Dewpoint and Heat Index explained

What is the “dew point”

The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must turn into liquid water. The dew point is always lower than, or equal to, the air temperature.

If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

If there is then further cooling of the air, more water vapor must condense out as even more dew, fog, or cloud, so that the dew point temperature then falls along with the air temperature.

While relative humidity is (as its name suggests) a relative measure of how humid the air is, the dewpoint temperature is an absolute measure of how much water vapor is in the air.  During the summer, it is the dewpoint temperature, not the relative humidity, that gives a better indication of how humid it feels outside. It is also a good measure of how much “fuel” is available to showers and thunderstorms, with a higher dewpoint representing more water vapor available for conversion to rain


The heat index given in weather readings is an index that combines air temperature and relative humidity, to give an indication of the temperature we are actually feeling.  This can also be known as the “apparant temperature”.  For example, if it is a hot day, say 30c, and there is a very high humidity reading, the actual temperature that you feel can be far more than 30c.

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