Andhra Pradesh receives 90% of rainfall from the South-West Monsoon as well as North-East Monsoon. South-West Monsoon is comparatively heavy in Northern parts of the State; the North-East Monsoon brings more rain to the southern areas of Rayalaseema. The coverage of monsoon periods is as follows:
The south-west monsoon enters the state in the month of June. The monsoon covers entire state and provides around 66 percent of state's rainfall except in the districts of Nellore, Prakasam, Chittoor and Kadapa.
The North-east monsoon generally enters the state in the month of October. It provides almost 24 percent of state's rainfall and mostly covers in the districts of Nellore, Prakasam, Chittoor, Kadapa and some parts of Anantapur.
Both winter and Hot Weather Periods put together contribute around 10 percent of state's rainfall. Most of the rains occur due to depression and cyclone in the month of May in the state.
In view of monitoring the drought situation in each mandal, the Directorate of Economics and Statistics, has calculated the normals for each rain gauge, (situated in every mandal) for the years 1985 to 1999. (i.e., 15 years).
The amount of rainfall received in a given area during a given time (i.e. 24 hrs periods to measuring rainfall every day at 8.30 hrs. IST) is defined as actual rainfall of a particular raingauge station for the day.
Average rainfall of a day for a particular district is calculated as
Sum of rainfall of all raingauge stations received in the district on a particular day divided by
Total No. of raingauge stations in the district
Rainy day is defined as a day on which 2.5 mm or more of rain is recorded. In arriving at the deficiency in rainfall in any given area the number of rainy days also taken into consideration
According to drought manual, a dry spell is a short period, usually 4 weeks (up to 3 weeks in case of light soils), of low rainfall or no rainfall. Thus, consecutive 3-4 weeks after the due date for the onset of monsoon with rainfall less than 50% of the normal in each of the weeks is defined as a "Dry spell".
During South-West monsoon (June to September) and North-East Monsoon (October to December) periods, rainfall data is monitored every day. The daily rainfall data is collected from all the raingauge stations through online / Fax and is being tabulated, computerized, and analyzed and reports sent to the departments concerned on the same day
Deviation of Rainfall is categorised in to 6 categories Viz.,
1)Large Deficient Rainfall : -99% to -60% Rainfall over normal is classified as Large Deficient Rainfall.
2) Deficient Rainfall : -59% to < 20% Rainfall over normal is classified as Deficient Rainfall.
3) Normal Rainfall : -19% to +19% Rainfall over normal is classified as Normal Rainfall.
4) Excess Rainfall : +20% to +59% and above Rainfall over normal is classified as Excess Rainfall.
5)Large Excess Rainfall : +60% and above Rainfall over normal is classified as Large Excess Rainfall.
6) No rain : There is no rain (-100%).