Air Pressure and Winds

Atmospheric Pressure

What causes air pressure to change in the horizontal?

Why does the air pressure change at the surface?

Atmospheric Pressure Horizontal Pressure Variations

It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air

Warm air aloft is normally associated with high atmospheric pressure

Cold air aloft with low atmospheric pressure

At surface, horizontal difference in temperature = horizontal pressure in pressure = wind

Basically the same thing happens above the surface … in the winds aloft

Two air columns, each with identical mass, have the same surface air pressure.

Since colder air is more dense, and takes up less space… it takes a shorter column of cold air to exert the same pressure as a taller column of warm air…

So… as column 1 cools, it shrinks, and as column 2 warms, it must expand.

What happens above these columns of air?

The pressure differences aloft causes the air to move from higher pressure toward lower pressure.

As air moves from the top of column 2 toward the top of column 1, the pressure at the surface drops,

And as air moves into column 1, the surface pressure rises.

At the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column. Warm air aloft is associated with high pressure and cold air aloft with low pressure.

LH

Surface level charts are modified to reflect atmospheric pressures AS IF they were at sea level (approximate adjustment: 10mb per 100 meters.a)Pressure at 4 cities.b)Pressure modified to one level (sea level)c)Shows isobars based on one level

Sea-level isobars drawn so that each observation is taken into account.Not all observations are plotted.

Sea-level isobars after smoothing.

The area shaded gray in the above diagram represents a surface of constant pressure, or isobaric surface. Because of the changes in air density, the isobaric surface rises in warm, less dense air and lowers in cold, more-dense air.

Where the horizontal temp-erature changes most quickly, the isobaric surface changes elevation most rapidly.

Changes in elevation of an isobaric surface (500 mb) show up as contour lines on an isobaric (500 mb) map. Where the surface dips most rapidly, the lines are closer together (steep gradient – think of topographic maps)

The wavelike patterns of an isobaric surface reflect the changes in air temperature. An elongated region of warm air aloft shows up on an isobaric map as higher heights and a ridge; the colder air shows as lower heights and a trough.

Surface map showing areas of high and low pressure. Notice that the wind blows across the isobars.

The upper-level (500-mb) map for the same day as the surface map. Notice that, on this upper-air map, the wind blows parallel to the contour lines.

Surface and Upper Level Charts Observation: Constant Pressure Surface

Pressure altimeter in an airplane causes path along constant pressure not elevation

May cause sudden drop in elevationRadio altimeter offers constant elevation

Newton’s Law of Motion

AN object at rest will remain at rest and an object in motion will remain in motion as long as no force is executed on the object.

The force exerted on an object equals its mass times the acceleration produced.Acceleration: speeding up, slowing down,

change of direction of an object.

Forces that Influence Winds Pressure Gradient Force: difference in

pressure over distanceDirected almost perpendicular to isobars

from high to low.Large change in pressure over short

distance is a strong pressure gradient and vice versa.

The force that causes the wind to blow.

The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure gradient force.

The closer the spacing of the isobars, the greater the pressure gradient. The greater the pressure gradient, the stronger the pressure gradient force (PGF). The stronger the PGF, the greater the wind speed.

Forces that Influence Winds Coriolis Force (Coriolis Effect)

Apparent deflection due to rotation of the EarthRight in northern hemisphere and left in southern

hemisphereStronger wind = greater deflectionNo Coriolis effect at the equator greatest at poles.Only influence direction, not speedOnly has significant impact over long distances

Except at the equator, an object heading either east or west (or any other direction) will appear from the earth to deviate from its path (as the earth rotates beneath it.) The deviation (Coriolis force) is greatest at the poles and decreases to zero at the equator.

The effect of surface friction slows down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure.

Air at the surface moves from high to low pressure == wind (pressure gradient force - PGF)

Above the level of friction, air (wind) will change it’s direction, due to the Coriolis Effect, until it balances with the PGF and flows parallel to the isobars at a steady speed.

Wind blowing under these conditions is called geostrophic.

Forces that Influence Winds Geostrophic Winds

Earth turning winds (turned by the earth’s rotation)

Travel parallel to isobarsSpacing of isobars indicates speed; close =

fast, spread out = slow

The isobars and contours on an upper-level chart are like the banks along a flowing stream. When they are widely spaced, the flow is weak; when they are narrowly spaced, the flow is stronger.

The increase in winds results in a stronger Coriolis force (CF), which balances a larger pressure gradient force (PGF).

By observing the orientation and spacing of the isobars (or contours) in diagram (a), the geostrophic wind direction and speed can be determined

Forces that Influence Winds Gradient Winds Aloft

Cyclonic: counterclockwiseAnticyclonic: clockwiseGradient wind parallel to curved isobars

Observation: Estimates AloftClouds indicate direction of winds, Allow locating pressure system -- consistent

with cloud location.

Winds and related forces around areas of low and high pressure above the friction level in the Northern Hemisphere. Notice that the pressure gradient force (PGF) is in red, while the Coriolis force (CF) is in blue.

Clouds and related wind-flow patterns (black arrows) around low-pressure areas. In the Northern Hemisphere, winds blow counter-clockwise around an area of low pressure.

In the Southern Hemisphere, winds blow clockwise around an area of low pressure.

Stepped Art

An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. - Solid gray lines are contours in meters above sea level. - Dashed red lines are isotherms in °C.

Forces that Influence Winds Winds on Upper-level Charts

Winds parallel to contour lines and flow west to east

Heights decrease from north to south

Surface WindsFriction reduces the wind speed which in

turn decrease the Coriolis effect.Winds cross the isobars at about 30° into

low pressure and out of high pressure

Winds and air motions associated with surface highs and lows in the Northern Hemisphere.

(Replacement of lateral spreading of air results in the rise of air over a low pressure and sinking over high pressure)