Here is an example of the eye candy Tom sent me. It show a rain shaft falling out of a cumulus cell near sunset. Just marvelous. There also appears to be some light rain falling out of the cloud to the left and mostly evaporating before reaching the ground. This is called virga.
Some folks confuse rain shafts with funnel clouds, but they are completely different animals, with the rain shaft simply a well-defined area of precipitation produced by the relatively small scale convective cloud updraft.
Below cloud base, rain shafts are usually associated with downward air motion (downdraft) due to two reasons: (1) the falling precipitation drags air downward, (2) if the air below cloud base is not saturated (relative humidity below 100%), then there is evaporation that produces cooling in the rain shaft. Cool air is more dense than surrounding warmer air and this results in a downward acceleration.
In the right conditions (very high cloud base, heavy precipitation, low relative humidity) such rain shafts can be associated with powerful downbursts that can produce winds of 50-100 mph as the downdraft hits the ground and spreads out. Such downbursts (also known as microbursts or macrobursts, depending on size) can bring down large jets (see image).
Fortunately, here in the Pacific Northwest we rarely get strong downbursts. Our cumulus clouds are low based, Northwest cumulus clouds/convection are wimpy, and we rarely have very dry air near the surface. In contrast, Denver Colorado often has ideal conditions for such dangerous events.
Finally, it is fun to examine the Langley Hill weather radar imager around 9 PM that day (8:58 PM to be exact). Look carefully near the southern tip of the Sound and you will see the echo from the cell photographed by Tom.