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Precipitation in Engineering Hydrology
Measurement of Precipitation | Mechanism of Precipitation


Definition of precipitation:

It is defined as the fall of moisture from the atmosphere to the earth surface in any physical form. Usually precipitation is in two forms:

  1. Liquid - rainfall
  2. Solid Precipitation
    1. Snow
    2. Hail
    3. Sleet
    4. Freezing rain
  • It is major factor controlling hydrology of a region.

  • It is major input of water to the earth surface.

Related Pages

Knowledge of rainfall in space and time is necessary for understanding soil moisture, groundwater, recharge and river flow. Data of rainfall (precipitation) is more readily available, for cities and longer period, than for any other component of hydrological cycle. In some countries precipitation data may constitute the only hydrological record.

The study of precipitation is thus of fundamental importance to hydrolysis but detail investigation of the mechanism of its formation is the domain of meteorologists and climatologists

Total amount of water in atmosphere represent only a minute proportion of world water budget. Atmospheric water accounts for less than 0.001% of world total supply of land, ocean and atmospheric water. But this small amount serves as a continuous source of supply.

Hydrologists are only interested in the distribution itself, in how much precipitation occurs and when and where it falls and their interest cease when precipitation reaches ground. The hydrological aspect of precipitation studies are conceived with the forces in which precipitation occur, its variation in both, space and time.


Factors affecting precipitation

Heavy precipitation occurs near the equator and decreases with the increase in the latitude i.e. Towards polar regions. Main source of moisture for ppt is evaporation from oceans. Therefore, ppt tends to be heavier near coastlines.

Since lifting of air masses is the cause of almost all ppt, amount and frequency of rain is genrally greater on windward side of the mountain. As downslope motion of air results in decease in humidity, thus the opposite sides of barriers  usually experience relatively light ppt. High amount of ppt is reported at higher elevations.

During the year

  • High ppt occurs in humid season i.e july, August
  • Low ppt occurs in dry season i.e May, June
  • Pressure variation in a particular area causes ppt.
  • Presence of mountains cause orographic ppt.
  • Temperature variation also causes ppt in a particular area.
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