Flashcards in Exam 2 Study Guide Deck (80):
Which four forces control the movement of air?
-Pressure Gradient Force
What causes the Pressure Gradient Force? In which direction does it act?
PGF exists anywhere pressure changes with distance. The pressure Gradient Force acts from high pressure to low pressure.
What causes friction in moving air?
Friction in air occurs when faster moving molecules collide with slower moving molecules or the earths surface, causing air to slow down.
In which direction does the force of friction act?
Friction acts opposite the direction of air motion.
What is the Coriolis Force?
The Coriolis force is an apparent force associated with the rotation of the earth.
In what direction does the Coriolis force act in the northern hemisphere?
It acts on objects in motion across the Earths surface, and it acts to the right of the direction of motion in the northern hemisphere.
What is geostrophic balance
Geostrophic balance is the force balance that exists when the horizontal pressure gradient force is equal and opposite to the Coriolis force.
What is hydrostatic Balance?
Hydrostatic Balance is the force balance that exists when the vertical pressure gradient force is equal and opposite to the gravitational force.
What is the jetstream
The jetstream is the band of strong winds that circles the globe in middle latitudes just below the tropopause. Its horizontal position is associated with strong horizontal surface temperature gradients in the troposphere.
Between what altitudes is the jetstream typically found?
The average altitude of the core of the jetstream is typically around 300 mb (30,000 ft), but as low as the 500 mb level
How is the pressure Gradient Force related to the wind speed?
In the absence of other forces, the pressure gradient force is proportional to wind speed.
What does a strong pressure gradient look like on a sea-level pressure map?
The pressure gradient force is strong where isobars are close together and weak where there are far apart.
Given that the vertical pressure gradient force acts upward, why don't air molecules fly off into space?
Gravity acts to hold air on the earth, balancing the upward pressure gradient force.
Name three turbulence that occur in the atmosphere.
Develops when air collides with ground objects
Develops when warm air collides with ground objects
Develops when wind speed changes with distance, and faster moving air mixes with the slower moving air.
On a hot summer day, air flows off the south coast of the United States and over the Atlantic Ocean. What should happen to the depth of the friction layer once the air moves over the ocean?
The Depth of the friction layer would decrease, since the mechanical turbulence is reduced because the water is smooth compared to the land surface.
What is a typical depth of the friction layer in the atmosphere?
The friction layer varies in depth. On a night with weak winds, the boundary layer over a large lake may only extend upward a few hundred meters, while the boundary layer over a city on a hot, windy afternoon may extend upward a few thousand meters.
Summarize the four key properties of the Coriolis force that are important to understand severe and hazardous weather.
1. Causes objects to deviate to the right of their direction of motion in the Northern Hemisphere
2. Affects the direction an object will move across the earths surface, but has no effect on speed
3. Is the strongest for fast moving objects and zero for stationary objects
4. Is zero at the equator and max at the poles.
What forces are important for horizontal air motion in the boundary layer?
-Horizontal Pressure Gradient Force
When air is in Geostrophic Balance What is the direction of the flow and isobars on a constant pressure map?
The direction of air is parallel to the isobars on a constant pressure map, when the air is in geostrophic balance.
When air in a geostrophic balance moves into a stronger pressure gradient, what will happen to the wind speed?
The wind will accelerate and will no longer be in geostrophic balance
What is a jetstreak?
A jetstream is a local speed maximum embedded in jetstream.
What is the relationship between the Low-Level Temperature gradient and the pressure gradient aloft?
Strong pressure gradients aloft are found over strong low-level temperature gradients.
Why are jetstreams found above regions of strong temperature gradients in the atmosphere?
Because air density is proportional to temperature, pressure decreases more rapidly with height in cold air compared to warm air. This causes pressure surfaces to slope downward toward cold air, leading to strong horizontal pressure gradients. Strong horizontal pressure gradients in turn lead to strong winds.
How does a change in the curvature of airflow affect the speed of the flow as air flows from ridge to a trough? Trough to ridge?
Air decelerates from a ridge to a trough and accelerates from a trough to a ridge.
What is a jetstreak?
A jetstreak is a region of exceptionally strong winds embedded in a jetstream.
Where Will a low pressure center form at the surface relative to an upper air trough and ridge?
