Freeze Watch Tonight

The National Weather Service has posted a Freeze Watch for essentially everyone north and west of DC tonight.


Freeze Watch map, courtesy of our local
National Weather Service office

With cool, dry air in place right now, temperatures will drop quickly tonight after the sun sets. A Freeze Watch is posted whenever temperatures are predicted to dip below freezing, thereby ending the growing season. (You won't see Freeze Watches in the middle of winter, when we've already had subfreezing temperatures and plants have already died.)
The map above clearly demonstrates the areas that freeze first, and those areas that tend to stay milder for a longer period into the fall. Here's a breakdown:

1. The mountainous areas of Virginia, including Clarke, Shenandoah, and Frederick counties get cold quicker because of the high elevation. The air is thinner at higher altitudes; literally, there are fewer air molecules in mountainous areas than at sea level. Thinner air doesn't hold heat as efficiently, which is why it's cooler in the mountains.
2. The counties in Maryland that are closest to the Mason/Dixon line are obviously the furthest north in the state, so it's easy to see why they're under the Freeze Watch.
3. The counties immediately north and west of the District, Arlington and Alexandria are under the Freeze Watch because they are less densely populated than the immediate DC Metro.
4. The DC Metro area is not included in the Watch, because the temperature is expected to stay above freezing due to the Urban Heat Island effect. When buildings, people, cars, and pavement are taking up most of an area's space, they prevent the temperature from dropping. Asphalt emits heat more slowly than grass and trees, so it stays warmer overnight. Also, people artificially heat their houses, which helps to keep the surrounding air warmer. People and vehicles emit heat, too. All those factors add up to a milder overnight temperature in town than in the suburbs.
5. The counties along the Chesapeake aren't under a Freeze Watch, either; this is because of the proximity to the water. Large bodies of water will very slowly release heat during the overnight hours, and this helps to keep the air temperature from dropping. Since humid air contains more water than dry air, a humid airmass will cool down more slowly, too. So, the temperatures right along the water don't drop as quickly overnight as those inland.

As I mentioned before, Freeze Watches are only issued if the area hasn't yet experienced temperatures below 32 degrees in the season. If a hard freeze does happen tonight in the highlighted counties on the map, then this will be the last Freeze Watch for those counties of this Fall.