Agronomy - Agrometeorology module 4

Classification of clouds
Clouds with vertical development

1.      Cirrus: Those clouds have a fibrous (hair like) appearance or a  delicate silky appearance or both.  These clouds do not give precipitation.

2.      Cirro-Stratus:  "Transparent, whitish cloud veil of  fibrous (hair like)  or smooth appearance, totally or partly covering the sky and generally; producing halo phenomena".  This type of cloud is so thin that it gives the sky a milky appearance.

3.      Cirro-cumulus:  Thin, white flakes, sheet or layer of cloud without shading.  Composed of very small elements in the form of grains, ripples etc.  This type of cloud is not common and is often connected with cirrus or cirrostratus. 

4.      Alto-Stratus:  A uniform sheet cloud of " Grayish or bluish cloud frequently showing a fibrous appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least wavely as through ground glass.  Alto-stratus does not show halo phenomena.  This type of clouds may cover all or large portions of the sky.  Precipitation may fall either as fine drizzle or snow.

5.      Alto-Cumulus: "White or grey, or both white and grey, patch, sheet or layer of cloud.  They have develt shedding on their under-surfaces.  Sometimes referred to as "Sheep clouds" or "Woolpack clouds".

6.      Nimbo-Stratus:  "Grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow.  Which in most cases reaches the ground.  It is thick enough throughout to blot out the sun.  It is a rain, snow or sleet cloud.  It is never accompanied by lightening, thunder or hail.

7.      Strato-Cumulus: "Grey or whitish or both grey and whitish patch, sheet or layer of cloud which almost always has dark parts, composed of tessellation's, rounded masses, rolls, etc.

8.      Stratus:  Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains, sky may be completely covered by this type of cloud.  Sum is visible through this cloud.

9.      Cumulus: "Detached clouds, generally dense and with sharp outlines, develop vertically in the form of rising mounds, domes or towers, of which the bulging upper parts often resembles a cauliflower They produce only light precipitation.

10.  Cumulonimubs: "Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers.  This type of cloud is associated with heavy rainfall, thunder, lightening, hail or tornadoes.

Cloud Seeding: 
It is the process by which the conditions of the cloud, (dimension, life time and size) are modified by supplying them with suitable nucleius at proper time and place.  For accelerating the warm rain process seeding with very large nuclei such as salt crystals can be used.  In the case of cold rain process, seeding with ice nucli such as silver iodide are used to make good the deficiency in the clouds.

Loss of water from living plants is called transpiration.  It can be stomatal, cuticular or lenticular.

Evapotranspiration (ET):
It is a combined losses of water through evaporation from the soil and transpiration from the plants.

Potential Evapotranspiration (PET):
It is defined as the amount of water which will be lost from an extensive water surface or soil completely covered with vegetation where there is abundant moisture in the soil at all times.

Condensation:  The physical process by which a vapour becomes a liquid or solid - opposited of evaporation.

Precipitation: Precipitation has been defined as water in liquid or solid forms falling to the earth.  Precipitation occurs in a variety of forms such as rainfall, snow, hail, fog and dew. 

Rain: It is "Precipitation of liquid water particles, either in the form of drops of more than 0.8 mm diameter or in the form of smaller widely scattered drops.

Sleet: Transparent, globular, solid grains of ice freezing of rain drops.

Hail: Precipitation in the form of halls or irregular lumps of ice.

Drizzle: It is "fairly uniform precipitation composed exclusively of fine drops of water (diameter less than 0.8 mm).

Snow:  It is "precipitation of white and opaque grains of ice. 

Hail:  It is  "Precipitation of small balls or pieces of ice (hailstorms) with a diameter ranging from 5 to 50 mm or sometimes more.

Dew: Dew is a common form of condensation in the environment.

            Frost is a form of condensation that occur on solid surfaces when the dew point is below freezing point. (0oC).
Fog: fog is condensed water droplets suspended in air in the lower atmosphere (surface of the earth)

            In general drought may be defined as a complex phenomenon which results from the prolonged absence of precipitation in conjunction with high rate of evaporation. 

Classification of Drought
            Drought can be broadly divided into three categories.
1.      Meteorological drought: is a situation when the actual rainfall is significantly lower than the climatologically expected rainfall over a wide area.
2.      Hydrological drought is associated with marked depletion of surface water and consequent drying up of lakes, rivers, reservoirs etc. Hydrological drought occurs when meteorological drought is prolonged.
3.      Agricultural drought is a condition in which there is no rainfall and insufficient soil moisture availability in soil to the crop.
4.      Atmospheric drought - It occurs when the rate of transpiration exceeds rate of absorption of water due to low RH, high temperature and moderate to high wind velocity even through available soil moisture is high in the soil.  The drought is temporary and reversible.
5.      Soil drought - Condition when the soil moisture supply exceeds - 15 hours (Permanent wilting point).  It is gradual and progressive.  It is highly detrimental than others.
6.      Physiological drought- even though the available soil moisture is high in the soil, the plants are not able to absorb.