What is pressure?
-the weight of the air above you
What is the relationship between pressure and height?
-pressure decreases with height
-pressure is heaviest at ground level
What is the equation of pressure?
pressure = force/area
-pressure depends on the area
How do airflow and pressure relate?
-Air flows from areas of higher pressure to areas of lower pressure
What is the average air pressure at sea level?
14.7 lb/in2 (almost 15 pounds)
or
1,013 mb
How is pressure measured?
With a barometer
-Mercury barometer
-Aneroid barometer
What is the recording instrument that provides a continuous record of pressure changes over time?
-Barograph
What are the atmospheric forces?
1. Pressure Gradient Force (P)
2. Coriolis Force (C)
3. Gravitation (G)
3. Friction (Fr)
How does P (pressure gradient force) push air?
-Pressure gradient forces air from areas of higher pressure to areas of lower pressure
How is the direction of the Pressure Gradient Force determined?
-Direction points from high to low pressure through the shortest distance possible
Pressure Gradient Force: Is the vertical pressure force stronger than the horizontal pressure force?
-the vertical pressure force is much stronger due to gravity
-Pv is 10,000 times bigger/stronger than Ph
What direction does the Vertical Pressure Gradient Force almost always point?
-From higher pressure to lower
-Upward (vertical)
What direction does the Horizontal Gradient Force almost always point?
-From higher pressure to lower
-Side to side (horizontal)
T/F: Air pressure is equal across all distances
False; Air pressure is relative to conditions/area
T/F: The Pressure Gradient Force has two components.
True; there is a vertical component and a horizontal component
What is the term describing the balance between the horizontal force and the vertical force in the Pressure Gradient Force?
Hydrostatic Balance
What is the Coriolis Effect?
-The effect of Earth's rotation on the direction of winds and currents
-modifies the direction of airflow
What direction does the Coriolis Effect flow?
-always to the RIGHT of motion
What is the size of the Coriolis force dependent on?
-the wind, speed of rotation, and the latitude
-Coriolis effect is ZERO at the equator and increases as it moves towards the poles (is max at the poles)
Which direction does the gravitational force flow?
-always points toward the center of the Earth
How is the gravitational force balanced?
-by the vertical gradient force = hydrostatic balance/equilibrium
What is the friction force?
-acts to slow moving objects, and affects wind speed
What direction does the Friction force flow?
-always acts OPPOSITE of motion
What determines the size of the Friction Force?
-wind speed; faster winds = more friction
-maximum at Earth's surface; depends on the 'roughness' of terrain
-Diminishes to zero at about 1km (3000 ft)
What is the Geostrophic Wind?
When a balance is reached between the Coriolis Force (CF) and the pressure gradient force (PGF)
-Here the wind flows in a straight path, parallel to
the contours at a constant speed
-an approximation to 'real wind'
What does the Geostrophic Wind assume?
1. Hydrostatic Balance is present
2. You are more than 1km above the surface (not useful for evaluating winds at ground level)
3. Wind speed and direction do NOT change
Which direction and size does the Geostrophic Wind flow?
Direction-flows parallel to lines with lower value contours on the left
Size(Speed)-inversely proportioned to isobar spacing
-small spacing = fast wind
-wide spacing = light wind
What are the Gradient Winds?
winds that flow along bends and cells of high or low pressure follow highly curved paths
-Ph and C are NOT balanced!
What direction do the Gradient Winds flow?
-parallel to lines with low pressure on the left (just like the geostrophic wind)
What determines the size of the Gradient Winds?
-inversely proportional to isobar spacing (small spaces = fast wind)
-faster than geostrophic wind around the Highs
-slower than geostrophic wind around the Lows
What is the differences between the Gradient Winds and the Geostrophic winds?
-Ph and C are NOT balanced in Gradient Winds!!
-Gradient winds flow faster than Geostrophic winds around High Pressure, and slower around Low Pressure
What direction do cyclones/cyclonic flow travel?
-MOVE AROUND LOW PRESSURE SYSTEM
-turn counter clockwise
-slower than geostrophic values
What direction do anticyclones/anticyclonic flow travel?
-MOVE AROUND HIGH PRESSURE SYSTEM
-turns clockwise
-"Aunti-Cyclone" memory tool
What is the difference between convergence and divergence?
-Convergence = joining/coming together
-the net inward transport of air causes a shrinking of the area it occupies (air piles up and increases in mass)
-Divergence = splitting apart/dividing
SEE FIGURE 6.22 P174
Convergence/divergence may occur near the surface due to what factor?
