluded front approaches
Cold air behind cold front is not dense enough to lift cold air ahead of warm front.
Cold front rides up and over the warm front.
4 Stages in Life Cycle of a Middle-Latitude Cyclone
•Cyclogenesis •Open Wave •Occluding •Dissipation
At this point two air masses of different densities (temperatures) are moving parallel to one another, but in opposite directions. Low pressure forms in the
middle as cyclonic flow occurs.
Under suitable conditions the frontal surface that is separated by two contrasting air masses will take on a wave shape. This wave can be up to several hundreds of kilometers long. Low pressure is deepening. No occlusion yet. Pressure at center continues to drop increasing windspeeds - pressure gradient
Bands of precipitation form.
Cold front catches warm front. Low pressure is deepest at this time (storm is mature). Occlusion develops.
Air from warm sector now all uplifted. Warm air removed from L, which reduces energy available for uplift and clouds / precipitation formation. Low pressure weakens, no more convergence.
Rapidly rotating column of air around intense low pressure that reaches the ground
•Maximum winds can sometimes exceed 300 miles per hour
•Usually 100-600m in diameter
•Sometimes referred to as twisters or cyclones
Formation of Tornado
What we do know:
-The most deadly tornadoes form in supercells
-Tornadoes can form in many types of severe weather events: cold fronts, squall lines, tropical cyclones
-Tornado formation is believed to be directed mainly around the mesocyclones in supercells
Forward Flank Downdraft (FFD)
This is generally the area of heaviest and most widespread precipitation. The precipitation core is bounded on its leading edge by a shelf cloud.
A vortex of air within a convective storm. Mesocyclone formation few miles up in the atmosphere, usually 1- 6 miles (2-10 km) across.
Tornado Formation Process
Mesocyclone formation • At first the mesocyclone is wide, short, and rotating slowly • Tighter wrapping of winds around the vortex - Lower Pressure • Column of rotating air narrows and stretches downward • Wall Cloud formation • Forms funnel cloud • Tornado forms when funnel clouds reaches the ground
A small-scale, persistent lowering from the base of a thunderstorm
a vortex of water vapor and air spinning at high velocity
• DO NOT REACH THE GROUND. WHEN THEY REACH THE GROUND, THEY BECOME LANDSPOUT/WATERSPOUT
Tornado Life Cycle
Organizing: Funnel extends downward and intensifies
Mature Stage: Most severe damage, greatest width
Shrinking: Decreased width, more tilt, narrow damage swath
Decay Stage: Rope like and contorted
Wall Cloud vs. Shelf Cloud
Shelf cloud appears on the leading edge of a storm (heavy precipitation), Wall cloud is usually found within or near the rear of the storm
• Wall clouds tend to slope inward, Shelf clouds tend to not have a slope
• Wall clouds are inflow features with air moving towards them whereas shelf clouds are an outflow feature with cool air moving away from the storm
F1 (lowest damage/speed, 65-85mph) - F5 (highest damage/speed, over 200mph)
Ordinary Single-Cell Thunderstorm
Develop in warm humid air masses •Short-lived •Only sometimes produce strong winds or hail •3 stages in lifespan: Cumulus, Mature, Dissipating
Cumulus Stage of a Thunderstorm
Convergence at the surface leads to uplift of air parcels
• Clouds are produced at the condensation lifting level
• Typically, thunderstorms initially develop cumulus clouds
• No precipitation, lightning, or thunder
• Strong updrafts keep droplets and crystals suspended
Mature Stage of a Thunderstorm
Most intense stage • Downdrafts aloft • Heavy precipitation, lightning, thunder.
• During the mature stage, updrafts exist side by side with downdrafts
• Clouds continue to enlarge
Dissipating Stage of a Thunderstorm
Begins 15-30 minutes after reaching mature stage
• Precipitation lightens
• Anvil top remains • Once downdraft begins, the vacating air and precipitation encourage entrainment of the cool, dry air surrounding the cell
Winds in excess of 58 miles per hour •Produce hailstones larger than 1.9 cm •Generate a tornado.
Form in wind-sheared environment
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