Precipitation and Types of Precipitation
Precipitation and types of Precipitation.
Before we start, you should have to know about Cloud and Hydrological cycle.
- Precipitation occurs when droplets of water or ice crystals become too large and heavy to be held aloft and fall as rain, snow, sleet or hail.
- The type of precipitation depends largely on how it formed and what the temperatureis in the cloud and below as it falls to earth.
- There are two hypothesis which attempt explaining the process of precipitation and those are:
1) Collision – Coalescence Hypothesis (merging).
2) Ice – Crystal Formation Hypothesis (Bergeron Findesein Hypothesis).
- In many cases, particularly in the tropics, clouds have temperature too high for the formation of Ice crystals.
- In such clouds, rain is formed by the collision and merging of water droplets.
- Condensation alone cannot yield rain because it produces lots of small droplets but no large drops. Thus coalescence is necessary.
- Apparently, coalescence is assured only if Atmospheric electricity is favourable I.e, if a positively charged droplet collide with a negatively charged one. The larger they grow, the faster they grow.
- The pressure exerted by the water vapour in the atmosphere is called vapour pressure. If the air is saturated then term saturation vapour pressure is applied.
- Saturation Vapour Pressure is greater above water surface then over an ice surface i.e, it takes more water vapour molecules to saturate air above water than it does above ice.
Ice crystals formation hypothesis
- Ice crystals and supercooled water droplets often co-exist in the cloud. These two are in direct competition for the available water vapour that is not yet condensed.
- There is lower Vapour Pressure around the ice crystals so they attract most of the vapour and the water droplets, in turn, tend to evaporate to replenish the diminishing supply of vapour.
- Therefore the ice crystals grew at the expense of the water droplets, until they arelarge enough to fall.
- As the crystals descend to the Lower, warmer portions of the cloud, they pick up more moisture and become still larger.
- They may then precipitates from the cloud as the snowflakes (or) they may be melted and fall as raindrops.
Different types of Precipitation
The mechanism with which the air is uplifted to result into clouds and precipitation is referred to as type of precipitation.
There are 3 major types of Precipitation.
(1). Convectional Precipitation.
(2). Orographic Precipitation.
(3). Frontal Precipitation.
- Convectional Precipitation is due to unequal heating of different surface areas.
- Convectional Precipitation is restricted in terms of latitudes ( especially lower latitudes), season (esp summer), time of the day (during only sun’s presence).
- Convectional Precipitation is generally showery with large raindrops descending fast and furiously but for only a short duration.
- It is due to a topographic barrier.
- On the windward side – as the air rises it cools adiabatically and condensation takes place and precipitation results.
- On the Leeward Side – as the air descends, it warms adiabatically and its Relative Humidity decreases and so results into rain shadow zone.
- Orographic Precipitation can occur any latitude, any season any time of the day if there are a barrier and moist air is forced to move up.
- In general Orographic Precipitation is likely to be gentler and prolonged ie., for a given amount of water it takes more time as compared to the Convectional Precipitation.
- A front is a zone of transition between the cold dry polar air and warm moist tropicalair.
- Fronts are generally restricted to middle latitude (temperature belt).
- In a Front, warm air rises over the cold air resulting in Clouds and Precipitation.
- The nature of Frontal Precipitation depends upon the types of front formed.
There are two major types of Fronts.
- Cold Front: Cold Front is the one in which cold air is aggressive and cuts off the warm air of the ground and lifts its up vigorously resulting into Precipitation more or less similar to convection Precipitation.
- Warm Front: Warm Front is one in which warm air is aggressive. In this front as warm air raises it spreads horizontally resulting into Precipitation which is more (or) less similar to Orographic Precipitation.