Pressure

WindsWind Instruments

Synoptic weather chart

Pressure

• Weight exerted on a unit area of the earth’s surface by the weight of a column of air above it

• Measured in millibars

• Air pressure decreases as altitude increases

• as gravity pulls air molecules toward the surface of the ground

• Barograph and barometer used to measure pressure – these are kept in the stevenson screen

Pressure- Barograph

How does it work?• Made up of 2 cylinders

• Larger cylinder has graph paper on it and it rotates, completing 1 cycle in 24 hours

• Smaller container has air removed, hence it expands and contracts with changing external pressure

• It has a pointer, which makes markings on the graph paper showing changes in pressure

• After 24 hours, the graph paper is changed

Pressure- Barometer

Wind

Wind refers to the movement of air over the surface of the Earth

It is caused by the movement of cool air that replaces rising warm air

Air moves from areas of high pressure to areas of low pressure

Wind Direction

A wind vane placed on a rooftop

Wind direction refers to the direction that the wind is blowing from

It is shown by a freely moving pointer on a wind vane

The wind vane is usually placed ona high, open place with little or no obstruction to the flow of wind

Wind Rose• A wind rose records the wind data for a

month

• The spokes indicate wind direction

• The number in the spoke represents the date when the wind blew from that direction

• The number in the centre represents the number of days without wind

Wind Speed

• Anemometer is used to measure wind speed

• It is placed on top of the tallest building in the weather station, to ensure no obstruction, giving accurate results

Wind Rose for wind speed

Tutorial on how to read wind rose: http://www.mtavalanche.com/weather/windrose

Synoptic Weather Chart

Synoptic Chart 1

Synoptic Chart 2

References

Waugh, D. (2000) Geography – An Integrated Approach, UK: Nelson.

Chong, M. (2001) Aspects of Physical Geography,

Singapore: Pekoe Books.