Hydrology, Hydrologic Cycle and Precipitation

HYDROLOGY

Hydrology is the science which deals with the occurrence, distribution and movement of water on the earth, including that in the atmosphere and below the surface of earth. Water occurs in the atmosphere in the form of vapor, on the surface as water, snow or ice and below the surface as ground water occupying all the voids within the geologic stratum.

Hydrologic Cycle 

Except for the deep ground water, the total water supply of earth is in constant circulation from earth to atmosphere, and back to earth. The earth’s circulatory system is known as the hydrologic cycle. Hydrologic cycle is the process of transfer of moisture from the atmosphere to the earth in the form of precipitation, conveyance of the precipitated water by streams and rivers to oceans and lakes, and evaporation of water back to the atmosphere.

Precipitation

Precipitation may be defined as the fall of moisture from the atmosphere to the earth surface in any form.

Forms of Precipitation

• Rain
• Snow
• Drizzle
• Glaze
• Sleet
• Hail.

Rain: It consists of water drops mostly larger than 0.5 mm and smaller than 6 mm in diameter. Drops bigger than 6 mm tend to break up as they fell.

       Type                                  Intensity (mm/hr)

    Light Rain                                       < 2.5         

    Moderate Rain                             2.5 to 7.5     

    Heavy Rain                                     > 7.5         

Snow: It is that type of precipitation which results from sublimation, i.e., water vapour directly changes into ice. It falls as white or translucent ice crystals often agglomerated into snowflakes. 

Average density of snow 0.1 g/cm³

The specific gravity of snow is often taken to be 0.1

Drizzle: They are tiny water droplets of size between 0.1 to 0.5 mm which fall with such slow settling rates that they occasionally appear to float.

As the size and intensity of Drizzle is very less, they appear to float in air.

Glaze: when the water droplet ( rain or drizzle ) come in contact with ground which have the temperature near about  0 °C or less than it, these water drops freeze and form ice coating on ground surface which is called glaze or freezing rain.

Sleet: When the rain drops fall through the layer of sub-freezing air near the earth’s surface the rain drops get frozen to ice stage. It is called sleet or grains of ice.

Hail: It is the precipitation in form of lumps of ice. The hail stones are produced in convective clouds mostly cumulonimbus. Their shape may be conical, spheroidal or irregular. The size of hail stones may be anything more than 5 mm. 

The specific gravity of hail stone is about 0.8

  Form of Precipitation                   Fall Speed ( m/sec)

       Drizzle Drops                                  1                     

       Rain Drops                                      10                   

       Hail                                                20-50