The dew point is the temperature to which air must be cooled to become saturated with water vapor.

Astrospheric processes hourly dew point from the Canadian RDPS.

When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air (such as a telescope), water will condense on the surface.

If the dew point temperature turns orange on the forecast, it means heavy dew or fog depending on other conditions. For example, if there is low wind speed, you're at lower elevation, and the sky is clear, then watch out for fog!

Using the dew point forecast on Astrospheric will allow you to determine how much dew protection you may need during a night of observing. The closer the temperature and the dew point temperature are, the higher the humidity, and the more you may need to use dew shields or heaters to keep your primary objective clear from dew.

Remember, even though the air may be warmer than the dew point, your telescope will continue to decrease in temperature on a clear night through radiation. That, coupled with the fact that water vapor needs a solid on which to condense, make your telescope a good place for dew to form. A slight breeze, dew protection, and/or setting up at a higher altitude away from a valley can help combat the formation of dew.