How and why do wind direction and speed vary on the Earth's surface?
Why does wind direction and wind speed vary across Earth’s surface?
1. Unequal distribution of air temperature
a) results in uneven distribution of pressure gradient
b) initiates horizontal motion of air and determines wind direction
2. Wind speed
a) influenced by strength of pressure gradient between two places
b) and friction due to Earth’s topography
3. Local and regional winds
a) land and sea breezes occur on a local scale
b) Northeast and Southwest monsoons occur on a regional scale and are influenced by the Coriolis force
Cooler air is denser and hence sinks resulting in an area of higher pressure. On the other hand, warmer air is less dense and hence rises create an area of lower pressure. This difference in pressure between the two areas forms a pressure gradient, causing air to move from the area of higher pressure to the area of lower pressure to fill in the void vacated by the rising air. Hence, there will be the formation of wind moving from an area of higher air pressure to an area of lower air pressure.
Local Winds
Day - As land heats up faster, the air over it is warmer and rise, resulting in lower air pressure. The air over the sea is cooler as the sea absorbs heat slowly. The air from the sea move towards the land as the air pressure over it is higher than over the land. This results in sea breeze.
Night- As land cools faster, the air over it is cooler and denser, resulting in higher air pressure. The air over the sea is warmer as the sea loses heat slowly. The air from the land move towards the sea as the air pressure over it is higher than over the sea. This results in land breeze.
(Note that the wind is named after where it is blowing from)
Regional wind
Monsoon winds are regional wind patterns that reverse direction seasonally and also affected by the Coriolis Effect produced by the rotation of the earth. The Coriolis Effect cause the wind to be deflected. In the northern hemisphere, the wind is deflected to the right and to the left in the southern hemisphere.
Jan - NE Monsoon
Between October and February, the southern hemisphere experience summer and an area of low pressure forms over Australia. The northern hemisphere experience winter and the low temperature causes the air to be cold and dense, resulting an area of high pressure over Central Asia. Air from Central Asia moves towards Australia due to the difference in pressure between Central Asia and Australia. The Coriolis effect deflects the wind to the right in the northern hemisphere thus it moves towards the equator as the Northeast monsoon. When the wind crosses the equator, the Coriolis effect deflects the wind to the left in the southern hemisphere and became the northwest monsoon.
July - SW Monsoon
Between June and September, the northern hemisphere experience summer and the air over Central Asia heats up, expands and rises, forming a region of low pressure over the area. The southern hemisphere experience winter and the low temperature cause the air to be cold and dense, resulting an area of high pressure over Australia. Air from Australia moves towards Central Asia as the southeast monsoon due to the difference in pressure between Central Asia and Australia. As the wind cross the Equator, the Coriolis Effect deflects the wind to the right and it became the southwest monsoon.
Demo on how wind is deflected to the right in the northern hemisphere due to the Coriolis effect from our Earth rotation.