Much Ado about Dew Point

When you watch the weather forecast you invariably see a listing of current conditions, which includes temperature, wind speed, relatively humidity, and dew point. If you’re like me you use those readings to determine how the weather is going to impact your comfort (hot, cold, humid, etc.).  But why is dew point important enough to be listed, and how does that impact your day?


Let’s start off back in school: dew point is defined as the air temperature below which water vapor in the air will condense into liquid water.  Your glass of ice water sweats because the surface temperature of the glass has dropped below dew point temperature.  Same goes for your wet yard on a summer’s morning. Dew point is the temperature at which dew will form, hence the name. So knowing the dew point temperature can be useful in determining if you will get wet shoes walking across the grass early in the day, but it goes beyond that.  Dew point is actually a better gauge of the humidity in the air and how comfortable that feels to you than does the relative humidity they give on TV.


Back to school: relative humidity (RH for short) is the amount of water vapor in the air given as a percentage of how much the air could hold (max is 100%).  It’s called relative because the percentage is relative to air temperature, which means the RH is changing by the hour throughout the day. Generally as the air temperature goes up, the relative humidity goes down, and your lowest RH will be around mid-afternoon and the highest after midnight.   Relative humidity doesn’t always give you a good read on how comfortable the day will feel, because a 90 degree day with a 50% RH is going to feel a lot worse than on a 40 degree day.  Dew point on the other hand is based on how much water vapor is actually in the air (more water vapor creates a higher dew point temperature), and will not change much through the day no matter the temperature. So it can be a more stable indicator of how the humidity will affect personal comfort.


So how do you use dew point? Most folks feel comfortable when the dew point is between 30 and 60 degrees. From 60 to 70 degrees it will begin to feel increasingly humid, and above a dew point of 70 degrees it will feel uncomfortably humid. Above 80 degrees it will feel oppressive and even dangerous for folks with asthma. On the flip side low dew point temps may foretell dry skin issues for some.


The difference between the actual temperature and the dew point temperature (called “the spread”) can be useful in determining if fog may form, which can happen when the air temperature drops within five degrees or less of dew point and is decreasing. Fog and low clouds will begin to clear out when the temperature-dew point spread is increasing. Aircraft pilots keep close tabs on dew point for these reasons and also to determine the likelihood of carburetor icing in light aircraft.   Frost on a clear night will occur when the temperature-dew point spread is 5 degrees or less and the dew point is colder than 32 degrees. Another name for dew point during the winter is “frost point”.   One more science note and I’ll quit: the formation of frost is not condensation but sublimation, where water goes from a vapor directly to ice and skips the liquid form.

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