Global Rainfall Patterns: Exemplifying the Dominance and Diversity
Global Rainfall patterns are highly correlated with atmospheric humidity while atmospheric humidity is closely related to temperature through the process of evaporation. The region having abundant surface water for evaporation and with high temperatures receives a higher amount of rainfall. Equatorial regions are one example of such a condition. Sub-tropical regions are also characterized by the above conditions but the western parts of the continent receive the least rainfall because of the anticyclone conditions due to the descent of air. Middle latitude also has favorable conditions for sufficient rainfall but polar areas receive their precipitation in the form of snowfall instead of rainfall.
The mean annual rainfall of the globe is 990 mm but this mean annual amount is unevenly distributed in the earth’s surface. Some places receive less than 100 mm of the mean annual rainfall (for example tropical hot deserts like Sahara, Thar, Atacama, Kalahari, etc.) while some receive more than 12000 mm of the annual rainfall at Cherrapunji. The equatorial region receives rainfall throughout the year but other areas are usually characterized by seasonal rainfall.
Diversity of Global Rainfall Patterns: (ZONAL DISTRIBUTION OF RAINFALL)
The cooling of ascending air is a prerequisite condition for the occurrence of rainfall. The air is lifted through the thermal convective mechanism, convergence of two extensive air masses, and obstruction of mountain barriers. It generally appears that air is lifted due to the convergence of two extensive air masses in most of the world.
Convergence of air mass is directly related to air temperature and air pressure. There are two major convergence zones of air masses i.e., trade winds converge along the equatorial low-pressure belt, and westerlies and polar winds converge along high latitude low pressure. On the other hand, winds descend near subtropical high-pressure belts diverge in opposite directions, and form anticyclones which introduce dry weather. Based on the above conditions Global Rainfall patterns/rainfall distribution is identified on the earth’s surface.
Equatorial zone of maximum rainfall– This zone extends up to 10°latitudes on either side of the equator and falls within intertropical convergence characterized by warm and moist air masses. The mean annual rainfall ranges between 1750 mm and 2000mm. Most of the rains are received through convectional rainfall accompanied by lightning and cloud thunder. There is daily rainfall in the afternoon. The intensity is very high as it occurs in the form of heavy showers.
Trade wind rainfall zone– It extends between 10°-20° latitudes in both the hemispheres and is characterized by northeast and south-east trade winds. These winds yield In the eastern part of the continents because they come from over the oceans and hence pick up sufficient moisture but as they move westward in the continents they become dry.
Subtropical zone of minimum rainfall- It extends between 20° and 30° latitudes in both the hemispheres, where descending air from above induces high pressure and winds diverge in opposite directions, with the resulting anti-cyclones formed. This condition is not conducive for rainfall and hence dry conditions prevail over large areas. It may be pointed out that tropical hot deserts are located in this zone where mean annual rainfall is below 250 mm. Most of the annual rainfall occurs during the summer months while the winter season is dry.
Mediterranean rainfall zone- It extends between 30° -40° latitudes in both hemispheres where rainfall occurs through westerlies and cyclones during the winter season while summers remain dry. Mean annual rainfall is 1000 mm.
Mid-latitudinal zone of high rainfall– It extends between 40°-50° latitude in both the hemispheres where rainfall occurs through westerlies and temperate cyclones. Mean annual rainfall ranges between 1000m and 1200 mm. The western parts of the continents receive more rainfall. It decreases from the western coastal areas of the island. The southern hemisphere records more rainfall than the northern hemisphere because of the dominance of the ocean in the former.
Polar zone of low precipitation– Precipitation decreases from 60° latitude poleward in both hemispheres. Mean annual precipitation becomes only 250 mm beyond 75° latitude. Most of the precipitation occurs in the form of snowfall.
OTHER FORMS OF PRECIPITATION (Diversity of Global Rainfall Patterns}
Snowfall– the fall of larger snowflakes from the clouds on the ground surface is called snowfall. Snowfall is the precipitation of white and opaque grains of ice. Snowfall occurs when the freezing level is close to the ground surface.
Sleet– it refers to a mixture of snow and rain but in American terminology, sleet means falling of small pallets of transparent or translucent ice having a diameter of 5 mm or less.
Hall– it consists of large pellets or spheres of ice. Hail is a form of solid precipitation wherein small balls or pieces of ice, known as hailstones, have a diameter of 5 to 50 mm from downward known as hailstorms.
Drizzle– the fall of numerous uniform minute droplets of water having a diameter of less than 5 mm is called drizzle. Drizzle falls continuously from low-status clouds but the total amount of water received on the ground surface is significantly low.