What Is Dew Point and How Does It Affect the Weather? Your Questions Answered!

Fact checked by Megan Soliman MD, MSc
Board-certified physician at Saba University School of Medicine

Have you ever wondered what ‘dew point’ means in a weather forecast? We’ve asked our experts to explain what a dew point is, how it’s connected to humidity, and how it affects weather.

What is the dew point in the weather?

Dew is the small droplets of condensation you see on the side of a cold glass of water on a hot day or on leaves and grass in the morning or evening. Dew forms when the temperature drops down enough for the water vapor in the atmosphere to condense. If it’s too cold outside, this condensation will appear as frost or ice. The temperature at which these water droplets or ice crystals appear is called the dew point.

In weather forecasting, the dew point is used to calculate relative humidity. It tells you how much water vapor is in the air and how humid it feels outside.

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How is the dew point calculated?

Dew point can be measured directly with a hygrometer, a device made up of a chamber with a mirror, light detector, and light beam. When the air in the chamber cools down to the dew point, tiny drops of water appear on the mirror and block the light. The light detector then senses that the light’s been blocked and indicates that the temperature has reached the dew point.

You can also calculate the dew point by knowing the temperature and the relative humidity.

Condensation on the window in humid weather with dew point

What is dew point temperature?

While it's partially true that you can tell how humid it is outside just by looking at the relative humidity level, it doesn’t tell you how comfortable it feels. You may feel a lot more hot and sweaty at 70 percent humidity than when the humidity level reaches 100 percent. Confusing, right?

That happens because the relative humidity shows the amount of moisture relative to the temperature but not the specific amount of moisture in the air. And this makes the dew point temperature much more accurate in telling you what it’s actually like outside.

A high dew point usually means humidity levels are also high, and it affects how comfortable we feel.

The high dew point can make it feel muggy or humid when it's hot outside. When it’s cold, the high dew point can make you feel even colder.

But what is the best dew point temperature? A dew point temperature less than 50 F shows that the air is dry, 50-68 F means it’s pretty comfortable, and anything above 69 F is considered very humid and uncomfortable.

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The way you experience the dew point depends on your metabolism and what weather conditions you’re used to. In general, in hot, humid weather, your body has a hard time cooling itself down. The sweat doesn’t evaporate from your skin as fast, so you may feel hotter for longer. In other words, the higher the dew point temperature, the longer it takes for your body to cool itself and the muggier it feels.

How does dew point affect weather?

Dew and condensation are a part of the natural water cycle. When dew point occurs at ground level, you see water droplets on plants and other surfaces. The same process occurs higher up in the atmosphere. As water evaporates, it combines with other vapor molecules and particles like dust, salt, and smoke. Then, when the temperature drops below the dew point level, the clouds are formed.

Water droplets on plants in dew point weather

Dew point also plays an essential role in the formation of fog. Just like clouds, fog is the result of condensation, but closer to the ground. When the temperature and dew point get within 2 or 3 F of each other, you may see fog appear. If the air temperature is below freezing, you may see ice fog — small ice crystals suspended in the air — that covers everything with a layer of frost.

Dew point in humidity: what causes humid weather?

As the sun heats the earth’s surface, water evaporates from bodies of water such as oceans, lakes, and rivers, releasing water vapor into the atmosphere.

Humidity tells us how much water vapor is in the air.

High humidity is often associated with precipitation, but 100 percent humidity doesn’t necessarily mean it’s raining — it means that the air has reached its maximum capacity for holding water molecules and can’t take any more.

In general, high humidity is more common during hot weather. The heat causes more water to evaporate, releasing more vapor into the air and increasing humidity levels.

Key takeaways

The dew point shows at what temperature water vapor in the air begins to condense. Dew point not only tells you how humid it is but how comfortable it feels outside. A high dew point usually means the humidity level is also high.

The higher the dew point, the more uncomfortable and sweaty you feel. When the dew point is high, your body is less effective in cooling itself down. So the risk of heat-related injuries, such as heat stroke and heat exhaustion, is higher too. Keep an eye on the dew point temperature and stay hydrated and avoid exercising when it’s getting higher.

August 10, 2022