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Weather Lab 6-8 / Week 05 / Thursday
4/6
Week 05 · Water in the Air

Thursday

Reading dew point and humidity
// Evaporation, condensation and the dew point
⏱ about 20 min

Thursday: Reading Dew Point and Humidity

Raven reads two weather reports off the lab screen. "Day one: temperature 30, dew point 30, relative humidity 100 percent."

"Day two: temperature 80, dew point 60, relative humidity 50 percent."

Rocket frowns. "So day one is the humid one. It says 100 percent!"

Raven taps the second line. "What do you notice about the dew points? Which day has more water in the air?"

Nova brightens slightly. "Think about what the dew point measures," she says, "and what relative humidity compares."

"Day two has the higher dew point," Rocket says. "So day two has more moisture, even though its percent is lower?"

Raven nods. "Percent can fool you. The dew point tells the truth."

Two ways to describe moisture

The National Weather Service defines dew point as the temperature air must be cooled to in order to reach saturation.

A higher dew point means more moisture is in the air. It is a direct measure of how much water vapor is present.

Relative humidity is a percent. It compares the moisture present with the most the air could hold at that temperature.

Warm air can hold more vapor than cold air. So relative humidity depends on both the moisture and the temperature.

That is why relative humidity by itself does not tell you how much water is in the air.

Two days from a weather office

The National Weather Service office in La Crosse, Wisconsin, explains dew point with this comparison. The numbers are in degrees Fahrenheit.

DayTemperatureDew pointRelative humidityHow it feels
One3030100 percentCold, and not muggy
Two806050 percentMuch more humid and muggy

Day one has 100 percent relative humidity, but the air is cold and holds little vapor. Its dew point is only 30.

Day two has only 50 percent relative humidity, but its dew point is 60. There is far more water vapor in that air.

The weather office's advice: to judge how dry or humid it will feel, look at the dew point. The higher the dew point, the muggier it feels.

READ THE TABLE
  • Read the question.
  • Tap your answer.
Which day has more water vapor in the air?
Which day feels muggier?
Why can relative humidity be misleading?
On day one, the temperature equals the dew point. What is the relative humidity?
StatementTrue or false?
A higher dew point means more moisture in the air.?
Relative humidity is measured in degrees.?
Warm air can hold more water vapor than cold air.?
100 percent relative humidity always means a muggy day.?
Dew point is a temperature.?
WHY THIS EXERCISEReading these two numbers correctly lets you judge a forecast the way a meteorologist does.
When the air temperature falls to the dew point, relative humidity is ____ percent. Type a number.
To judge how muggy it will feel, look at the ____ point.
On day two, what was the dew point? Type a number.
WHY THIS EXERCISEPulling one value from a table is a skill you will use for every real data set.
HOW THE ICY GLASS CONNECTS TO THE DEW POINT
  • ?Warm room air holds some water vapor.
  • ?The air touches the icy glass and cools.
  • ?The air reaches its dew point and becomes saturated.
  • ?Extra vapor condenses into drops on the glass.
WHY THIS EXERCISEYesterday's lab and today's numbers describe the same event.
Draw two thermometers side by side for day one and day two. Mark the temperature and the dew point on each, and shade the gap between them.
Try it
Find today's temperature and dew point for your area in a weather report, with a grown-up.
Are they close together or far apart? Predict whether tonight will bring dew, and check in the morning.

Excellent reading. Tomorrow you will find dew point and condensation all around your home.

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