Precipitation


Water falling in solid or liquid form e.g. rain, snow, and hail..

Uses of Precipitation Data

      • Runoff estimation analysis
      • Groundwater recharge analysis
      • Water balance studies of catchments
      • Flood analysis for design of hydraulic structures
      • Real-time flood forecasting
      • low flow studies

Mechanism Producing Precipitation

Three mechanisms are needed for formation of precipitation.
1. Lifting and Cooling – Lifting of air mass to higher altitudes causes cooling of air.
2. Condensation – conversion of water vapor into liquid droplets.
3. Droplet Formation – Growth of droplets is required if the liquid water present in a cloud is to reach ground against the lifting mechanism of air.

Types of Precipitation

Depending upon the way in which the air is lifted and cooled so as to cause precipitation, we have three types of precipitation, as given below:

        • Cyclonic Precipitation
        • Convective Precipitation
        • Orographic Precipitation

Cyclonic Precipitation:

Cyclonic precipitation is caused by lifting of an air mass due to the pressure difference. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation.

1. Frontal precipitation:

It results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side. A front may be warm front or cold front depending upon whether there is active or passive accent of warm air mass over cold air mass.2.

2. Non-frontal precipitation:

If low pressure occurs in an area (called cyclone), air will flow horizontally from the surrounding area (high pressure), causing the air in the low-pressure area to lift. When the lifted warm-air cools down at higher attitude, non-frontal cyclonic precipitation will occur.

prcp1

In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, forceful rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.

prcp1

In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front. Warm front precipitation is generally light to moderate.

Convective Precipitation

Convective precipitation is caused by natural rising of warmer, lighter air in colder, denser surroundings. Generally, this kind of precipitation occurs in tropics, where on a hot day, the ground surface gets heated unequally, causing the warmer air to lift up as the colder air comes to take its place. The vertical air currents develop tremendous velocities. Convective precipitation occurs in the form of showers of high intensity and short duration.

Orographic Precipitation

Orographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward and hence rise up, causing condensation and precipitation. All the precipitation we have in Himalayan region is because of this nature. It is rich in moisture because of their long travel over oceans.

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The Hydrological Cycle


The Hydrological Cycle
(also known as the water cycle) is the journey water takes as it circulates from the land to the sky and back again.

The sun’s heat provides energy to evaporate water from the earth’s surface (oceans, lakes, etc.). Plants also lose water to the air – this is called transpiration. The water vapour eventually condenses, forming tiny droplets in clouds.

When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and water returns to the land (or sea). Some of the precipitation soaks into the ground.  Some of the underground water is trapped between rock or clay layers – this is called groundwater. But most of the water flows downhill as runoff (above ground or underground), eventually returning to the seas as slightly salty water.

This Information page provides an understanding of the hydrological cycle.  It describes the principal stages of the cycle, with a brief description of each stage.  A diagram gives a clear visual explanation.  The links between the hydrological cycle and the duties of a water utility to supply clean water and dispose of dirty water are also explained.

hydrologic_cycle

What is the Hydrological Cycle?

The total amount of water on the earth and in its atmosphere does not change but the earth’s water is always in movement. Oceans, rivers, clouds and rain, all of which contain water, are in a frequent state of change and the motion of rain and flowing rivers transfers water in a never-ending cycle. This circulation and conservation of earth’s water as it circulates from the land to the sky and back again is called the ‘hydrological cycle’ or ‘water cycle’.

How does the Hydrological Cycle work?

The stages of the cycle are:

  • Evaporation
  • Transport
  • Condensation
  • Precipitation
  • Groundwater
  • Run-off

Evaporation

Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. The sun’s heat provides energy to evaporate water from the earth’s surface. Land, lakes, rivers and oceans send up a steady stream of water vapour and plants also lose water to the air (transpiration).

Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.

Transport

The movement of water through the atmosphere, specifically from over the oceans to over land, is called transport. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets.

Clouds are propelled from one place to another by either the jet stream, surface-based circulations like land and sea breezes or other mechanisms. However, a typical cloud 1 km thick contains only enough water for a millimetre of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater than this.

Most water is transported in the form of water vapour, which is actually the third most abundant gas in the atmosphere. Water vapour may be invisible to us, but not to satellites which are capable of collecting data about moisture patterns in the atmosphere.

Condensation

The transported water vapour eventually condenses, forming tiny droplets in clouds.

Precipitation

The primary mechanism for transporting water from the atmosphere to the surface of the earth is precipitation.

When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is triggered and water returns to the land (or sea). A proportion of atmospheric precipitation evaporates.

Groundwater

Some of the precipitation soaks into the ground and this is the main source of the formation of the waters found on land – rivers, lakes, groundwater and glaciers.

Some of the underground water is trapped between rock or clay layers – this is called groundwater. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers, which empty into lakes, rivers and the oceans.

Under special circumstances, groundwater can even flow upward in artesian wells. The flow of groundwater is much slower than run-off with speeds usually measured in centimetres per day, metres per year or even centimetres per year.

Run-off

Most of the water which returns to land flows downhill as run-off. Some of it penetrates and charges groundwater while the rest, as river flow, returns to the oceans where it evaporates. As the amount of groundwater increases or decreases, the water table rises or falls accordingly. When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface.

Different surfaces hold different amounts of water and absorb water at different rates. As a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. Flooding is very common during winter and early spring because frozen ground has no permeability, causing most rainwater and meltwater to become run-off.

NOTE:
Courtesy: http://www.euwfd.com/html/hydrological_cycle.html