CH. 18 – AIR PRESSURE & WINDS

Air Pressure

Weight of atmosphere above a

given place

Standard barometric pressure =

1013.2 millibars (mb)

Low pressure <1013.2 mb

High pressure >1013.2 mb

Wind

Horizontal movement of air

Factors affecting wind speed & direction

1) Pressure Gradient Force (PGF)

- air at surface always moves from

high pressure to low pressure

* PGF is only force that generates

wind!

Pressure Gradient Force

Caused by the differential heating

of Earth’s surface

- causes high & low pressure

centers to develop

Map w/ isobars

Pressure Gradient Force

Isobar = line that connects points

of equal air pressure

Ideally, air flow would be

perpendicular to isobars

- shortest distance to low pressure

Pressure Gradient Force

Spacing of isobars indicates

gradient

Gradient = change in pressure

over certain distance

(similar to “slope” of land)

Factors affecting wind speed & direction

2) Coriolis Effect

- caused by Earth’s rotation on its axis

- free-moving objects are deflected from their path of motion

Ex: wind, water currents

Coriolis Effect

Northern Hemisphere – deflected

to right of its path of motion

Southern Hemisphere – deflected

to left of its path of motion

Coriolis Effect

a) Affects only wind direction, not

wind speed

b) Is affected by wind speed

Faster winds = _______ deflection

Slower winds = ______ deflection

Coriolis Effect

c) Strongest at poles, weakest at

equator

Hurricanes can’t form between

5oN and 5oS

Factors affecting wind speed & direction

3) Friction

Important only w/in a few km of

Earth's surface

- slows wind speed & reduces

Coriolis Effect

Friction

Strongest over land, weakest over

water

QUESTION:

Which diagram represents air flow

over land?

Surface Circulation Patterns

Associated w/ high & low pressure

centers

Low pressure center = cyclone

Air flow around low pressure =

cyclonic air flow

Surface Circulation Patterns

High pressure center =

anticyclone

Air flow around high pressure =

anticyclonic air flow

Cyclonic/Anticyclonic air winds

Weather Generalizations

Low pressure center

- rising air

- cloud formation & possible ppt.

- converging surface winds

Weather Generalizations

High pressure center

- descending air

- clear skies

- diverging surface winds

Surface Circulation Patterns

Movement of mid-latitude cyclone

w/ changing wind directions

- very common occurrence in

Midwest

Barometric Tendency

Used for short-term weather

predictions (several hours

ahead)

1) Falling – approach of low

pressure (clouds)

Barometric Tendency

2) Rising – approach of high

pressure (clear skies)

3) Steady – maintain status quo

Weather Proverbs (handout)

Global Air Circulation

George Hadley (1735):

On a uniform non-rotating Earth:

Heated equatorial air rises

Upper-level air cools & sinks at

poles

Global Air Circulation

Global Air Circulation

Creates low pressure at equator

Creates high pressure at poles

Global Air Circulation

However:

Air flow is deflected by Coriolis

Effect

Breaks circulation pattern into four

global pressure zones

Global Pressure Zones

Equatorial Low (0o)

- hot humid air rises & cools

adiabatically

- produces clouds & ppt.

Ex: Tropical rain forests (Amazon

& Congo)

Global Pressure Zones

Subtropical High (30oN & 30oS)

- air aloft “piles up” from the

Coriolis Effect

- cold air sinks

- produces prevailing trade winds

at surface

Subtropical High

Descending air heats adiabatically

- creates arid environments

Ex: Sahara Desert (N. Africa),

Arabian Desert, Australian

deserts

Hadley

Cell

Global Pressure Zones

(90oN & 90oS)

ColPolar High dest air aloft sinks

towards poles

- “polar easterlies” winds blow

towards lower latitudes

Global Pressure Zones

Subpolar Low (60oN & 60oS)

Cold air from poles meets warm

air from subtropical highs

- air converges and rises

- produces stormy region known

as the “polar front”