Even though the seasonal distribution of precipitation is an important attribute of the climate of a region, its diurnal variation is no less important a climatic feature. It affects communication, controls the weather conditions and many other outdoor activities.
The diurnal cycle of precipitation influences the precipitation effectiveness in agriculture, because a large part of the rainfall during the warmer part of the day is lost through the process of evaporation.
Two main types of diurnal variation of precipitation can be recognised: (a) continental or inland type, and (b) maritime or coastal type. Like the annual variation of precipitation, all sorts of departures from these two types are observed.
There are many local factors, such as the form of coastline, the topography of coastal areas, the presence of rivers, lakes and irrigated fields, etc. which influence the diurnal cycle of precipitation.
The continental type has its maximum of precipitation during the late morning or afternoon. In this type topography is the main cause of local differences in diurnal variation. The maximum amount of precipitation falling in the afternoon is mainly due to the convective activity which over land areas is strongest in the early afternoon.
Thus, there is a tendency to produce a precipitation maximum about the warmer part of day. Since convection is more vigorous during summer than during winter, some inland locations have maritime diurnal regime in winter and continental regime in summer.
In the tropics, because of the intensity of solar radiation and the greatest convective activity, there occur the most pronounced afternoon maxima of precipitation.
Convectional showers over some areas are produced much earlier than over others. This is due to the variations in slope and exposure to the prevailing or local winds.
Drainage conditions, moisture of soil, colour of the surface or types of vegetation may also cause variations in the diurnal precipitation regime. That is why most of the inland stations have a distinct precipitation minimum in the morning and a maximum in the afternoon.
According to Haurwitz and Austin, there are certain coastal regions which show a precipitation maximum during the night or in the early morning. Such locations have the maritime type of diurnal variation of precipitation.
In certain areas, a change in the direction of prevailing winds may cause a change in the diurnal regime of precipitation. This is the main reason why coastal stations in the monsoon lands experience large seasonal variations in the diurnal precipitation regime.
The maritime or coastal type shows a maximum of precipitation during the night or in the early hours of morning. This type of variation is caused by nocturnal convection. This is because of a steepened lapse rate.
During night, the upper troposphere cools off by radiation losses, mainly from cloud tops, while the surface layers of atmosphere lying in close contact with the water surface remain warm. Thus, the vertical temperature gradient is greater at night.
On the other hand, during day the lower layers of atmosphere are warmed up by directly absorbing the solar radiation, while the water surface is heated slowly.
Thus, atmospheric stability is created at low levels which are opposed to precipitation. As a result of this, convective activity over the ocean is greater at night than during the day.
However, the two types of diurnal precipitation regime are seldom found in ideal form except only at a few stations. Local factors strongly modify the diurnal precipitation distribution.