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The weather in the DC Metro area as of late has been pretty dry. Despite the rain with Tuesday's cold front, we had Fire Weather Watches and Red Flag Warnings earlier this week. This means that the weather conditions were making it easy for small fires to start. When one of these fire weather advisories is in effect, we urge people to not throw lit cigarettes out the car window, to use extreme caution when building campfires (or to avoid campfires altogether), among other common-sense precautions.

The two main contributors to fire weather are dry air and wind. We had both of those conditions in abundance yesterday. But isn't this supposed to be the time of year with heavy rain and severe thunderstorms? And, if dry air and wind are the main factors for fire weather, why don't we normally see the same warnings in place during heat waves in the summer?

The answer, believe it or not, lies in the trees.

Right now, all the trees in the DC Metro area are in the process of forming and growing their leaves. Many of them are still flowering (you're welcome, allergy sufferers) and have leaves that are just miniature versions of the fully-grown leaves we'll see in just a few weeks. Since the leaves aren't fully developed yet, our climate is missing its "moisture buffer", so to speak! As I'm sure you know, tree leaves have a lot of water in them. A process called transpiration distributes moisture from the leaves into the air surrounding them, which helps give the atmosphere extra humidity. Since the leaves are still very small on our massive tree population in the area, there is very little transpiration taking place! Without the extra humidity provided by transpiration, dry air entering the DC Metro stays drier. This, in turn, makes it more likely that fire-conducive weather will happen when it's dry and windy.

Trees don't just have a big effect on the possibility of fire weather. The process of transpiration can also have a big effect on temperature! Water leaving a leaf causes evaporational cooling in the air surrounding it. In addition, humid air requires more heat energy to maintain the high amount of water in the air as a gas (water vapor). So, a humid atmosphere insulates against big swings in temperature. This is why you'll often have summer nights when the temperature stays in the 70s, or even 80s, well after dark. Without our trees, we wouldn't have such a strong barrier against a temperature drop. If you've ever been to a desert, then you have experienced this radical temperature change first-hand, and you know how different it is from the night-and-day cycle here in the DC Metro area! Finally, just as bare trees can allow temperatures to cool off more quickly, they also make it easier for the air to heat up. When warmer air surges into the DC Metro area in the summer, the leaves and other foliage actually absorb some of the heat and mitigate the warmup. But in late March or early April, there's no foliage to stop the warmer air. So, we can get a "dry heat" reminiscent of the heat in the Southwest. This happened last April during the Cherry Blossom festival. We had a 4-day stretch where we reached 80°, 86°, 91°, then 86° again! At least the beautiful blossoms provided some shade from the early-season heat!

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