When rainfall occurs locally several factors can determine its intensity and or duration. Rainfall intensity is measured in mm/hr as total rainfall / storm duration (hrs). It is important to realise that heavy rainfalls usually lasts for a few hours or minutes whilst moderate rainfalls lasts for several hours or days.
Some cloud produce more and heavy rainfalls than others. Low lying clouds such as nimbostratus produces heavy rainfalls that can last for hours. Towering cumulonimbus clouds that stretch from below to high up in the atmosphere produces very copious rainfalls that lasts for several minutes. These clouds usually generate hailstorms because of their high convectional currents. Conversely, cloud types such as altostratus and cumulus may bring gentle drizzles whilst types such as stratocumulus generate no rainfall at all.
Uplift mechanism is the method air parcels are forced to rise and condense. Lifting mechanisms such as convectional and orographic uplift may generate heavy rainfalls with thunder and lightning or even hailstorms. Cold Fronts which undercuts and lifts up warm air can also causes heavy and copius downpours. These rainfalls tend to be short lived. Uplift mechanism such as along warm and stationary fronts produces fair and moderate rainfalls that can last for several hours.
Degree of Stability
If the uplifted parcels of air are stable, i.e cooling faster than the atmosphere, drizzles can occur or does not rain at all. Conversely, if the parcels of air are warm and cools slowly than the atmosphere (instability), intense rainfalls tend to occur that are usually short lived.
Availability of Condensation Nuclei
Minute particles such as dust and ash act as centres where water vapour accumulates and turns into tiny water droplets (clouds). If condensation nuclei is sparse rainfall tend to low. But, if condensation nuclei is abundant, e.g after fireworks or volcanic eruptions, heavy rains tend to occur.
Time of the Day
Heavy rainfalls usually occur around midday due to intense heating of the ground causing convectional uplift. Rainfalls tend to be low in the morning but can last for several hours.