The middle ear, which is located just behind your ear drum, is an air pocket inside the head that is vulnerable to air pressure changes. Normally, the Eustachian tube, which connects the middle ear to the back of the nose, helps equalize pressure between the middle ear and the outer ear. Eustachian tube size and function varies from person to person, and children tend to have narrower tubes than adults. When the eustachian tube is blocked, during a common cold or from allergies, pressures cannot equalize, and a painful vacuum develops inside the middle ear. Often this will clear with time, but sometimes fluid builds up, blood vessels burst, infection occurs or the ear drum ruptures.
Hearing is usually muffled, and in severe cases, can be affected permanently.
Vertigo, a sensation of the world spinning, can also occur. Swallowing and yawning can help open the eustachian tubes and equalize pressure - so chewing gum or letting a mint or lifesaver melt in your mouth during take offs and landings may help. Be sure not to sleep during descents - you'll miss your opportunity to yawn. Take a breath of air. Using your cheek and throat muscles, force the air into the back of your nose as if you're trying to blow your nose.