There are many differences between warm air and cold air! The warmth or coldness of a material is measured by its temperature. So, things that feel hot are a high temperature, and things that feel cold are a low temperature. But, what temperature is really telling you is how fast the atoms within the material are moving.

When things are high temperature, the atoms have a lot of energy, and they bounce around very quickly. When things are a low temperature, the atoms still bounce around, but more slowly. When something hot (where the atoms are bouncing around quickly) is brought into contact with something cold (where the atoms are bouncing around slowly), the quick bouncing atoms bump into the slow bouncing atoms and give them some of there energy. When this happens, the hot object cools down, and the cold object heats up. An example of this would be putting a hot pan into a sink full of cold water. The pan cools down, and the water heats up. Just like the pan and the water, air is made of atoms too!

So, the atoms in the hot air are moving around more quickly than atoms in cold air. And, if hot and cold air mix, the fast atoms bump into the slow atoms, and eventually everything is the same average speed and the same temperature.

Another effect of temperature is to change the density of materials. The density is a measure of how tightly packed the atoms are to each other. When something heats up, the atoms bounce around more quickly, and they push each other apart more strongly. This gives each atom more space, and since every atom has more space, the material gets bigger--it expands. So if you take a balloon full of cold air and heat up the gas inside it, the balloon gets bigger because the cold air becomes hot and expands, which pushes against the balloon and makes it bigger. Here is an experiment you can try at home: fill a balloon with air at the hottest part of the day.

Record how wide the balloon is. Then, leave the balloon outside over night. The next morning, as early as you can wake up, measure the size of the balloon again. Since the temperature at night and in the early morning is much colder than the temperature during the hottest part of the day, the balloon should be smaller because the density of the air inside decreases when it's cold. Try it and see!