What causes the dew point to rise and fall?

Question

Is it the wind speed, direction, temperature, etc.

I would much rather know what causes it to rise over what causes it to fall.

Answer 1

The dew point is the temperature at which the moisture in the air is 100% saturated and when cooling even further, it has to condensate in small water droplets. Temperature is the only factor here, not the wind.

At Standard Atmosphere (SA) the average on earth, the temperature is 15 Celsius and the pressure is 1013 hPa. The air cools down with 0.65 Celsius per 100 meter of altitude and the air cannot contain more than 14 grams water per cubic meter.

Knowing the temperature at the surface and the dew point, enables me, e.g. to guess the ceiling of the clouds. The difference between the two is called the spread. The larger the spread, the dryer the air and the higher the clouds will be. If the spread is none, i.e. same ambient temperature and dew point temperature then the air is 100% saturated as it is in fog.

If the temperature at the surface is say 15C and the dew point is 11C, the spread is 4C. Since air adiabatically cools down at an average of 0.65C per 100 m, or 2C per 1,000 feet, the ceiling can be expected to be 2,000 ft. Sorry if I mix metric and feet but, as a pilot, we use both; feet being used for altitude.

In the old days, seafarers used the wet & dry bulb thermometer. The wet one was covered with wet cotton. Evaporation kept it cooler. Sailors were then measuring the sea temperature and if it came to under the average of the two thermometers, fog could be expected.

I don't know how they measure dew point today but not too long ago, meteorologists at airports were still using the wet & dry thermometer but they had it at the end of a rope they rotated in order to create wind and cool the wet bulb. Then they read its temperature and after a correction for the altitude, called it the dew point temperature.

So, the answer your question (sorry, it was long! :-) what makes the dew point temperature to rise or fall is the content of moisture in the air.

Answer 2

Dew point mainly depends upon the moisture content in the air;Indirectly temperature also helps to increase the dewpoint temperature as warm air can hold more water vapour than cold air and more water vapour means increase in dew point temperature.