Types of Precipitation

Precipitation occurs when local air becomes saturated with water vapor, and can no longer
maintain the level of water vapor in gaseous form. This occurs when less dense moist air cools,
usually when an airmass rises through the atmosphere. However, an airmass can also cool (e.g.
through radiative cooling, or ground contact with cold terrain) without a change in altitude.
Convective precipitation occurs when air rises vertically through the (temporarily) self-
sustaining mechanism of convection. Stratiform precipitation occurs when large air masses rise
diagonally as larger-scale atmospheric dynamics force them to move over each other. Orographic
precipitation is similar, except the upwards motion is forced when a moving airmass encounters
the rising slope of a landform such as a mountain ridge.
1. Convectional:
Convection occurs when the Earth's surface, especially within a conditionally unstable or moist
atmosphere, becomes heated more than its surroundings and in turn leading to significant
evaporation. Convective rain and light precipitation are the result of convective clouds, for
example cumulonimbus or cumulus congestus. In the initial stages of this precipitation, it
generally falls as showers with rapidly changing intensity. Convective precipitation falls over a
certain area for a relatively short time, as convective clouds have limited vertical and horizontal
extent. Most precipitation in the tropics appears to be convective; however, it has been suggested
that stratiform and convective precipitation often both occur within the same complex of
convection-generated cumulonimbus.
Graupel and hail indicate convection; when either are present at the surface, it is indicative that
some form of precipitation is extant at the freezing level, a varying point in the atmosphere in
which the temperature is 0°C.In mid-latitude regions, convective precipitation is often associated
with cold fronts where as it is often found behind the front, occasionally initiating a squall line.

2. Cyclonic:
Frontal precipitation is the result of frontal systems surrounding extratropical cyclones or lows,
which form when warm and tropical air meets cooler air. Frontal precipitation typically falls out
of nimbostratus clouds.
When masses of air with different densities (moisture and temperature characteristics) meet, the
less dense warmer air overrides the denser colder air. The warmer air is forced to rise and, if
conditions are right, creates an effect of saturation, causing precipitation. In turn, precipitation
can enhance the temperature and dewpoint contrast along a frontal boundary, passing weather
fronts often result in sudden changes in environmental temperature, and in turn the humidity and
pressure in the air at ground level.
Warm fronts occur where warm air pushes out a previously extant cold air mass. The warm air
overrides the cooler air and moves upward dud. Warm fronts are followed by extended periods
of light rain and drizzle due to the fact that, after the warm air rises above the cooler air (which
remains on the ground), it gradually cools due to the air's expansion while being lifted, which
forms clouds and leads to precipitation.
Cold fronts occur when a mass of cooler air dislodges a mass of warm air. This type of transition
is sharper, since cold air is more dense than warm air. Precipitation duration is often shorter and
generally more intense than that which occurs ahead of warm fronts.
A wide variety of weather can be found along an occluded front, but usually their passage is
associated with a drying of the air mass.

3. Orographic:
Orographic or relief rainfall is caused when masses of air are forced up the side of elevated land
formations, such as large mountains (often referred to as an upslope effect). The lift of the air up
the side of the mountain results in adiabatic cooling, and ultimately condensation and
precipitation. In mountainous parts of the world subjected to relatively consistent winds (for
example, the trade winds), a moister climate usually prevails on the windward side of a mountain
than on the leeward (downwind) side. Moisture is removed by orographic lift, leaving drier air
on the descending (generally warming), leeward side where a rain shadow is observed.
The table below shows the types of rainfall based on their intensity:
 Forms of Precipitation:

There are many different forms of precipitation i.e., rain, snow, hail, and sleet for example—yet
they all have a few things in common. They all come from clouds. They are all forms of water
that fall from the sky. Additionally, they all affect life on Earth, causing some people to leap with
glee while making others scowl, mumbling about umbrellas or snow shovels, causing garden
flowers to grow or causing massive crop damage.
The most common forms of precipitation:
➢ Rain:
Drops of liquid water fall from the clouds when water vapor condenses around dust particles in
the clouds, forming tiny droplets that eventually get too big for the cloud to hold so they fall,
growing larger as they collect more water on their way down.

➢ Snow:
Snow is ice that falls from the sky. Each snowflake is a delicately complex arrangement of ice
crystals. A snowflake forms when water vapor sublimates, or turns directly from a gas into its
solid form, ice.

➢ Hail:
Hail is ice that falls from the sky, often in round shapes. Hailstones form within thunderstorm
clouds when upward moving air keeps pellets of frozen water from falling. The pellets grow
larger as drops of very cold water hit them and freeze. Eventually the balls of ice become so
large and heavy that they fall to the ground as hailstones. The largest documented hailstone
weighted more than one and a half pounds! Scientists estimate that it reached a speed of more
than 80 mph as it fell toward Earth.