A low pressure center will form on the east side of the trough under the left exit region of the jetstreak
How does surface friction modify the pressure at the center of low and high pressure systems?
Surface friction causes air to flow into lows, increasing the central pressure, and out of highs, decreasing their central pressure.
If an air column is heated through latent heat release, how will the surface pressure change?
The surface pressure will lower when air column is heated. This is due to high pressure aloft, and air diverging from the air column.
If an air column is cooled through radiative cooling, how will the surface pressure change?
The surface pressure will increase when an air column is cooled. this is due to low pressure aloft, and air converging into the air column.
Explain how a change in wind speed can create divergence or convergence?
If air decelerates, it must pile up, or converge, across the region of deceleration. Similarly, if air accelerates, it must spread out, or diverge across the region of acceleration.
Is the jetstream is flowing in a curved pattern, where would regions of convergence and divergence be found?
Convergence would occure between the ridge and the downstream trough, and divergence between the trough and the downstream ridge.
If a jetstreak is present in the upper level flow, where would regions of convergence and divergence be found?
Divergence would occur in the right entrance and the left exit regions, and convergence in the left entrance and right exit regions of the jet streak.
How do flow curvature and jetstreaks combine to produce divergence and convergence at jetstream level?
When a jetstreak is in the base of a trough, flow curvature and jetstreak processes combine to produce convergence west of the trough axis under the left entrance region of the jetstreak, and divergence east of the trough axis under the left exit region of the jetstreak.
What is the relationship between a jetstreak and the jetstream?
A jetstreak is a local region of exceptionally fast winds embedded within the larger jetstreak flow.
Where is the entrance region of a jetstreak? the exit region?
The entrance region is where air is flowing into the jetstreak, and the exit region is where air is flowing out of the jetstreak.
Assume a ridge/trough/ridge configuration is present in the jetstream and a jetstreak is located in the base of the trough. Where in this flow pattern will the maximum divergence aloft be found? Convergence?
When a jetstreak is in the base of a trough, flow curvature and jetstreak precesses combine to produce divergence east of the trough axis under the left exit region of the jetstreak, and convergence west of the trough axis under the left entrance region of the jetstreak.
How does friction disrupt geostrophic balance near the surface?
Friction slows the winds, reducing the coriolis force and causing the pressure gradient force to deflect air toward low pressure.
How does surface roughness influence air flow in the friction layer?
The greater the surface roughness, the greater the deflection of air from high to low pressure.
If surface isobars are oriented south with low pressure to the west, approximately what direction will the surface wind be blowing?
Toward the northwest.
Where and in what seasons do semi permanent high pressure centers form at the surface?
Places that are relatively cool compared to the region around them. In the northern hemisphere: Winter: Canada and siberia.
Summer: North Atlantic and pacific Oceans.
Does air rise or decend in low pressure systems?
-Lows - Air rises
-Highs - Air decends
Consider a situation in which upper level divergence exceeds low level convergence. Is the surface system a high or low, and is the system becoming stronger or weaker?
If there is low level convergence there must be a low at the surface, if the upper level divergence exceeds the low level convergence, more air is being removed from the column than added so the low is strengthening.
Consider a situation in which upper level convergence exceeds low level divergence. is the surface system a high or a low, and is the system becoming stronger or weaker?
If there is low level divergence there must be a high at the surface. if the upper level convergence exceeds the low level divergence, more air is being added to the column than removed so the high is strengthening.
Why is cloudiness typically assosiated with surface low pressure systems?
Air rises and cools in low pressure systems, leading to adiabatic cooling, increasing relative humidity and cloud formation.
Why are clear skies typically associated with high pressure systems?
Air sinks in high pressure systems, leading to adiabatic warming, decreasing relative humidity and clearing skies.
How large and how deep is a typical airmass?
Area is Colorado size or much bigger; depth ranges from 1 or 2 km to the depth of the troposphere.
What characteristics make a region a good source for airmasses?
Good source regions are flat and have relatively homogenous surfacecharacteristics. Ex. Mountain surfaces are complex, and thus don't give rise to homogenous air masses. Oceans are flat and wet, and de give rise to homogenous airmass.
What is the relationship between air masses and fronts?
