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Learn what the dew point is and how to calculate it.

Understand the characteristics of the dew point temperature and how it’s a useful aspect of forecasting the weather.

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Dew Point Definition

I live in the desert where it’s not very humid. When it’s hot in the summer and I tell the temperature to someone that lives somewhere more humid than where I’m at, it never fails that they will say ‘at least it’s a dry heat.’ While there is some truth to that, it’s not much comfort when the temperature is over 110 degrees!That said, the amount of moisture in the air is a very important aspect of our weather. Humidity is the amount of water vapor in the air. We often describe the humidity as a percentage that is actually a ratio comparing the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold.

When humidity is expressed as a percentage, it’s called relative humidity.So, for example, relative humidity of 50% means that the air is holding half of the water that it possibly can. If the relative humidity is 100%, it’s holding the maximum amount of water possible.

When that happens, it means that clouds can form, and it can start raining. The temperature of the air when that occurs is called the dew point temperature, and it’s an important part of humidity.

Dew Point Characteristics

The dew point is useful in weather forecasts for a number of reasons:

  • Air temperature must fall to the dew point.

    That means that the dew point temperature is always less than the air temperature. Also, air temperatures can drop no lower than the dew point. This helps meteorologists predict temperature lows in a weather forecast.

  • A very high dew point can indicate severe weather. A high dew point means that the air is unstable and that thunderstorms may develop.
  • The dew point is a reflection of the humidity.

So, to give you some examples, a very dry place with low humidity, like the desert I live in, would have very different dew points than a humid place with high humidity, like Florida.

If it was 90 degrees in the desert, a typical dew point would be less than 50 degrees. If it was 90 degrees in Florida, the typical dew point would be more like 70 or 80 degrees.In more humid places with higher water vapor content, it’s harder for the sun to heat the air. This means that there is less of a difference between high and low temperatures in a day than there is in dry environments. Also, warm air has the ability to hold more water than cold air.

This is why it’s more humid in the summertime.

Dew Point Calculation

You can physically determine the dew point temperature with a device called a hygrometer. For it to work, a smooth, shiny surface, like a mirror, is cooled until water vapor in the air begins to condense on it. When that happens, you have the dew point temperature.The humidity can be found by comparing the dew point with the temperature of the air. Because dew point reflects the relative humidity, you can also use an equation to determine it.

The equation looks like this:Relative Humidity = Mixing Ratio / Saturation Mixing Ratio x 100It may look complicated, but it’s really not. The top number, or mixing ratio, is the actual amount of water vapor in the air. The bottom number, or saturation mixing ratio, is just the amount of water vapor in the air when it’s saturated. That means it contains the maximum amount of water vapor that it can.The equation is pretty easy to figure out because the mixing ratio is the same for each degree of temperature.

There are charts that have been made that show you what the saturation mixing ratio is for each degree of temperature.

Graph of relative humidity


If you know the temperature, you find it on the bottom and match it to a value on the left column. This tells you the saturation mixing ratio.Here’s an example. If you know that the mixing ratio is 22.

3 and the saturation mixing ratio is 36.5, then the relative humidity is 61%, calculated as 22.3/36.5 x 100.

However, the equation can also be used to calculate the dew point. The dew point is the temperature when the air is completely saturated or the relative humidity is 100%. In the equation, for the relative humidity to be 100%, the mixing ratio would be equal to the saturation mixing ratio. So, if you know that the mixing ratio is 11.1 and you want to know the dew point, you know that the saturation mixing ratio is also 11.1.

Then, you can simply look at the chart and get the dew point temperature of 15.6 degrees C (60 degrees F).Really, the most important part about dew point and humidity is how it makes the weather feel to us. This is called sensible temperature.

If it’s hot and humid, it actually feels warmer than the thermometer says, and the sensible temperature is high. On a hot and dry day, the air actually feels cooler, and the sensible temperature is low. The opposite is true when it’s cold. The sensible temperature is low when it’s cold and humid, and it’s high when it’s cold and dry.

Lesson Summary

Humidity is a term that describes the amount of water vapor in the air.

When it’s expressed as a percentage, it’s called relative humidity. The temperature when the relative humidity is 100% is called the dew point, and it’s at this point that clouds and rain can develop.The dew point is an important aspect of weather forecasts. It indicates when the weather will stop being sunny, and when there may possibly be stormy whether.

Because the temperature cannot drop below the dew point, it helps predict low temperatures in a forecast.Dew points vary a great deal based on how humid an area is. Dew points can be measured with an instrument called a hygrometer. Dew points can also be calculated with an equation for relative humidity that compares the actual water vapor in the air to the maximum water vapor the air can hold.

Humidity and dew points are important to us because the way it feels outside is affected by them. This is called sensible temperature, and it means that whether it’s humid or dry makes it feel a different temperature than the thermometer says.

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