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Wet Grass In the Morning Might Be From Dew Not Rain

Have you ever woken up in the morning and wondered if it rained overnight because the grass was so wet? Sometimes it isn't rain at all, but dew that leaves the grass damp.

Is dew or rain to blame for wet grass? (Photo via Shutterstock)

Dew is moisture that forms because of condensation. Condensation occurs when water vapor — the gas form of water — turns into water in its liquid form. At this time of year, dew is just as likely as rain to be the reason for wet grass in the morning, but whether it forms at all is related to the weather overnight.

The dew we see and feel on the grass in the morning is linked to how cloudy the sky was the night before. When the sky is clear or mostly clear overnight, the ground cools, and warm air radiates up toward the sky. If the ground cools enough, dew forms on the grass. When it's cloudy overnight, the ground retains more of its heat, and not as much is radiated up to the sky. The heat that does radiate off the ground is reflected back toward Earth's surface when it hits the clouds in the sky. This means dew is less likely to form.

Basically, dew is most likely in the morning following a warm, sunny day that led into a cooler night with clear skies. The presence of dew on the morning grass — or lack thereof — has even led to a saying about what it means for the day's weather. "When dew is on the grass, rain will never come to pass. When grass is dry at morning light, look for rain before the night," is how the saying goes. 

There's some truth to the saying, but it is not a foolproof way to predict the weather. Dry grass in the morning usually means it's either cloudy or windy, which can also mean rain may be on the way. A clear night means dew is more likely on the grass, but it's also an indicator of a rain-free day ahead.

If you’ve ever watched the weather report on the TV news, you've probably heard meteorologists talk about the dew point and what it means for how pleasant the weather will be. The dew point is the temperature at which dew will form. The dew point varies from day to day depending on factors such as air temperature, time of day and location.

The dew point and humidity are related. The dew point is the temperature air needs to be cooled to for the humidity to be 100%. At the dew point, the air cannot hold any more water in the form of water vapor. Any increase in moisture in the air will be as water in its liquid form, either as fog or rain. 

Humidity is not always a good indicator of how comfortable it will be outside, but the dew point is. In general, if the dew point is 55 or lower, the air will feel dry, making it feel comfortable outside. Dew points between 55 and 65 mean the air will feel a bit humid and muggy, especially in the evenings. Dew points of 65 or greater mean there's a lot of moisture in the air, making it feel very humid. Dew points in the 70s are those oppressively humid days we experience every summer in northern Illinois. 


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