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How to Determine a Dewpoint on a Map

The dew point, or dewpoint, is a temperature measurement indicating how much the air must cool to become saturated with water vapor. (See Reference 1) In warm weather, this is the point at which dew forms on surfaces such as grass, bushes and automobiles. Higher dew points correspond to higher water vapor levels in the air. The chance of rain and severe thunderstorms is higher when the dew point is high. The dew point is also used to predict fog. (See References 2) If it falls below freezing, it is called the frost point because the water vapor creates hoarfrost or frost.

Things You'll Need

A current dew point map
Current relative humidity reading

Instructions

- 1
  Locate your city on a current dew point map online or in a newspaper. Online maps are generally more accurate because they are updated frequently throughout the day.
- 2
  Observe the map color at your location. Look for the map color key, and match the color at your location to the key. Most keys change colors in increments of 10-degrees in dew point. The degree reading for your location's color is the dew point for your area.
- 3
  Compare your location's dew point to the following general guideline in the summer: a dew point of 60 degrees to 65 degrees is slightly humid, 65 degrees to 70 degrees is very humid and over 70 degrees is tropic-like humidity. (See Reference 3) Step outside and experience what the dew point feels like.
- 4
  Compare your location's dew point to the relative humidity for your location. The higher the relative humidity, the closer the dew point is to the actual temperature. The dew point is the same as the current temperature when the relative humidity is 100 percent. If the dew point remains at the same degree level, and the temperature rises, the relative humidity decreases.
  
  The dew point is used by general aviation pilots to determine the possibility of icing on the carburetor, and for calculating the height of the cloud base. Painters and individuals who perform body work on vehicles use the dew point to determine whether they can safely paint, because paint doesn't dry well in highly saturated air.

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