what is the dew point of rain?

Question

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Answer 1

When you are in the rain, fog, of flying in the clouds, the dew point temperature is the same as the outside temperature because the air is saturated with moisture.
In the old days, seafarers used to have a so-called wet & dry bulb thermometer on board. One would be dry and the other, envelopped in wetted cotton. The instrument had to be outside, exposed to wind but not to the direct sunlight. The dew point was then estimated as being the average of the two readings. When the air became saturated, both thermometers were reading the same temperature as the cooling effect of the wet cotton didn't work anymore.

In aviation, the air temperature and dew point temperatue are always given in the METAR so that the pilot can evaluate the relative humidity of the air. If the spread between the two readings is very little then I am sure I will use often the carburettor heating of my engine because it will humid air means often danger for carburettor icing.

At the average Standard Atmosphere of 15 degrees centigrade, a cubic meter of air can sustain about 14 gr. of water. It is then said th be saturated and the relative humidity is 100%

Answer 2

Dew points are measured in degrees, not percentages. Humidity and dew points are definitely related. What you are probbaly thinking of is if the temperature and the dewpoint are the same. If that is the case, then there is usually rain in the area. If the humidity and dew point are the same, it doesn't mean much. Dew point and humidity could both be low with the temperature high, and that would mean quite the opposite, with very dry air instead of clouds and rain.