You might be thinking why is it more humid in the morning, when on hot summer days the humidity levels fall as the day progresses. After all, surely the humidity should stay the same, or even get higher? However, humidity is a complicated concept to grasp and many people do not truly understand it at all, which can lead to a number of confusing beliefs about levels of humidity.
If a meteorologist on the television weather forecast is talking about humidity levels, they are referring to the amount of water vapor in the air—not rain drops, hail, or snowflakes. This might seem a simple concept to understand, but because there is so much confusion about what humidity actually means, even some weather forecasters still get it wrong—and they are meant to be the experts!
Humidity levels are related to energy levels and the two key factors that determine the levels of humidity are the amount of thermal energy available and the availability of water. This means that on a warm day there is more water vapor in the air because there is more thermal energy to fuel the evaporation process. Warm air has more kinetic energy, or speed, than cold air. Warm air molecules colliding with slower moving water molecules cause the water molecules to gain kinetic energy, which frees some of them to become a free moving gas molecule.
Relative humidity is an expression of how much energy is available to release the liquid water molecules from their neighbors in the water source. If the relative humidity levels are reported to be 50%, this means that 50% of the available energy has been used to evaporate water from the sea, river, stream, or ground, but 50% is still left to do some more evaporation.
But why is it more humid in the morning?
During the day, the temperature normally rises as the sun heats up the ground and surrounding buildings. As the ambient temperature rises, so does the level of kinetic energy in the atmosphere. Even if there is no more water vapor in the atmosphere, the relative humidity levels fall simply because there is twice as much kinetic energy available.
Why does this happen?
Relative humidity is expressed as a percentage of what energy has been used to evaporate water, so when the available energy doubles, the percentage of energy used is half what the original figure was. As the sun warms the air, there is more energy available, and therefore less overall energy has been used.
The Dew Point temperature is often considered to be an easier way to measure levels of humidity. When air cools sufficiently to allow condensation to take place, this is called the Dew Point temperature. As air cools and energy levels drop, relative humidity increases. At the point where 100% relative humidity is reached, the Dew Point temperature has also been reached, so at 100% relative humidity, dew point temperature equals atmospheric temperature. In the morning, there is less of a difference between the dew point temperature and atmospheric temperature—hence the relative humidity is higher.