Why the Ground is Important to Weather (and Weather Forecasting)

Have you ever walked along a path that winds through fields, woods, or pavement and noticed a difference in temperature along the way? Read on to find out how the characteristics of the land surface can influence the weather.

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If you've ever boiled a pot of water (hello fellow pasta fans!), you know that you can't, or at least shouldn't, fill the pot all the way to the top or you'll have a mess on your hands. That's because as the water is heated from below, it expands. You'll see lots of bubbles and turbulent mixing as the water level rises, hopefully not overflowing the pot.

What does this have to do with the atmosphere?

Air is a fluid. During the day it is heated from below, just like the water in the pot. In the morning the sun rises, heating the ground and then the air directly above it. The air warms, expands, and rises, causing mixing in the layer of air closest to the ground. This mixed layer, called the planetary or atmospheric boundary layer, grows taller as the sun continues to warm the ground throughout the day.

It's hard to see this mixing, but you may have experienced it. If you've ever flown in a plane on a sunny day, you may have felt a lot of shaking and bouncing of the plane right after take off. Bump, bump, bump, bump...and then suddenly...smooth. You're out of the boundary layer and one step closer to your peanuts and 2 oz soda.

The sun heats the ground, which heats the air from below, causing mixing in the boundary layer and possibly a bumpy takeoff!

The characteristics of the ground can play an important role in determining how much heat and moisture are supplied to the atmosphere. This in turn affects how tall the boundary layer grows. It also influences the formation of clouds and rain.

For example, if we replace a grass patch with a city, the sun's energy will go primarily into raising the temperature of the land, making the air warmer and creating a drier, warmer, and taller boundary layer.

Cities are often warmer and drier than their surroundings. You may even notice the cloud base is a bit higher than in the country.

The opposite is true if we instead replace the city with a cornfield. The land is likely wetter so more of the sun's energy will go into evaporating water. This means that the boundary layer will be cooler, more humid, and the top of the layer will be shallower, or closer to Earth's surface.

Irrigated fields are usually cooler and more humid than their surroundings.

We know that these processes are important for predicting the weather. That's why our weather models include a dataset that defines each location's primary land use. The model uses this dataset to treat each land use differently when calculating how water and energy are transferred between the ground and atmosphere. In the end, the temperature, humidity, and wind flow in and near a forest will be different than over grass or a city, just like you'd experience in real life.

Want to learn more about this topic? This blog is a snippet of content from a longer public presentation for the University of Albany. You can view the full presentation here.

Thanks for reading!