Why Does Dew Form on Grass Overnight?

Well, we’re here to put your curiosity to rest. A scientific explanation tells us why dew forms on cool mornings and where that water covering your ground surface is actually coming from.

Today we’re diving into everything dew and addressing some common questions about dew’s impact on lawn care.

What Does Dew on The Grass Mean?

The air around us always contains some water vapor. This water vapor is simply liquid water in the form of a gas. Dew forms when that water vapor in the atmosphere undergoes a process of condensation thanks to the ground (or any surface, really) cooling.

Think of how beads of water form on the sides of an ice-cold drink you’re enjoying on your porch. Your glass goes through a cooling process thanks to that ice, causing those water drops – the condensation – to form.

Water molecules in the air are constantly settling on blades of grass and other flora in your yard, and some of these water molecules will stick around, forming a very thin film of water. While much of this water film will evaporate, the rate at which it evaporates depends on several factors.

These factors include the temperature of the water film, the temperature of the atmosphere, and the object’s temperature on which the dew may or may not form.

Whenever an object gets sufficiently cold and adequate moisture in the air, condensation will exceed the evaporation rate. The watery film on those blades of grass will bead into dewdrops.

If you listen to a weather report, pay close attention to the expected temperature and the dew point. The dew point temperature indicates the temperature at which the air must be cooled in order for it to reach a relative humidity of 100% and become saturated with water vapor. When a surface is cooled beyond that point, the water in the air condenses, forming beads of liquid water – or dew.

When the dewpoint and temperature are relatively close, you will likely be waking up to a lawn covered in dew the following morning.

Is Dew Enough to Water Grass?

Is Dew Enough to Water Grass

It’s understandable that when you wake up to find your grass seemingly soaked with dew, you might think that moisture is enough to satisfy your lawn’s needs. Think again, though. Morning dew on grass simply isn’t enough to quench your lawn’s thirst for water.

While the dew is present, however, is actually a great time to water your lawn.

Why? The reason is simple. There is already lots of moisture in the air. The temperatures tend to be cooler, so you won’t lose as much water to the effects of evaporation.

Most of the dew remaining on your lawn will likely be burned off as soon as the sun rises high enough to gain strength, meaning it will do very little to nourish your lawn.

Is Morning Dew Enough for Fertilizer?

Many homeowners also wonder if the moisture delivered to their lawn by morning dew is enough to help fertilizer be more effective at nourishing their lawns. The answer is yes but with a catch.

Whenever fog and dew form overnight and in the early morning, they can provide enough moisture to assist the fertilization process. The fog and dew combine with the salt component contained in most fertilizers, allowing your lawn to absorb the fertilizer through the blades of grass.

On the other hand, because there is usually a buildup of salt in your soil, your lawn can experience dry spots because of insufficient water delivery. In that case, fog and dew aren’t enough to combine with fertilizer and boost its effectiveness.

You’ll generally be better off providing additional watering to make sure that your lawn adequately absorbs all fertilizer.

How Can You Tell if Dew is Formed?

For the most part, dew will only form at night and in the early morning when temperatures drop, and objects like the blades of grass in your lawn begin to cool off. However, it is also possible for dew to form any time the dewpoint temperature is reached.

To be sure that what you’re seeing on your lawn is actually dew (and not the leftover water droplets from your sprinklers, for example), you can check the weather forecast to find out what the dewpoint temperature was the previous night and whether it was close to the actual temperature.

Then, if you don’t recall your sprinklers running and you don’t remember a rainstorm, you can be pretty confident that the layer of liquid water covering your grass is, in fact, dew.

There’s More to Dew Than Meets The Eye!

Besides just being pretty, dew plays an essential role in the ecosystem.

In fact, it functions as a critical source of water for many arid ecosystems, contributing to the daily, seasonal, and annual water balance. It helps to combat the deficit between precipitation and evapotranspiration – the sum of eater evaporation and transpiration from a surface area to the atmosphere, according to this definition from the U.S. Geological Survey.

While it can do great things for arid ecosystems, you now know it simply can’t provide all the water your lawn needs to stay green and healthy. So, next time you wake up to a lawn covered in dewdrops, just know that it’s the perfect time to break out the sprinklers and give your lawn the deep watering it needs to stay healthy.

Plus, now you’re prepared with an answer the next time your kids are wondering why it’s dewy in the morning.

Thanks, science!


Author: Matt Hagens

Hi, I’m Matt the owner of Obsessed Lawn. I love to be outside working on my lawn, planning my next project. I created this website to help people like you find the best products for yard care and great advice. Learn more about me and find me on Facebook.