Winds and convection

Movement of air

➔ Air flows from dense to less dense

➔ Air flows from dense to less dense.➔ We feel moving air as wind.

dense air less dense air

wind

cold air warm air

Air expands upon heating warm air is less dense than cold air.

How do we get uneven air density? http://www.youtube.com/watch?v=8qvlp4Lhva4&feature=endscreen

heating

candle

Facts about air movements• Air moves from dense to less dense area.

• Warm air is less dense than cold air.

• Warm air rises (because of low density).

wind

warm cold

dense less dense

Convection

Convection

• Warm air rises and is replaced by cooler air from the sides.

• After rising, the warm air cools, flows to the sides (high up) and eventually sinks back to the ground away from the heat source making a loop. warm and cold air is constantly in motion

heat

warm airrises

risen air cools,flows to the sides

Cooled air sinks back to the ground

Cooled air sinks back to

the ground

air flows to replace air that is rising at heat source = wind

air flows to replace air that is rising at heat source = wind

Sea Breeze

Land Breeze

Day

Night

Sea breezes

Day: • Land warms more quickly than water.• Air over the land heats up and rises.• Cooler air from the sea flows in to replace

risen air (= sea breeze).• Risen air cools and sinks over the water.

Land breezes

Night:• Water cools more slowly than land.• Air over the water warmer than air over the

land -> air over water rises.• Cool air from the land flows in to replace risen

air.• Risen air cools high up and sinks over the land.

Air circulation due to uneven heating of the earth by the sun.

Equator

• At Equator:

• Therefore: Air flows in from the sides. WIND

• The rising air eventually and flows to the .• Away from the equator:

.

warm air rises

cooled air sinks

sidecools

• Convection causes six atmospheric circulation cells

• The air flow close to the earth surface is wind we experience

• to the right on northern hemisphere;

• to the left on southern hemisphere.

Because of earth rotation: Coriolis effect -- deflects moving air masses:

Prevailing winds at the earth surface:Close to the poles: Polar east winds

Around the equator: easterly

Trade winds

In middle latitudes:

West winds

Mid-latitude

Atmospheric convection• Atmospheric convection creates 6 atmospheric

circulation cells (three on each hemisphere).• The air flow close to the earth surface of each

circulation cell represents the main wind in the respective region. (= prevailing wind)

• In middle latitudes (e.g. Montreal) winds come predominantly from the west.

• The Coriolis effect diverts moving air masses to the right on the northern hemisphere and to the left on the southern hemisphere.

Measuring winds: wind direction

• Weather vane• Points in the

direction the wind is coming from.

• Back half must be larger than front half.

Where does the wind come from?

• Coming from:south west

• Going to:north east

Measuring winds: wind speed

• Wind speed is measured with an anemometer.

• The faster the wind the faster the anemometer turns.

Measuring winds: wind sock

• On bridges to warn cars of heavy winds• Indicates wind force and direction

Measuring facts about winds

• A weather vane indicates the direction of wind; it points in the direction of the wind’s origin.

• A weather sock indicates direction and force of wind.

• An anemometer measures the speed of wind in km/h.

Memory check page 343