Precipitation

Cause

Sea is the main source of our rainfall. Sea water evaporates and the water vapor is absorbed in the air streams. The moisture-laden air keeps the water vapor absorbed until its' temperature below the dew-point temperature. The water is then be precipitated as rain, hail or snow.
Causes of precipitation can be classified as follows:
1. Convective precipitation
Convective precipitation regarded as fall in air mass temperature due to convection. The warm moist air rising and cooling to form cloud, then precipitated as rain subsequently.

2. Orographic precipitation

As Ocean air streams passing over land and being deflected upward by coastal mountains, water is being cooling below saturation temperature and spilling moisture. Most rain is deposited on windward slopes. This phenomenon is classified as Orographic precipitation.

3. Cyclic precipitation
As air tends to move into low pressure area, air is displaced upward, to cool and precipitate.

4. Frontal precipitation
Frontal rain is associated with the boundaries of air masses where one mass is colder than the other. This intrudes cool wedge under it, raise warm air and form clouds and rain.

Forms of precipitation

1. Rain

_{The term "rainfall" is used to describe precipitation in the form of water drops of sizes larger than 0.5 mm diameter. The maximum size of a raindrop is about 6 mm diameter. For raindrop size larger than 6mm, it tends to break up into drops of smaller sizes.}

2. Snow/ice

_{Snow acts as a form of storage as it has the capacity to retain water. Density = the quantity of water contained, varies from 0.005 to 0.6 for newly fallen snow and highly compressed snow respectively.}

3. Fog

The view of Victoria Harbour always covered by fog.

4. Dew

(picture from http://wallpapers.free-review.net)

5. Condensation

_{Condensation produces precipitation from humid air flows over ice sheets and in temperate climates by condensation in the upper layers of soil.}

Calculations of Precipitation

1. Arithmetic mean
_{When the rainfall measured at various stations in a catchment show little variation, the average precipitation over the catchment area is taken as the arithmetic mean of the station values. For P1, P2, ..., Pn are the rainfall values in a given period in N stations within a catchment, the mean precipitation can be estimated by the following equation:}
2. Thiessen-Mean Method
_{In this method, lines is drawn between pairs of gauges , then bisecting lines with bisectors. Assuming all the area enclosed formed the intersecting perpendiculars has had rainfall of the same amount as the enclosed gauge. Therefore, mean precipitation can be calculated as:

For N stations.

General form:

(Ai/A) is also called the weighted average factor for each station.}

Example:

Question: Compute the mean precipitation of the basin below using Thiessen network. (All dimensions are in meter)

basin

Answer:

3. Isohyets method
_{Isohyets are the contours of equal rainfall depth. Area between successive isohyets are measured and assigned an average value of rainfall. So we can get the overall average for the area from weighted averages. This is the best method as local effects are taken into account.}

4. Estimation of missing data
_{As missing data often happens, we need to fill in the gaps in station's record. For given annual precipitation values, P1, P2, ..., Pn at n stations 1, 2, ..., n respectively. Pa is the missing annual precipitation at station A. We can use the normal ratio method to find out Pa:}