-Friction
Convergence/divergence may occur aloft in the atmosphere due to what factor?
-Wind direction and speed
-air will diverge if you change its direction
-air will converge if you change its direction as well
-example: interstate lanes mergeing
Convergence/divergence contributes to what?
-Vertical motion of air (rising air, subsiding air)
Convergence/Divergence can occur near the surface due to what?
Friction
Convergence/Divergence can occur aloft due to what?
Variations in wind direction and speed
What effect do mountains have on covergence/divergence?
-air passes over a mountain, compresses vertically, and produces divergence of the air aloft
-air reaching the other side of the mountain, vertically expands and produces convergence of air aloft
What is the circulation system of the atmosphere?
Earth's highly integrated wind system
What are the dynamics of the circulation system of the atmosphere?
The General Circulation system is one large system comprised of many smaller parts
-Large weather patterns persist longer than their smaller counterpart (ex. Dust Devil vs Cyclone)
Local winds are an example of which type of wind?
-Mesoscale winds
cause = temperature and pressure differences that result from variations in topography/surface conditions
What are the four types of Local Winds?
1. Land and Sea Breezes
2. Mountain and Valley Breezes
3. Chinook (Foehn) Winds and Katabatic (Fall) Winds
4. Country Breezes
What is the difference between a Land Breeze and a Sea Breeze?
-Land Breeze: AT NIGHT, land cools off more rapidly than the sea; Airflows off of the land
-Sea Breeze: DURING THE DAY, cooler air over the water moves inward toward an area of low pressure
What is a Lake Breeze>?
smaller-scale sea breeze
What is the difference between a Valley Breeze and a Mountain Breeze?
-Valley Breeze: DURING THE DAY, air along a mountain is heated and glides UP the mountain slope (WARM Air moves FROM valley TO mountain)
-Mountain Breeze: AT NIGHT, air along a mountain slope is rapidly cooled, which then drains into the valley (COLD Air
What are the Chinook (Foehn) Winds?
-warm dry winds (Chinooks), move DOWN the slope of a mountain; created when a strong pressure gradient develops
-produced a record Temperature rise
-can rapidly melt snow cover ("chinook" = snow eater)
What are the other Local Scale Winds?
Santa Ana Winds: Occur in Southern California; contribute to wildfires*
Katabatic ('Fall') Winds: cold dense air situated over a highland area begins to move into lowlands
Country Breezes: associated with large URBAN areas; on a clear night, light wind bl
What is the Global Circulation Model?
-developed largely from the average worldwide pressure/wind distribution
-A general pattern of pressure/wind
Caused by: differential heating of Earth's surface
What is the Single-Cell Model of Circulation?
-Produced by George Hadley
-Large temperature contrasts between the poles and the equator creates a large convection cell
-Upper level air flowing toward the poles and surface level air flowing toward the equator
What does the Single-Cell Model fail to consider?
-Earth's rotation
What is the Three-Cell Model of Circulation?
-comprised of three cells: Hadley, Ferrell, and Polar
-Considers Earth's rotation
What are the trade winds?
-flow toward the equator that is deflected by the Coriolis Effect
-Used by sailors to move goods between hemispheres
What are the prevailing westerlies?
-winds that blow from west to east in the middle of the latitudes
-Used by Ben Franklin to study storms
What are the polar easterlies?
-cold winds that blow from the east to the west near the North and South Poles
What is the difference between the prevailing westerlies and the polar easterlies?
-Prevailing westerlies blow W to E in the middle latitudes
-Polar easterlies blow E to W near the poles
What is the equatorial low?
-Also known as the Intertropical Convergence Zone (ITCZ)
-region of ascending moist, hot air that experiences abundant precipitation
-Location of where the trade winds converge
What are the subtropical highs?
-Location in which the prevailing westerlies and the trade winds originate and go their separate ways
-High pressure zones where a subsiding air column produces weather that is normally WARM and DRY
What are the polar highs?
-Location in which the polar easterlies originate
-Result from surface cooling
-This higher than average surface pressure is caused by the cold, dense air near the poles
Where do WE live: The Prevailing Westerlies or the Polar Easterlies?
-The Westerlies
What are jet streams?
narrow bands of strong winds; fast streams of air
-occur near top of Troposphere
What causes jet streams?
