Temperature, humidity and dewpoint are words that get thrown around a lot when people talk about the weather, thus complicating the simple question of how you should dress today. What are the differences between these terms, and how do they relate to each other? Let’s examine them and find out.
Temperature is easy: How hot is it? Admittedly, the humidity and dewpoint can make it feel hotter than it is, but the temperature itself as a simple, concrete number based on how much heat is in the air at this exact moment. It is the founding factor of the other two points.
Humidity (more precisely called relative humidity): How much water vapor is in the air, compared to how much the air can actually hold? This number is properly presented as a percentage. This does not affect temperature, merely how we perceive it.
That said, temperature does affect humidity, because hot air can hold more water vapor than cold air; this means that the relative humidity is on a sliding scale. For example, 50 percent humidity on a summer’s day would feel very uncomfortable, while 90 percent humidity in the winter is quite nice. Because of this, many scientists and meteorologists feel that talking about humidity to be confusing to laymen, and prefer to give the dewpoint instead.
Humidity and dewpoint are related, as both measure the amount of the water in the air. Dewpoint, however, is defined as the lowest temperature at which dewdrops will form, based on how much water vapor is currently in the air. If the dewpoint is the same or higher than the current temperature, then dew (or fog) will occur. It uses the same scale of degrees that the temperature does: for example, if the dewpoint and temperature are both 50F, dew occurs.
Many now prefer to use the dewpoint because it is more concrete than relative humidity. Unlike our previous example, a dewpoint of 60F is always uncomfortable, 70F is even more uncomfortable, and so on, no matter what the exact temperature actually is at the moment.
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