Re: Why (assuming wind isn't a factor) does it suddenly start raining 'harder'?

Marv,

Rainfall rate can be dependent on several different factors. Meteorologically, the rate of precipitation can depend on the strength of the lift, the degree of instability, the amount of available moisture, and the temperature structure of the rain-producing clouds, among others. The degree of instability and lift will determine whether rainfall will be convective (showers and thundertorms) or stratiform (steady and light) in nature.

In addition, the rain-producing system will undergo changes in rainfall rates as it goes through the different stages of maturity. Early in the life of a thunderstorm, the updraft is dominant and little if any rainfall occurs. In the mature stage, the updraft and downdraft coexist, and heavy rain often occurs. As the storm dies, the downdraft becomes dominant. Heavy rain can continue for a while longer, but will soon taper off. The life cycle of a typical thunderstorm is less than one hour.

Another interesting point to consider: cloud-to-ground lightning is often observed in areas of heavy downpours, and sometimes occurs where intense precipitation is about to begin.

Other factors which may explain the change in rainfall rate that you experience: usually, either you or the storm (and maybe both) are in motion. Therefore the rate of precipitation will vary as your position changes relative to the storm. Storms have been observed to move at speeds of 50 mph or more!