Chp. 07 Atmospheric Pressure & Wind

Pressure: Few direct impacts on our daily life----- Rheumatism may flare up. A bad tooth may ache.

H L The wind is a direct response to pressure But it does not blow directly from H – L.

Atmospheric pressure. The “weight” of all. the molecules above Atmospheric pressure The “weight” of all the molecules above a horizontal area What is the pressure on the surface of the moon?

James Burke discusses silver mining problems which led to ....

Continued

Continued

Mercury is often used to measure pressure: Air pressure Blood pressure

Aneroids are often used in home weather forecast stations

Barograph (aneroid barometer)

Chat with your neighbor and discuss why this fellow is exiting this way.

To see what this means we go to the next slide

The station pressures are barometric readings along the hills and valleys in the top diagram.

Weather maps are used to describe pressure patterns Weather maps are used to describe pressure patterns. The lines connect points of equal pressure. We interpret the patterns to help explain the weather (including winds).

A slice of the U.S. across the Rockies. Oakland Winnemucca Salt Lake City Denver Dodge City ? Nashville Athens A slice of the U.S. across the Rockies. Since pressure drops about 10mb for every 100m of height, mountains cause big low pressure features.

This is a mess! What should we do? This is a map of the staion pressures for this date. Guess where the mtns are. This is a mess! What should we do?

The trick is to “correct” each station to sea level by adding about 10mb for every 100m above sea level. Thus the mtn lows are “built up”.

Now the map looks normal

It is insightful to evaluate atmospheric behavior at selected height surfaces.

The governing equations are simpler if constant pressure surfaces are used. This is the reason the only constant height surface used is the surface map.

Changes in elevation of a constant pressure sfc (500 mb) 570 564 558 552 Changes in elevation of a constant pressure sfc (500 mb) show up as contour lines on a constant pressure (500 mb) map. Where the sfc dips fastest the lines are closest.

October 6 500 mb Flow

Because warm air takes more space than cold air, the distance between constant pressure surfaces is greater when the air between them is warmer.

The wavelike patterns of a constant pressure sfc reflect the changes of air temperature. The ridge (high heights) and the trough (low heights) correspond with warm and cold temps.

Newton’s 2nd law: F=ma Or F/m=a Force per unit mass equals (or causes) accelerations Accelerations are changes in speed and/or direction

These forces are important for air flow GRADIENT These forces are important for air flow

p This causes the wind to blow L Pressure Gradient Force = , the change in air pressure for some specified distance Closely spaced lines represent large values of pressure gradient, thus high winds!!

Closely spaced contours indicate a strong pressure gradiant force and high winds.

High pressure systems have widely spaced contours and weak winds.

What do you predict the winds will be like in these two regions? Sketch direction and speed

See how much you know?

Forces which influence horizontal winds Pressure Gradient Centripetal Wind Friction Coriolis Force

Coriolis Example

Only an apparent force The Coriolis Force is proportional to : wind speed the spin of the system (earth) the distance from the equator (latitude) If it was a real force it could change the speed of the wind. It only can change the direction!

Pressure Gradient Direction of initial motion Low Pressure Isobars at the surface or height contours at other levels Coriolis Force only exists when the speed is nonzero High Pressure

Upper Air Wind Forces

Pressure Gradient Low Pressure Isobars at the surface or height contours at other levels Wind (“final” motion!) Coriolis Force High Pressure

Geostrophic Flow For the upper air surfaces (no friction) when the contours are more or less straight and parallel the flow is called Geostrophic Flow

L L H This region has ~ geostrophic flow

Surface flow Must consider the effects of friction

The wind speeds up as it gets away from friction.

The surface winds cross the yellow isobars

Again compare the surface and upper levels.

Can the wind direction tell you where the low is? Look at this diagram and try to invent a rule.

What causes the wind to follow curved paths? When the PGF is greater than the Coriolis Force (e.g., close contours in a low) inward flow occurs.

L L H

Centripetal acceleration pulls the riders inward to make the path circular.

The imbalance toward the center of motion is called the centripetal force.

Important consequences

Later we’ll put this together