Wednesday, December 15, 2004

Why Is It Often Warmer On Cloudy Days?

Why would clouds make it warmer outside? First we have to know what a cloud is made of. We see them all the time, we know rain comes from them, and we've probably pointed out some that look like animals or other shapes, but we may not know exactly how they are made.

It starts out when water from lakes, rivers, ponds, streams, and other water on the ground evaporates into the air. To say that the water evaporates means that some of the water leaves the ground after being heated by sunlight, and is carried by the air in very tiny droplets or as water vapor (gas) that cannot be seen with the naked eye.

You may have heard that warm air rises. The warm air close to the earth that carries the water vapor is an example of this. As the air rises, it cools, losing the heat that kept the water droplets suspended in the air. This cooling causes the water vapor to condense (turn back into liquid droplets). These droplets land on tiny specks of dust in the air. In other words, the water is no longer carried invisibly by the air, but instead, it clings to the dust particles. Groups of these wet particles make clouds.

How do clouds make it warmer outside if a cloud is only water droplets clinging to dust in the air? Actually, the clouds don't make it warmer outside; clouds keep it warmer outside.

A blanket of clouds covering the sky is a little like a blanket that you use to stay warm in the winter: the blanket holds in heat. Sunlight is the source of energy for the earth, and some of that energy is in the form of heat. On days when there are no clouds, heat from the sun can enter and leave the atmosphere without anything getting in the way. When it is cloudy, the clouds absorb (hold) some of the heat so it can't escape. Clouds also reflect some of the heat back towards the earth.

So, on a cloudy day, some energy from the sun gets into the atmosphere through the clouds, but can't get out again. When this happens, the heat builds up during the day, so it gets warmer outside. On days when there are only a few hours of daylight, the sun doesn't have much time to send some of its heat energy through the clouds, and not very much heat builds up. That's why, even on cloudy days, it's still cold out in the winter.

Sometimes it gets even warmer on cloudy days because of advection. Advection is the movement of heat, cold, and moisture when air moves (when there is wind). When warm air from a tropical climate moves into a cooler area on a cloudy day, the clouds keep in the heat just like they keep in the heat from the sun.

Since warm air can hold water vapor better than cooler air can, clouds that are very high up in the cooler part of the atmosphere do not have as much water, and so they are not as thick. Since thinner clouds let in more light and heat energy from the sun, it is not as warm on a day with high, thin clouds as it is on a day with a low, heavy cloud cover.

To summarize, clouds are made of droplets of water that cling to specks of dust in the air. Some heat energy from the sun can make it into the atmosphere through the clouds, but the clouds trap the heat in by reflecting and absorbing it, causing the air inside the cloud blanket to warm up throughout a cloudy day. Warm air moved by advection can bring heat into an area that will be held in if there are clouds. High, thin clouds do not hold in heat energy well as lower, thick clouds do.

Little Lion Experiment:

In order to see how clouds form on particles in the air, you will need:

  • black sheet of paper
  • glass jar or mug
  • flashlight
  • clear bag with ice
  • confectioner's sugar


  1. Tape a black sheet of paper to one side of a glass jar or mug.
  2. Have an adult boil about a cup of water then fill the jar or mug part of the way with boiling water.
  3. Immediately sprinkle a small amount of confectioner's sugar into the top of the jar.
  4. Quickly cover the jar with the bag of ice. Turn out the lights and shine the flashlight into the jar.

You should be able to see a cloud forming inside of the jar as the warm water evaporates, rises, and condenses onto the particles of confectioner's sugar when it reaches the higher air cooled by the ice. You may also see the water condense directly onto the sides of the jar or mug instead of onto the particles, but keep in mind that there is no container for real clouds; only dust particles.

This experiment was adapted from Lesson Exchange;