The role of water in your weather forecast

October 25, 2016


Jordan Smith
Zenith News

When we think about the weather, it’s often about precipitation. Does it look like rain today? Will it snow tomorrow? Are we going to have a humid summer?

But water is everywhere, affecting the weather even when we can’t see it. You may notice the terms “dew point” and “relative humidity” in weather reports. If, for example, the amount of water vapor in the air remains unchanged as a hot day cools in the evening, the air becomes saturated, causing the water vapor to condense into dew. So the temperature at which the air becomes saturated is called the dew point.

The dew point is measured in degrees, just like temperature. It’s calculated by the air temperature and the amount of water vapor in the air. When it condenses around tiny particles, called condensation nuclei, it forms fog or cloud droplets.
A new mass of drier or wetter air moving into a region can change the dew point. So can wind to a lesser extent.


Generally, a dew point temperature of 60 degrees or higher will feel humid, and 70 or above is downright uncomfortable.
Relative humidity is a measurement of the water vapor in the air, compared to how much would be in the air if it were saturated at the current temperature. Relative humidity is measured by a percentage rather than degrees. Anything below 40 percent is drier than normal, and anything above 70 percent is wetter than normal. While more exact than the dew point, relative humidity is less accurate in measuring how uncomfortable the humidity will feel.

A reading of 100 percent can still be drier than a lower reading, depending on the circumstances. For example, a temperature and dew point of 50 degrees would be 100 percent relative humidity, while a temperature of 90 degrees and a dew point of 60 degrees would read 36 percent relative humidity—even though the latter involves more water vapor in the air.

If the temperature were only nine degrees with a dew point of seven degrees, the relative humidity would read 91 percent—even though it involves less water vapor in the air. So even a high relative humidity won’t necessarily make you hot and sweaty, although it certainly could, given the right conditions.

To give you another example, heating your home raises the air temperature, but the dew point remains unaffected. So if your living room is at 70 degrees while the dew point stays at seven degrees, you’ll have an indoor relative humidity of only eight percent.

Anything below 30 percent will usually feel pretty dry, which is why your skin may feel dried out when you need to turn on your heater. It is also important to keep the relative humidity inside your home below 50 percent to avoid problems like mold.

On the cold side of precipitation, snow is not frozen rain; they’re completely separate phenomena. Frozen rain is sleet. Snow is ice that crystallizes around a nucleus. Water droplets need freezing nuclei in order to become ice, and the best freezing nuclei are ice crystals, which begin forming at around 32 degrees. As they form, they begin attracting slow-moving water molecules, causing bigger and bigger ice crystals.
But 32 degrees isn’t so much the point at which liquid water freezes, but rather the point at which ice begins to melt.


Supercooled water, for example, remains liquid, even at or below 32 degrees. Tiny water drops, or even drizzle, can occur without any such freezing nuclei to cling to, resulting in supercooled water, which can pose a serious hazard in the winter. Drizzle, for instance, when striking the ground, trees, powerlines, etc. will instantly freeze as they use what they landed on for a freezing nuclei, resulting in ice storms.

Speaking of precipitation, it should be pretty close to normal for November, with temps only two or three degrees above average. But November is November, and a lot of that precipitation may come in the form of snow, rather than rain as we approach Thanksgiving.

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