CLEVELAND, Ohio - It's fall, which means the mornings are cool. And we're all waking up to a glistening layer of water over our lawns and cars.
We get that it's dew, but how exactly does it form? A cleveland.com reader (or at least the 3-year-old daughter of a cleveland.com reader) posed the question.
Understanding dew point temperature
First, it's important to understand what "dew point temperature" is. This is the temperature air must cool to in order for saturation to occur.
At saturation point for that specific temperature and pressure, the air holds as much moisture as possible. Therefore water starts to condensate, forming tiny droplets.
The higher the dew point, the higher the moisture in the air, the easier it is for condensation to occur. That's why humidity comfort levels are based on dew point. Also, the higher the dew point, the less the air has to cool to reach it.
The lower the dew point, the lower the humidity, and the more comfortable it'll feel outside.
Formation of dew
Dew usually forms the night following a warm day and will stick around until early the next morning.
Why a warm day? The warmer air is, the larger the capacity for it to hold moisture. So, the warmer the air, the more moisture, or water vapor, there is.
Overnight, the Earth radiates heat from the ground into the air. As the Earth radiates this heat, the ground cools.
As the ground cools, it cools the layer of air closest to the surface. Basically, the same way your stove heats a pot.
As this thin layer cools, it will eventually reach its dew point. Once the dew point is reached, the air will condensate. Since blades of grass are right on the ground, they create a perfect surface for the water to condensate onto.
The air closest to the ground will cool to its dew point much faster than the air 10 or so feet up (where temperature readings are taken), since the cooling is happening from the ground up.
So, the air farther from the ground will reach its dew point sometime right before sunrise, when the air is at its coolest. The air closest to the ground will reach its dew point much quicker, so dew can actually form overnight.
The exact time depends on the temperature and moisture conditions of the atmosphere and soil.
Conditions of formation
Dew is less likely to form on cloudy nights, because clouds act like blankets for heat.
With clouds, this heat cannot escape, and is instead re-radiated, or reflected, off the clouds and back to the surface. Since the air, and therefore the surface, will stay warmer, it is harder for it to cool to its dew point.
How come some days there's more dew than others?
Light winds are one factor. Light winds reduce dew because they prevent the moist air at the surface from mixing with the drier air above, higher in the atmosphere. Stronger winds would mix these two air masses together, therefore reducing the overall moisture available to condensate at the surface.
The soil moisture is another factor. The moister the soil, the heavier the dew will be. This will happen when it's been raining for a couple of days. Very dry regions that haven't seen rain in over a week are much less likely to have dew formation.
Why does dew form on only some surfaces?
Some surfaces have a tendency to stay warmer overnight -- the street, for example, or sidewalks. Concrete tends to hold heat for long amounts of time. So even at night after a warm day, the concrete will still be warm. Therefore, the air above it will not cool to its dew point in order to form dew.
Dew will only form on surfaces that cool relatively quickly.
Why is this important?
Apart from the annoyance of having to clean off your car in the morning, dew can actually significantly impact agriculture. Dew makes soil very moist, decreasing the amount of evaporation from the soil. Dew can also make mowing a lawn much more difficult because the grass will tend to stick together in large clumps.
Keep checking cleveland.com/weather for twice daily weather updates for Northeast Ohio, and don't forget to submit any weather questions you may have!
Kelly Reardon is cleveland.com's meteorologist. Please follow me on Facebook and Twitter @kreardon0818.
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