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I know "dew point" measures dry air. But what does the number represent? And why is it called "dew" point? I used to think a dew point of 62 meant the air will turn to dew at 62 degrees, but that's not right, is it?

Posted March 19, 2007

MIKE MOSS SAYS:       W. T.,   The number of a dew point, for example the 62 you mentioned in your question, represents the temperature at which a given air mass would be saturated with water vapor, so that any additional water vapor added to the air, or any further cooling of the air, would lead to condensation in the form of cloud or fog droplets (or if the air is in contact with a surface like the top of a car or a blade of grass, the condensation would take the form of dew on that surface).

The dew point is actually proportional to the amount of water vapor in a given parcel of air, so that an increasing dew point means there is a greater concentration of water vapor molecules present and vice versa. So, for example, air with a dew point of 62 is more "moist" than air with a dew point of, say, 35. In that sense, dew point is an indicator of absolute or specific humidity, both of which depend only on the concentration of water vapor in the air. This is not to be confused with "relative humidity," usually given in percent, which depends not only on the amount of water vapor present but also on the temperature. Relative humidity is a ratio of the amount of water vapor actually present in the air or in a given space compared to the amount that would be required to reach saturation. If a given volume of air or space contains only half the water vapor molecules required to become saturated, for example, the relative humidity would be 50%.

The dew point can tell us where moist or dry air is located and how it is changing. In combination with the temperature, it can also tell us how close the air is to being saturated. A few examples might help to illustrate the concept. Suppose we have air in place with a dew point of 62 degrees. If the temperature is 80 degrees, the relative humidity would be 54%, far from being saturated. If we were to cool the air to 62 degrees with no other changes, the air would then be saturated (relative humidity would reach 100%) and further cooling would cause some condensation to occur. On the other had, if we kept the temperature at 80 degrees but added water vapor to the air via evaporation so that the dew point climbs to 80 degrees, the air would likewise be saturated (relative humidity 100%) and a further increase in water vapor would also reault in some condensation of liquid water. Conversely, if the dew point remains at 62 but the temperature climbs to 90 degrees, the relative humidity would fall to 39% even thought the actual amount of water vapor in the air remains the same. Likewise, if the temperature stayed at 80 but drier air moved into the area so that the dew point fell to 53, the relative humidity would also fall to 39%.

Hopefully, these examples aren't too unclear. The basic idea is that the higher the dew point, the more moisture is present and vice versa, and the greater the difference between temperature and dew point, the lower the relative humidity and vice versa. Also, as for your final question, just keep in mind that if the temperature falls to match the dew point or any lower, the air itself doesn't turn to dew, but some of the water vapor present may do so.



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