Pressure and Winds

Today’s objectives:

-we will be able to define key terms in Gateway 2

-we will be able to explain the various causes of climate change

• Air pressure (atmospheric pressure): the downward force exerted by the air’s weight per unit area on the Earth’s surface– about 1kg / cm2

• Relationship between pressure & altitude?Negative relationship!Low pressure occurs when altitude is higher.

Air molecules are far apart and air becomes thinner.High pressure occurs when altitude is lower.

Air molecules are closely packed and air becomes denser.

Pressure and winds

Extra: Isobars – lines on the weather map joining together

places of equal atmospheric pressure• http://www.bbc.co.uk/schools/gcsebitesize/geography/weather/weathersystemsrev2.shtml

6. Pressure and winds• Wind: the horizontal movement of air from a region

of high pressure to a region of low pressure

Area of high pressure develops at low altitude. Differences in pressure causes

cool air to rush in as wind. Movement of air mass from high to low pressure area.

Ground gets heated, the air above ground gets heated & rises. An area of low pressure develops at low altitude

Warm air rises and coolsCool air sinks as it is dense.

6. Pressure and winds• Winds can occur on:

– Small scale (2)• Sea and land breezes

– Large scale (1)• Monsoon winds

6. Pressure and winds• Sea and Land Breezes

– http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm

Sea Breeze (Day) – cooling effect on coastal areas in the day.

1. Ground gets heated, up faster than the sea. 3. Sea does not get

heated as quickly as land.

4. Cooler and denser air above sea sinks to form an area of high pressure.

2. Air gets heated & rises to form an area of low pressure.

5. Cool air over sea (high pressure) blows towards the land (low pressure), replacing warm air on the land, forming a sea breeze.

Land Breeze – wind that blows from land to the sea at night.

1. Land loses heat faster than the sea at night. 3. Sea

loses heat slowly.

Warmer air above the sea rises to form an area of low pressure above the water surface.

2. Air over the land cools and sinks to form an area of high pressure.

5. Cool air over land (high pressure) blows seawards and replaces warm air over sea (low pressure), forming a land breeze.

6. Pressure and winds• Large-scale (monsoon) winds:

– seasonal movements of large air masses across the Earth, as the Northern and Southern Hemispheres experience different seasons.

• Why?• Caused by the extreme heating and cooling of large

land masses.

• Winds are deflected as they move across Earth due to the Coriolis effect

• http://www.youtube.com/watch?v=mcPs_OdQOYU• http://www.youtube.com/watch?v=iv5WL1W4-WI

Coriolis Effect and Winds (S)

Summer

Winter

SW Monsoon (June – Sept)

High Pressure

Low Pressure

6. Pressure and windsSouthwest Monsoon (June to September)

Seasons Northern Hemisphere: Summer (hot) Southern Hemisphere: Winter (cold)

Pressure difference North: Indian sub-continent (low pressure) South: Australian continent (high pressure) Hence, general direction of wind is from S to N.

Results: Cold air from Australia blows toward India. Known as SE Monsoon Winds are deflected to the left due to Coriolis effect. As winds enter into the Northern Hemisphere, they are deflected to the right now

known as SW Monsoon Picks up moisture from Indian Ocean and the Bay of Bengal, and brings heavy rain to

SW India and Bangladesh Indian subcontinent’s weather is hot and wet these months!

SW Monsoon(June to September)

Southeast Monsoon

Southwest Monsoon

Northern HemisphereSummer (Warm)

Southern HemisphereWinter (Cold)

Indian sub-continent (Low pressure)Wet and hot

Australian continent (High pressure)

N

W E

S

Summer

Winter

NE Monsoon (Oct – Jan)

High Pressure

Low Pressure

6. Pressure and windsNortheast Monsoon (Oct to Jan)

Seasons Northern Hemisphere: Winter (cold) Southern Hemisphere: Summer (hot)

Pressure difference North: Asian continent (high pressure) South: Indian sub-continent (low pressure) Hence, general direction of wind is from N to S.

Results: Cold air blows across Asia toward India Known as NE Monsoon Winds are deflected to the right due to Coriolis effect. Winds are deflected to the left as they enter into the Southern Hemisphere Now

known as NW monsoon Winds that blow across Asia are cold and dry, hence they bring little moisture. Indian subcontinent’s weather is cold & dry these months!

NE Monsoon(October to Jan)

Northwest Monsoon

Northeast Monsoon

Northern HemisphereWinter (Cold)

Southern HemisphereSummer (Warm)

Asian continent (High pressure)

Indian sub-continent (cold and dry)Southeast Asia like Malaysia (wet)

N

W E

S

Extra: Measuring Wind Direction• Measured using a wind vane• Freely moving pointer & four points (NSEW)

– Points to direction wind is coming from

• Placed in high open places free from obstructions

Extra: Wind sock

• Also used to measure wind direction

Extra: Recording Wind Direction

• Wind rose which records:1. Wind frequency (no. of days)2. Wind direction

• Centre circle and 8 arms representing the 8 main compass directions– Centre circle: no. of calm

days– No. of squares: no. of days

wind comes from each direction

number of calm days5number of calm days5

5 E

S

N

NW

SW

NE

Chapter 6: Understanding the Elements of Weather

• Helps prepare for whatever weather conditions that may occur.

• Allows the classification of the world’s regions according to different types of climate.