Weather 101: Wet-Bulb Temperature

Weather 101

Why This Phenomenon is Critical for Winter Weather Forecasting

Temperatures are definitely critical for winter weather forecasting. They can mean the difference between seeing a winter wonderland or a cold rain in your backyard. Dry air is not often monitored as closely, but in winter weather situations it can play a major role. This is due to a process meteorologists call wet-bulbing. The wet-bulb temperature is temperature air will cool to when water is evaporated into unsaturated air.

Visualizing Evaporational Cooling

You actually have probably experienced this process before when you step out of a body water such as your shower or a swimming pool. There is a brief amount of time after you exit the water where you feel a slight chill to the air. This is because the water droplets on your skin evaporate into the surrounding air. They are able to do this by stealing energy from your body in order switch from a liquid to gaseous state. The result is you feel cooler due to the release of heat.

Evaporational Cooling in the Atmosphere

This same process of energy transfer occurs in the atmosphere. When you have dry air in place at the surface, the falling precipitation has to fight through that layer to make it to the ground. It does eventually reach the ground, but at the expense of the temperature.

As the precipitation falls into the drier air it evaporates. Just like our shower example, in order for the evaporation to occur, energy must be taken by the water droplets from an outside source. In the atmosphere this source is the surrounding air. As the energy is absorbed the air temperature cools due to the loss of energy.

Wet-Bulb Temperature Explainer

During the fall and spring months this process drops the temperature maybe a degree or two. This limited drop is because the air isn’t all that dry at the surface, so the amount of time it takes to saturate the column is a lot less.

During the winter, the temperature drop is more extreme due to a couple of factors. The first is that the air can become bone dry during the winter months. This means that it takes longer to saturate the air and costs more energy from the atmosphere for the precipitation to reach the ground.

The second is that during the winter, sublimation (transition from solid to gas) is often involved. This process requires even more energy than evaporation, so the temperature drop is greater due to the larger amount of energy consumed in order to transition the snowflakes from a solid state to a gaseous state.

If you enjoyed learning about the wet-bulb temperature on this edition of Weather 101, you can find other interesting weather topics on our website under the Weather 101 tab.

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