Hygrometers measure the amount of moisture or humidity in the air, but there are a number of different types of hygrometers, and they display their results in different ways. Most hygrometers display their readings in the amount of water in a given volume of air, making the units milliliters per cubic centimeter. Alternatively, some hygrometers measure relative humidity, which is expressed as a percentage with no units.
Dew point temperature is a measure of humidity, but it is reported in degrees Fahrenheit or Celsius. For example, if the temperature is 80 degrees Fahrenheit, and the humidity is very high, the dew point may be 79 degrees Fahrenheit. This means that if the air cools just one degree, it is not able to hold the water it is already holding and water condenses from the air.