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Weather Lab 6-8 / Week 05 / Wednesday
3/6
Week 05 · Water in the Air

Wednesday

Station Lab: Water from thin air
// Evaporation, condensation and the dew point
⏱ about 20 min

Wednesday: Station Lab: Water From Thin Air

Rocket lines up two glasses on the Station Rooftop table. One holds ice and cold water. The other holds plain water from the tap.

"Same room, same air," he says. "If the air is full of water, both glasses should sweat."

Raven starts a timer. "What do you notice after five minutes?"

Rocket leans in. "The icy one is already fogging up. The other one is dry."

Nova hovers between them. "Same air touched both glasses," she says. "What was different about the air that touched the cold one?"

"It got cold," Rocket says slowly. "Cold enough to let the water out."

Raven writes a new word at the top of the data sheet: dew point.

Your mission

You will chill one glass with ice and leave another at room temperature. Then you will watch which one collects water from the air, and how much.

This is a fair test. The only thing that changes between the two glasses is the temperature.

The National Weather Service uses this same demonstration in its JetStream lessons.

  • Two matching glasses, jars or empty cans
  • Ice cubes (crushed ice works faster)
  • Cold water and room temperature water
  • A timer or clock
  • A paper towel
  • Your station log
Safety first
Nothing is heated in this lab. No kettle, no stove, no hot water.
A grown-up handles the ice tray and anything sharp, such as an opened can edge.
Nothing is tasted. The water in the glasses stays in the glasses.
Wipe up drips so the floor does not get slippery.
  1. Dry both glasses with the paper towel and set them side by side.
  2. Fill the first glass with ice, then add cold water.
  3. Fill the second glass with room temperature water, no ice.
  4. Start the timer. Look at the outside of both glasses every five minutes.
  5. Each time, write "dry", "fogged" or "drops running" for each glass in the data table.
  6. After 30 minutes, touch the outside of each glass. Which is wet?
  7. Pour the water down the drain, dry the glasses and put them away.
TimeIcy glassRoom temperature glassRoom feels (dry, normal, humid)
5 minutes
10 minutes
20 minutes
30 minutes
PREDICT BEFORE YOU LOOK
  • Read the question.
  • Tap your answer.
Which glass do you predict will collect drops on the outside?
Where will the water on the outside of the glass come from?
What is the one thing that changes between the two glasses?

What the glass shows

Air touching the icy glass cools below its dew point. The water vapor in that air condenses onto the glass as liquid drops.

The dew point is the temperature air must cool to before it is saturated and cannot hold any more vapor.

The room temperature glass does not cool the air, so nothing condenses on it.

How much water collects depends on the humidity. The more water vapor the air holds, the more can condense.

On a humid day, the icy glass drips. On a very dry day, it may only fog lightly.

What the lab showsTrue or false?
The water on the outside of the icy glass came from the air.?
Both glasses collected the same amount of water.?
Air touching the icy glass cooled below its dew point.?
More humid air leaves more water on the glass.?
WHY THIS EXERCISEThe cold glass is a tiny model of a cloud: cooled air gives up its vapor as liquid drops.
The temperature at which air becomes saturated is the ____ point.
In a fair test, how many things change between the two glasses? Type a number.
What is the change of state that put water on the icy glass? Type one word.
WHY THIS EXERCISENaming the change connects your lab to the water cycle model.
Draw both glasses after 30 minutes. Show the drops on one and label where the water came from.

Great lab work. Tomorrow you will read real dew point numbers from a National Weather Service office.

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