Pressure and Winds Flashcards
What is pressure?
The weight of the atmosphere
How is pressure measured?
millibars
What are isobars?
Isobars connect points of equal pressure
What do circles in isobars represent?
The circles represent cells of high & low pressure
What might cause pressure variation across the surface of the earth?
Uneven heating of earth’s surface
What are the 4 types of surface pressure conditions across the surface of the earth?
Thermal Lows and Highs and Dynamic Lows and Highs
What is thermal Low pressure and what is it caused by?
Thermal Low Pressure is caused by SURFACE HEATING. Warm air rises (convection) as it expands & becomes less dense than surrounding air creating a Thermal Low at surface
Where might thermal low pressure occur on a very large scale? (hint: where is there heating on a very large scale?)
At the equator. There is more direct sunlight on the equator than any other place, and the equator receives 2.5 times more energy than the poles
What is thermal high pressure?
When air cools, it contracts, becomes denser than surrounding air and sinks, creates a Thermal High at surface
Where does thermal high pressure happen on a large scale?
The poles
What are dynamic highs in lows and where do they occur?
They are changing and moving, and occur in the mid-latitudes
What causes winds?
Differences in pressure across space
What are the three physical forces that act on wind?
- pressure gradient force
- Coriolis Force (Effect)
- Friction
What is the pressure gradient force (PGF)
It is perpendicular to isobars. PGR causes air to flow from high low
What does the spacing of isobars indicate in relation to PGF?
The spacing of isobars indicates strength of PGF
What does closely spaced isobars mean?
Steeper pressure gradient (closely spaced) = stronger wind
What kind of winds are driven mainly by PGF?
Most local and many regional scale winds
Land and Sea Breezes are one example of local winds. How do Land and Sea Breezes form?
Local Winds: Land & Sea Breezes (PGF) form because of land/water differences in temperature and pressure
What are sea breeze conditions in the daytime?
Look at picture on slide number 14 on powerpoint
What are sea breeze conditions in the nighttime?
Look at picture on slide number 15 on powerpoint
What is the basic reason for land-sea breezes?
Differential heating of land and water (remember? land heats up and cools down faster)
This difference in temp. creates pressure differences and thus breezes
What is another type of local winds?
Mountain-Valley Breezes
Will mountian-valley breezes go up or down the mountain during the day
Valleys heat up, so air rises upslope
What happens to mountain-valley breezes during the night?
Radiational cooling at night causes colder denser air to sink by gravity
Where do regional-scale winds occur?
Regional-scale winds (a little bigger than local-scale) occur over large areas at the scale of mountain ranges rather than a single mountain (local)
What are cold katabatic winds?
Cold, dense air flows down slope due to gravity
(just like water!).
Often very strong winds as high as 100 mph
Winds often channel, converge, and compress, like streams into a very strong “river” of air
What is some evidence of katabatic winds?
- pattern of snow exposing ridges and building up on the lee side, and 2. the streamers of new frozen sea ice
What is polynya?
Pockets of open water
What are other regional-scale winds beside katabatic winds?
Chinooks, Santa Ana Winds
What are chinooks?
Warm, Dry Wind: Chinooks occur on dry “leeward side” of the Rocky Mts., warm by compression and are called “snow eaters”– they melt or evaporate snow (sublimation)
What are the effects of Chinooks?
- Wood may split due to extreme dryness
- Fire hazards are increased—May cause violent grass or forest fires, or cause small fires to spread.
- irritability, sleeplessness
- Susceptible people may get migraine headaches or suffer nervous disorders
- Psychologically, some people feel “better”
What are Santa Ana Winds (Devil Winds)?
Warm dry winds affecting coastal S CA esp. in fall.
- Strong H Press. Over US west/sw = cool, dry, “heavy” air
- Flows H to L
- air heats by compression (~5°F /1000 ft.) as air descends coast mountains
What are some negative effects of Santa Ana Winds?
Dust, Heat, Fire!
What are some positive effects of Santa Ana Winds?
- Surfers love them because they make big waves.
- Increased ocean circulation near shore = nutrients, fish
- Blows off smog over LA Basin
In summary what are the wind associated with Pressure Gradient Force (PGF)?
Local scale •Land-Sea breezes •Mountain-Valley breezes Regional scale •Cold Katabatic winds, e.g., Mistral and Bora •Warm winds, e.g., Chinooks, Santa Anas
What is the Coriolis force?
Coriolis is not actually a force, but an “apparent” DEFLECTION due to the rotation of the earth…
Dos the earth rotate faster at the equator or the poles?
Earth rotates faster at equator than at the poles
Why is the Coriolis effect important?
Its important to pilots because if they take a direct path to a location, say NYC, they would end up completely off course (like Des Moines)
Where does the Coriolis effect deflect winds to?
The right in Northern Hemisphere
The left in the Southern Hemisphere
What is the effect of Coriolis like at the equator? At the poles?
Effect of Coriolis is ZERO at the Equator
Strongest at Poles!
How does the Coriolis Force affect the PGF and what does it create?
It balances the PGF and sometimes creates “Geostrophic” winds
What do geostrophic winds do?
“Geostrophic” winds flow parallel to the isobars in the upper Troposphere ……NOT at surface because FRICTION
What acts upon surface winds?
Friction
What does friction do?
Friction SLOWS down atmospheric motion AT SURFACE where there is roughness
How does friction affect the coriolis effect?
Friction slightly overrides the coriolis force and causes wind at the SURFACE to flow across isobars at an angle, but always from H to L
What does friction cause surface winds to do?
Friction causes surfaces wind to spiral forming
Dynamic Low and High pressure “cells”.
What are anticyclones?
High pressure cells
What are cyclones?
Low pressure cells
Where do these high and low pressure cells created by surface winds travel?
These cells TRAVEL across the mid-lats from W to E!
What are the two rules for pressure cells?
- Surface Flow is ALWAYS OUT of Highs and IN to Lows (Remember flow High to Low like water)
- Spirals are opposite in Northern and Southern Hemispheres
What happens to air in a cyclone?
Low pressure converges air at the surface
What happens to air in an anticyclone?
High pressure diverges air at the surface
What are some examples of Katabatic winds?
Mistral and Bora
