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Air Pressure

Pressure exerted by weight of air. Exerted in all directions, pressure pushing down balances pressure pushing up

Standard Sea Level Pressure

1013.2 millibars

Barometer

Device used to measure air pressure

Aneroid Barometer

Small, flexible metal box called aneroid cell (capsule)

Wind

Air flows in areas of high pressure to low pressure

3 Factors Combining to control wind

Pressure difference, Coriolis effect, and friction

Pressure Gradient

Amount of pressure change occurring over a given distance

Closely spaced isobars

Indicate steep pressure and high winds

Widely spaced isobars

Indicated weak pressure gradient and light winds

Coriolis Effect

How Earth's rotation effects moving objects. In Northern Hemisphere, objects deflected at right of path of motion. Southern Hemisphere deflected at left. All air deflected at right angle. It influences wind direction, strongest at poles, strongest wind= greater deflection of Coriolis effect

Friction

Slows air movement, which changes wind direction

Jet streams

Fast moving "rivers" of air that occur above friction layer. Move in east-to-west direction

Cyclones

Centers of low pressure. Air rises in center

Anticyclones

Centers of high pressure. Air subsides (sinks) at center

Cyclonic vs. Anticyclonic winds

Pressure gradient, friction, Coriolis effect applied to pressure center in Northern Hemisphere, winds blow inwards and counterclockwise around a low pressure. Southern Hemisphere opposite. Both Hemispheres friction causes inward or outward in cyclone or anticyclone

Fair Weather associated with anticyclone (high pressure)

Descending air, clear weather, dry conditions

Low pressure (cyclone) system in US moves from West to East

Produce bad weather, paths are unpredictable, takes a week to cross country

Weather and Air Pressure

Rising air= cloud formation and precipitation

Weather Forecasting

Weather reports emphasize locations and possible paths of cyclones and anticyclones, low pressure produce bad weather at any season

Non-rotating Earth

Air at equator would rise and move toward poles

Horizontal convergence

Net inward movement of air causes area occupied by air mass to shrink

Horizontal divergence

Net outward movement of air causes area occupied by air mass to increase

Subtropical High

Dry, sinking near equator

Polar high

Dry, sinking near poles

Equatoria low

Wet, rising air near equator

Subpolar low

Wet, rising air near poles

Trade winds

2 belts of wind blowing constantly from east to west on north and south sides of subtropical highs from equator

Westerlies

Dominant west-to-east motion- characterized regions on poleward side of subtropical highs

Polar eateries

Blow polar high to sub polar low. Not constant

Polar front

Stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin

Influence of continents

Only truly continuous pressure belts is sub polar low in South Hemisphere (no land masses to disrupt flow)

Monsoons

Seasonal reversal of wind direction associated with large continents, especially Asia. In winter, wind blows from land to sea ( seasonal high-pressure system) In summer, wind blows from sea to land (seasonal low pressure= rainy)

Local winds

Caused by topographic effects or variations in surface composition-land and water

Land and sea Breezes

In sumer mountains, land surface is heated more intensely during daylight hours than water

Sea breeze

Originates in day and flows towards land. Most intense in mid-afternoon

Land breeze

Originates in evening and flows towards water

Valley and Mountain Breezes

Warm air moves up mountain during day, cool air moves down during night

Prevailing wind

Wind that blows more often in one direction than another

Wind vanes

Indicate wind direction and 0˚=N, 90˚=E, 180˚=S, 270˚=W

Anemometer

Instrument that measures wind speed

El nino

Name given to periodic warming in central and eastern Pacific. Cold water is replaced with warm equatorial water. Major el nino can cause extreme weather

La nina

In eastern Pacific. Colder temp's than average

Nina and Nino

Weather patterns can affect fish industry, climate, and farming industry

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