“Did it rain last night?” Have you ever asked that question after waking up to see a wet lawn in the early morning? Maybe it did rain last night, but then again… maybe it didn’t!

Rain or shine, there’s always a chance of waking up to a wet lawn if the weather conditions are just right. Those tiny droplets you see clinging to the blades of grass are called “dew.”

Dew is liquid water droplets that form on grass, spider webs and other things in the early morning or late evening. Dew only forms under certain conditions.

If a warm, clear day is followed by a cool, clear evening, dew will likely form. On a normal warm day, water evaporates from the warm ground into the air. That means it turns from a liquid into a gas called “water vapor.”

When evening comes, the warm ground continues to radiate heat into the air. As the ground begins to cool, the air will not be able to hold all the moisture.

At a certain point — a temperature called the “dew point” — water vapor in the air will begin to condense (turn back to liquid water) faster than water is evaporating.

When this happens, dew forms on surfaces that aren’t warmed by the heat radiated from the ground. That’s why you mainly see dew on things like grass, leaves and even car roofs.

If the temperature gets low enough in the evening, dew may undergo yet another transformation. When temperatures are low enough, dew may freeze to a solid form that we then call “frost.”

If you’re wondering why it’s important that the evening is clear for dew formation, it’s because cloudy nights hold heat closer to the ground. If the evening is cloudy, the clouds reflect heat back to the ground. When this happens, the ground doesn’t cool off enough for dew to form.

If you pay attention, you may notice that on mornings when you find dew on the grass, there’s probably not much, if any, dew on the soil of flower beds. Like clouds on a cloudy evening, the plants in a flower bed keep the ground warm enough to prevent dew.

In some areas, certain species of plants depend on dew. For example, in areas that have lengthy dry seasons, such as deserts, plants like lichen and pine seedlings depend on dew to survive.


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    • SUPER question, Karen! We’re thinking you might want to explore the answer to that question on your own. You will need a journal, clock, and your own eyes to explore the answers. Stop back by and let us know what you discover! :)

    • We are so glad you’re WONDERing with us, K/1! Frost will form if the local temperature is below freezing (32 degrees F). Thanks for stopping by Wonderopolis! :)

    • Great question, sifiso! You might be able to find more information by doing research at your library. Thanks for WONDERing with us! :)

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Have you ever wondered…

  • Why is the grass wet in the morning?
  • What is dew?
  • Why doesn’t dew form when it’s cloudy?

Wonder Gallery

fresh thick grass_shutterstock_71481652Vimeo Video

Try It Out

Hmmm… what shall we dew now? How about a couple of experiments? Get together with your family and give these simple experiments a try to see condensation at work firsthand!

Find the dew point with a soda bottle

You’ll need a two-liter plastic bottle, a saucer, a thermometer and water. Remove the bottom of the plastic two-liter bottle.

Place the bottle (leave the cap on) in a saucer filled with water. Make sure all of the bottom edge of the bottle is underwater. Leave the bottle on the saucer overnight.

In the morning, remove the bottle and quickly place a cup of ice water with a thermometer in it on the saucer. Put the bottle back over the glass on the saucer.

Watch what happens: You’ll soon see condensation (drops of water) form on the outside of the glass. Look to see at what temperature this begins to happen. That’s the dew point for your experiment!

Steamy breath

Find a mirror or a window to use. Stand close and take a very deep breath. Exhale your hot breath onto the mirror or window.

What happens? Did it steam up and make the mirror or window foggy? That’s condensation! Your hot breath contains water vapor.

When it hits the colder mirror or window, some of the water vapor condenses back into thousands of tiny liquid water droplets!


Still Wondering

Use Smithsonian’s History Explorer’s The Water Nearby lesson to learn more about the bodies of water closest to your home using Google Earth!


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