Fronts are boundaries between air masses.
What are the three criteria used to classify fronts?
-Thermal and moisture characteristics of the air masses
-direction and movement of the air masses
-whether or not the boundary between the air masses is in contact with the ground
What are the six different types of fronts?
- warm front
-Upper Level Front
List at least five variables that can be used to identify the position of fronts on weather maps
Temperature, dewpoint, wind speed, wind direction, pressure. Also, characteristic frontal weather: a line of showers or thunderstorms; a transition from clear to cloudy sky.
Where do cold air damming typically occur?
Cold air damming: Along east side of Appalachian Mountains. Cold air is also damming along east side of Rockies.
How does an occluded front develop?
Occluded fronts develop when a front overtakes a warm front, so that the warm air is no longer in contact with the surface, forming the trowal
What is the difference between a warm occlusion and a cold occlusion?
-Warm occlusion, air north of warm front is colder than air behind cold front
-cold occlusion, air behind cold front is colder than air of warm front.
Where is warm air found in the vicinity of an occluded front?
Aloft, not in contact with surface, again, this warm air at hight is called the 'trowal'
What is a dry line?
A dry line is an airmass boundary that is characterized by a sharp moisture difference, rather than a temperature difference, between the two airmasses.
Why are dry lines most commonly found in Texas, Oklahoma, and Kansas?
In the southern great plains, air flowing eastward from the high desert pleateauregions of the southern U.S. and mexico often encounters moist air flowing northward from the gulf of Mexico.
Explain why air can be lifted along a dry line when there is no temperature difference between the airmasses on either side of the boundary.
Dry air behind dry line is denser than poist air ahead of dry line.
How do the surface temperaure and dewpoint temperaure vary as an upper level front passed over head.
Surface Temperature and dewpoint show little or no variation since the front passed well over your head.
What type of front is often associated with flash flooding events?
Explain why maps depicting surface temperature contours are not very helpful in identifying dry lines and upper level fronts.
Dry lines and upper level fronts generally have little or no temperature contrasts at the surface.
What is the lifetime of a typical extratropical cyclone?
Several days to well over a week. They intensify for only 1-2 days though.
What is a typical size of an area affected by mature extratropical cyclone?
Several Hundred to a thousand miles across, up to a third the area of the contigous U.S.
What does a cyclone look like on a satellite images?
Where are warm fronts, cold fronts and dry lines found relative to the center of an extratropical cyclone?
-East (warm front)
-West/South (Cold Front)
-South (Dry Line)
What is the role of waves in the jetstream in the development of extratropical cyclones?
Waves create divergence aloft, reducing surface pressure and leading to formation of the surface low.
Name all of the boundaries that can trigger thunderstorm development south of a low pressure system
Upper level front, Dry Line, Cold Front
Where does upslope flow occur relative to the center of a low pressure system on the plains?
Northwest of cyclone center primarily
What is the trowal and where is it located relative to the center of an extratropical cyclone?
Trough of warm air aloft? a wedge of warm air aloft that wraps around a low pressure center to its N,NW and W. The comma head so to speak.
What type of weather may be found in the head of an extratropical cyclone?
-Head - Rain, Ice Pellets, Freezing rain, Snow
-Tail - Showers and thunderstorms
What types of fronts can be responsible for the tail of a comma cloud
Upper level front, dry line or cold front.
What is the warning, definition of a blizzard?
winds> 30 kts(35 MPH) and falling or blowing snow reducing visability to
Where do blizzards mostly occur in the United States?
Northern plains, OR North Dakota, South Dakota, Minnesota, Nebraska,
Why do ground blizzards occur?
Light Fluffy snow, loose snow is blown by fast winds, reducing visibility.
What geostrophic featurs of North America and surrounding water bodies support the development of blizzards over the Great Plains? What is the role of each of these features in creating conditions for bizzards?
-Mountains in Western U.S. - Block warm air from pacific from reaching interior and channel cold air from Canada southward.
-Gulf of Mexico - Provides moisture necessary for cloud and snow formation
A blizzard warning is issued for your area, what type of weather conditions should you expect?
Winds in excess of 30 knots (35 mph), falling or blowing snow, and visibility reduced to less that 1/4 mile for at least three hours.