Climatology

Local winds


Local winds are winds occur in a smaller spatial and temporal scale. These winds blow for a relatively short period of time and distance. Micro climate is the predominant factor in forming these winds. Local winds can be classified into five types, namely –

1. Land breeze

2. Sea breeze

3. Valley breeze or katabatic wind

4. Mountain breeze or anabatic wind

5. Monsoon winds



1. Sea breeze

A sea breeze is associated with coastal areas which blow during the day time. Land breezes develop due to pressure gradient which exists between the ocean and adjacent land mass. Land masses have a smaller specific heat and thus heats up more rapidly than water masses. However, land masses radiate its heat much quicker. During the morning and early afternoon the sun heats up the land surface, resulting in temperature changes of up to 10˚C. Therefore, coastal areas will have lower atmospheric pressure than the adjacent ocean. Air starts to flow from the sea towards the land (on-shore flow). Many holiday makers visiting the coastal regions would have noticed that beach conditions often deteriorates in the afternoon as winds start to pick up with cloud development over the coastline. Sea breezes can also develop over smaller water bodies such as lakes.



2. Land breeze

A land breeze is associated with coastal areas which blow during the evenings. Sea breezes develop due to a pressure gradient that exists between the ocean and the land. Ocean masses have a greater specific heat and thus heats up much slower but retains the heat much longer than land masses. Temperatures in oceans can change by only 0.5˚C. A pressure gradient develops between the ocean and adjacent coastal area. Air starts to flow from the land towards the ocean (off-shore flow). Land breezes can also develop over smaller water bodies such as lakes.



Sea breezes and land breezes result in elevated levels of humidity, temperature moderation and high precipitation in coastal areas.

3. Valley breeze or anabatic wind


Valley breezes develop during the day when the valley sides and the air in contact warm. The warm air which is less dense start to rise to form a gentle upslope wind.



4. Mountain breeze or katabatic wind

At night the valley slopes and the air in contact cool down rapidly. Cooled air is denser and heavy, thus start to sink down from the mountain to the valley. A mountain breeze can also be called a katabatic wind or gravity wind.

The ideal conditions for the development of a mountain breeze are an elevated plateau and steep downhill slopes. When snow occurs on the plateau, cold air moves down the slope. When cold air accumulates on the valley floor temperatures can plummet resulting in the formation of a frost pocket.



Katabatic winds are associated with cold winds and are called different names in different parts of the world. Along the northern coast of the Adriatic Sea, cold polar air flows from Russia and is called the bora. The bora can sometimes be very violent and can reach wind speeds of up to 100knots. A less violent wind which develops in France’s Rhone Valley and flows out to the Mediterranean is called the mistral.



Warm katabatic winds are the Chinook which blows from the Rockies, the föhn wind blows from the Alps and the Santa Ana blowing from the desert in South California.
5. Monsoon winds

Monsoon is derived from the Arabic word “mauzim” which means season. The main reason for the development of monsoon winds is the difference in heating of land and water masses which results in pressure gradient differences. During the winter season in the Northern Hemisphere the air over the Eurasia continent cools down significantly and becomes much colder than over the ocean at India. Cool, dry air start to flow from the interior, with a higher pressure, over the Himalayas towards the sea with a lower pressure. As the air descends towards the coast it heats up adiabatically and reaches the coast as a warm, dry wind. This monsoon is known as the Northeastern monsoon or winter monsoon.

During the summer season in the Northern Hemisphere the land heats up more than the oceans. Warm, humid air flows from the higher pressure region over the ocean towards the lower pressure region over the interior. As the humid air rises up the Himalayas it cools, condenses and result in heavy rain. This monsoon wind is known as the Southwestern monsoon or the summer monsoon.