Humidity and Precipitation: Distribution of Precipitation, Regional Variations, Seasonal Variations

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Distribution of Precipitation

The spatial distribution of precipitation is not uniform all over the world. The average annual precipitation for the world as a whole is about 97.5 cm, but the land receives lesser amount or rainfall than the oceans. The annual precipitation shows marked difference on the land. The main features of the distribution of precipitation can be explained with the help of global pressure and wind belts, distribution of land and water bodies, and the nature of relief features. Let us discuss the regional and seasonal distribution patterns of precipitation.

Regional Variations

On the basis of average amount of annual precipitation, the main precipitation regions in the world are:

  • Regions of Heavy Precipitation: The regions which receive over 200 cm of annual precipitation are included in this category. These regions include equatorial coastal areas of tropical zone and western coastal regions of temperate zone.

  • Regions of Moderate Precipitation: The regions which receive 100-200 cm of annual precipitation are included in this category. These regions lie adjacent to the regions of heavy precipitation. Eastern coastal regions of subtropical zone and coastal regions of the warm temperate zone are included in this category.

  • Regions of Less Precipitation: This category includes regions which receive precipitation between 50-100 cm. These regions lie in the interior parts of tropical zone and eastern interior parts of the temperate zone.

  • Regions of Scanty Precipitation: The areas lying in the rain shadows or leeward side of the mountain ranges, the interior parts of continents, the western margins of continents along tropics, and high latitudes receive precipitation less than 50 cm. These regions include tropical, temperate, and cold deserts of the world.

Rainfall Map of the Earth

Rainfall Map of the Earth

After studying the map showing the annual average precipitation distribution of the world, we came to the following conclusions:

  • Precipitation is greatest in the equatorial region and decreases towards the poles.

  • Precipitation is heaviest in the coastal regions and decreases towards the interior of the continents.

  • Eastern coastal areas of tropical lands and western coastal areas of temperate lands receive heavy precipitation including equatorial regions.

  • Precipitation is very heavy on the windward side of highlands while very dry conditions prevail on the leeward side.

  • Coastal areas adjacent to cold currents are drier than coastal areas near warm currents.

  • The western margin of tropical land and polar region receives scanty rainfall.

Seasonal Variations

It is important to study the seasonal variations of precipitation in the world. The nature of precipitation especially of those regions where seasonal fluctuations in the amount of precipitation are very common includes arid, semi-arid or sub-humid regions. The facts related to seasonal variations are as follows:

  • The equatorial regions and the western parts of temperate lands receive precipitation throughout the year. The former receives conventional type of rain while the later gets cyclonic cum orographic type through westerlies.

  • About 2 % land areas of the world receive precipitation only in winter. These include Mediterranean regions of the world and Coromandel Coast of India. Due to the seasonal shift in pressure and planetary wind systems, the Mediterranean region do not get precipitation in summer as it come under sub-tropical high-pressure belts and trade winds which become dry while reaching the western margins of continents.

  • The remaining parts of the world receive precipitation only in summer. Most parts of the world experience marked seasonal variation in precipitation. The scanty precipitation during short growing season in high latitudes is more effective than that of heavy precipitation in lower latitudes. Likewise, precipitation in the form of dew, fog and mist in some parts like central India and Kalahari Desert has an appreciable effect on standing crops and natural vegetation.

Factors Affecting Rainfall Distribution

  • Moisture supply to the atmosphere is the main factor in determining the amount of rainfall in any region. Equatorial and rest of the tropical region have highest evaporation and hence highest supply of moisture. Coastal areas have more moisture than interior parts of continents. Frigid regions have very low evaporation having very scanty precipitation.

  • Wind direction in the belts of trades and westerlies winds is very important. Winds blowing from sea to land cause rainfall. Land bearing winds are dry. Winds blowing from higher to lower latitudes will get heated and give no rain while those blowing from lower to higher latitudes will get cooled and cause rainfall. Sub-tropical deserts have very little rainfall because they have offshore winds.

  • Warm ocean currents are associated with warm moist winds which cause rainfall while cold currents have cold dry wind and hence no rainfall.

  • Presence of mountain across the direction of wind causes more rainfall on the windward side, thus creates rain shadow on the leeward side.

  • Pressure belts are closely related with wind direction and rainfall. Areas of low pressure attract rain bearing winds while areas of high pressure do not.

In brief, it can be said that

  • Water vapour is an important component of atmosphere. It is responsible for global heat balance, atmospheric phenomena, and sustaining plant and animal life on our planet. The water vapour present in the atmosphere is called humidity, which is expressed as absolute humidity and relative humidity. Water vapour enters into atmosphere through a process called evaporation. Temperature of the air controls the amount of moisture it can hold at a given volume. The air which holds the moisture to its full capacity is called saturated air and the temperature at which it reaches saturation point is termed as dew point. Condensation is a process of changing of water vapour into liquid or solid state. It happens when temperature of an air falls below dew point. Condensation occurs near the ground as dew, mist, or fog and at higher levels of clouds.

  • Falling down of atmospheric moisture is called precipitation which occurs due to continuous condensation. Drizzle, rainfall, snowfall, sleet, and hail are various forms of precipitation. The rainfall occurs in three different ways conventional, orographic, and cyclonic.

  • The distribution of precipitation in the world shows marked regional and seasonal variations. Some regions receive heavy rainfall while others scanty precipitation. Some regions receive precipitation throughout the year while others only in the winter or summer.

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