NOT less friction in atmosphere!!
-moving from North to South temperature change is concentrated in a narrow zone = polar front
-Strong horizontal pressure gradients are found over Polar Fronts and cause jet streams
What are the two main jet streams called?
-Polar jet stream: occurs along a polar front; generally flows from West to East
-Subtropical jet stream: occurs over the subtropics during winter; slowest of the jet streams; flows from West to East
What is a monsoon?
NOT a rainy season!!
-a wind system that reverses direction twice a year
What area of the world is most affected by monsoons?
Southeast Asia
During the 'dry season', winds flow which direction?
-Winds flow FROM the land to the sea
During the 'wet season', winds flow from which direction?
-Winds flow FROM the sea to the land
What is the El Nino and the La Nina?
-Unusual sea surface temperatures in the tropical portion of the Pacific Ocean
El Nino: warming of ocean water in the Eastern tropical Pacific ocean
La Nina: cool water extends from Eastern tropical Pacific Ocean farther west
What is the Southern Oscillation?
the swapping of high and low pressure zones in the South Pacific under El Ni�o conditions
What are air masses?
large bodies of air that have uniform temperature and moisture
Where do the characteristics of air masses come from?
The Source Region (the underlying surface)
-air masses must remain over their source region for a long time in order to take on characteristics of the underlying surface
What is an ideal source region?
-one that is characterized by weak winds and a uniform surface
What are the four types of air masses?
-continental polar (cP); cold, dry
-continental tropical (cT); warm, dry
-maritime polar (mP); moist, cold
-maritime tropical (mT); moist, warm
Where do continental (dry) air masses form?
-Over land
Where do maritime (moist) air masses form?
-Over oceans
Where do polar (cold) air masses form?
-in colder regions
Where do tropical (warm) air masses form?
-in warmer regions
Why don't air masses form over mountains / coastlines?
-The surface is NOT uniform
Why don't air masses form over the middle latitudes of the United States?
-Jet streams move air too quickly
What are the two most frequent air masses in Iowa?
-(cP)Continental Polar air from Canada
-(mT)Maritime tropical air from the Gulf of Mexico
When are air masses modified?
-when they move out of their source regions
What front gives us our coldest weather in Iowa?
Continental Arctic air mass (cA)
What protects the western U.S. from icy arctic air?
The Rocky Mountains
What type of air mass would be responsible for daily afternoon thunderstorms in the US?
mT - Maritme Tropical from the Gulf of Mexico
What is Lake Effect Snow?
-cold dry air masses move across the Great Lakes during the winter causing Lake Effect Snow
-cold air mass is warmed and moistened by the lake and encounters great friction once it arrives at the shore
-winds begin to pile up (converge) and produce snowfa
What air mass is usually associated with Lake Effect Snow?
-(cP) continental polar masses that move over warm lakes
What is a front?
a transition zone between two air masses
What type of front produces storms?
-a cold front
What are the 5 types of fronts?
-warm fronts, cold fronts, stationary fronts, occluded frons, and drylines
What is a warm front?
Warm air that moves forward and rises over top of the cooler air
What type of air mass are warm fronts usually associated with?
-mT (maritime tropical)
Which type of front is dominated by subsiding air within a cP air mass?
-a cold front
What is a stationary front?
Where two air masses meet, but neither one advances.
When does an occluded front form?
-when a rapidly moving cold front overtakes a warm front
What is a polar front?
the stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin
-produces the polar jet stream
What leads to convergence/divergence of air?
-variations in speed or direction
Convergence/divergence may result in what?
-cyclogenesis = the strengthening of a cyclone
What is the Polar Front theory?
a theory that explains the life cycle of mid-latitude cyclones and their associated fronts
What is the life cycle of a mid-latitude cyclone?
1. Formation: two air masses clash
2. A Wave Develops
3. Mature Stage: air masses swirl into each other
4. Occlusion: occluded front forms; storms weaken
5. Storm Dissipates
In an anticyclone, what must happen with convergence and divergence?
-divergence must be exceeded by convergence aloft
Why is divergence aloft essential to cyclogenesis?
-because it effects the wind speed of cyclones
When does speed divergence occur?
-When air enters a zone of high wind speed, it accelerates and stretches out
When does speed convergence occur?
-When air enters a zone of slower wind speed, an air pileup results
What causes directional divergence/convergence?
-change in wind direction
What contributes to the formation of surface low/high pressure systems?
flow aloft