PRESSURE SYSTEMS IN THE WORLD
- Cyclone
- It’s a low pressure system characterised by low pressure at the centre and increases outwards.
- Starts in areas where air ascends from the ground to the atmosphere and descends at high altitude.
- It’s of two types. Tropical cyclones e.g. hurricane, typhoon and willy willies and depressions which are characterised by temperate latitudes.
- The movement of wind is anticlockwise in the N. hemisphere and clockwise in the S. hemisphere.
- Anticyclone
- A high pressure system characterised by high pressure at the centre and decreases outwards.
- • It starts in areas where air is descending from the atmosphere onto the ground and then blows outwards on the ground.
- The movement of wind is clockwise in the N. hemisphere and anticlockwise in the S. hemisphere.
Table of Contents
Local Winds
-Which occur regularly for a short period of time affecting a limited area.
-Modify the weather of the area they blow to.
- Sea Breeze
– A light and gentle wind which blows from the sea to the adjacent land.
How it Forms
- During the day land is heated faster than the sea.
- Air over the land is warmed and rises.
- Air from the sea moves to the land to replace the rising air.
- The rising air from the land cools and descends over the sea at high altitude.
- Circulation continues until the pressure difference is reversed at night.
Effects on weather
It takes cooling effect on land on a hot afternoon.
- Land Breeze
-A light and gentle wind which blows from land to the sea during the night.
How it Forms
- At night land loses heat faster than the sea.
- Air over the sea is warmed and rises.
- Air from the land moves to the sea to replace the rising air.
- Rising air from the sea descends over land at high altitude.
- Circulation continues until pressure difference is reversed during the day.
Effects on weather
It causes early morning showers through moisture brought towards land at high altitude.
- Anabatic winds (Valley Breeze)
– Cool local winds which blow from the valley to the hill tops during summer afternoons.
How it Forms
- During the day hill tops are heated more than valley bottoms.
- Air over the hill tops is warmed and rises.
- Cool air over the valley move up to the hill to replace the rising air.
Effect on weather
– Cause afternoon showers on hilly grounds.
- Katabatic/Descending Winds
– Cold local winds which blow from hill tops to the valley during the night.
- During the night hill tops lose heat faster than the valley.
- Air over the valley is warmed and rises.
- Cool air over the hill tops move to the valley by gravity to replace the rising air.
Effect on Weather
– Takes chilly conditions on valley bottoms.
- Harmattan Winds
-N.E winds which originate from Sahara and blow across W. Africa between November and March taking dry conditions there.
- Fohn Winds (Alps)
-Local cold winds which slide down the leeward side of the mountain at high speed and are warmed producing a temperature rise.
Due to the high speed and temperature they are associated with wild fires.
They are known as Chinook in Rocky Mountains, Santa Anas in California and Mistral in France.
Factors influencing Wind Flow (Speed and direction)
- Pressure Gradient
If the pressure difference between high and low pressure areas is high the winds blow at high speed (strong) but if it’s low they blow at high speed (are gentle).
- distance between Places of High and Low Pressure
if the high and low pressure areas are near each other winds blow at high speed but if distant from each other winds blow at low speed.
- Rotation of the earth
Rotation of the earth deflects winds to the right in the N. hemisphere and to the left in the S. hemisphere.
- Frictional Force
If the surface of the earth is rugged or has obstacles such as hills, mountains, valleys or vegetation the wind is blocked causing speed reduction and its direction of flow is also changed.
See also
PROBLEMS ENCOUNTERED IN FIELD WORK