➢ Sleet:
Sleet is like slush falling from the sky. Sleet forms when raindrops freeze into ice as they fall to
the ground. They are usually smaller and wetter than hailstones.
➢ Drizzle:
Drizzle is a fine sprinkle of tiny water droplets of size less than 0.5mm and intensity greater than
1mm/h. The tiny drops forming a drizzle appear to float in the air.
➢ Glaze (Freezing Rain):
Drizzle is a fine sprinkle of tiny water droplets of size less than 0.5mm and intensity greater than
1mm/h. The tiny drops forming a drizzle appear to float in the air.
 Factors Effecting Runoff
When rain falls onto the earth, it just doesn't sit there, it starts moving according to the laws of
gravity. A portion of the precipitation seeps into the ground to replenish Earth's groundwater.
Most of it flows downhill as runoff. Runoff is extremely important in that not only does it keep
rivers and lakes full of water, but it also changes the landscape by the action of erosion. Flowing
water has tremendous power—it can move boulders and carve out canyons.
Runoff of course occurs during storms, and much more water flows in rivers (and as runoff)
during storms. For example, in 2001 during a major storm at Peachtree Creek in Atlanta,
Georgia, the amount of water that flowed in the river in one day was 7 percent of all the
streamflow for the year.

Meteorological factors affecting runoff:

➢ Type of precipitation (rain, snow, sleet, etc.)
➢ Rainfall intensity
➢ Rainfall amount
➢ Rainfall duration
➢ Distribution of rainfall over the watersheds
➢ Direction of storm movement
➢ Antecedent precipitation and resulting soil moisture
➢ Other meteorological and climatic conditions that affect evapotranspiration, such as
temperature, wind, relative humidity, and season.

Physical characteristics affecting runoff:

➢ Land use
➢ Vegetation
 ➢ Soil type
➢ Drainage area
➢ Basin shape
➢ Elevation
➢ Slope
➢ Topography
➢ Direction of orientation
➢ Drainage network patterns
➢ Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or alter runoff from
continuing downstream

Some Details of Factors:

1. Physical Factors

➢ Time of year
In temperate climates, where seasonal change is evident, runoff levels can vary greatly
throughout the year.
In summer, runoff levels can be low due to a reduction in rainfall. Soil saturation levels will be
low and therefore any rainfall at this point can easily infiltrate into the ground.
However, intense baking of the soil by the sun can lead to the soil becoming effectively
impermeable and summer storms can lead to high levels of runoff as the rain is unable to soak in.
This can lead to flash floods.
In winter, precipitation may be in the form of snow and the water may be stored on the ground
due to low temperatures. Warmer temperatures in spring may lead to snowmelt and this can lead
to the soil reaching field capacity quickly. Further meltwater will therefore run over the surface.
➢ Storm conditions:
Intense storms with heavy rainfall can lead to soils quickly becoming saturated. This can happen
in two ways:
• Prolonged rainfall – moderate to high volumes of rainfall over a sustained period can
eventually saturate the soil leading to runoff.
• Intense rainfall – heavy rain in a short period can bounce and then flow over the Earth’s
surface as runoff.
➢ Vegetation cover
Vegetation can intercept precipitation and reduce runoff. Leaves and stems can capture rain and
prevent it from reaching the ground. It will eventually reach the ground but the process will have
been slowed by the vegetation and therefore water will infiltrate into the ground rather than
runoff.
Dense vegetation with proteoid roots (hairy roots with a large surface area) can absorb large
volumes of water even in storm conditions, which will prevent runoff.
➢ Soil saturation levels
Where field capacity is reached in the soil, no more infiltration can take place and therefore
runoff will occur.

➢ Topography & relief

In “v” shaped valleys on steep slopes, runoff will increase due to gravity.
On flatter surfaces runoff is less likely to happen as water will be able to infiltrate into the soil
more easily.
Equally, depressions in the Earth’s surface and an undulating relief can allow water to collect
and reduce runoff.

2.Human factors
➢ Agricultural land use
Initially, agricultural land use can have the same impact as vegetation cover, in that crops can
intercept precipitation and reduce runoff. However, intensive agriculture where irrigation may be
used, can cause waterlogged soils and therefore lead to runoff.
Heavy use of agricultural machinery can compact the soil and reduce its infiltration capacity,
making runoff more likely.
➢ Urban land use
Changing greenfield surfaces to impermeable concrete and tarmac as construction takes place
can increase the level of runoff in an area.