The Urban Heat Island Effect

View of Washington, DC from space

During the fall months as temperatures begin to cool it is quite common to see a larger range in daytime temperatures.  When the air is drier, it’s easier for it to cool and heat up than it is when the air is more humid.

Another factor that contributes to a high degree of variability between morning lows and daytime highs is the geography of a given region.  Given uniform atmospheric conditions, a rural area will cool off more quickly than an urban area.  Conversely, an urban area will heat up more quickly than a rural area.  This is known as the “Urban Heat Island” effect.  

The Nation’s Capital experiences the “Urban Heat Island” effect often during the fall months.  On the water near the center of town, National Airport is often much warmer at night than the more rural Dulles Airport since the rural area does not capture heat during the day that way the concrete and buildings do in a big city.  Since there is more vegetation around Dulles Airport that absorbs less heat, Dulles also cools off more quickly at night.

We just saw a good example of this yesterday.  Although only a relatively short geographic distance exists between the two airports, yesterday’s morning low at National Airport was 46 degrees while it cooled to a frosty 33 degrees at Dulles.  Being a quiet weather day, this significant difference in temperature can be attributed to the “Urban Heat Island” effect.