← Back to course
Weather Lab 6-8 / Week 05 / Station Day
6/6
Week 05 · Water in the Air

Station Day

Station Day: Dew hunt
// Evaporation, condensation and the dew point
⏱ about 20 min

Station Day: Dew Hunt

Saturday starts gray and damp at Station Rooftop. Beads of water sit on the railing, the rain gauge and the wind flag.

"Dew on everything," Rocket says, running a finger along the rail.

Raven checks the morning report. "Temperature 52, dew point 51. What do you notice about those two numbers?"

"They are almost the same," Rocket says. "So the air is right at saturation. That is why everything is wet."

Nova hovers above the gauge without touching it. "What do you think happens to the dew once the Sun climbs higher?" she asks.

"It evaporates," Raven and Rocket say together, and Raven adds a column to the log: dew gone by.

  1. Water vapor is an invisible gas, always present in the air.
  2. Evaporation turns liquid into vapor and takes energy. Condensation turns vapor into liquid, as on a cold can.
  3. The Sun's energy and gravity drive the water cycle: the Sun lifts water as vapor, gravity brings it down.
  4. The dew point is the temperature air must cool to before vapor condenses. A higher dew point means more moisture.
  5. Relative humidity is a percent that depends on temperature too. The dew point is the better guide to how humid it feels.
◇ FAMILY DEW HUNT
Early in the morning, walk around outside together and find every surface with dew: grass, a car roof, a railing, a leaf.
Count the dewy surfaces, then note which surfaces stayed dry. Were the dry ones under cover or warmer?
Check the morning temperature and dew point in a weather report and write both in the log.
Go back at midday and count again. Talk about where the dew went and what moved it.
For a grown-up
The dew hunt stays on the ground, on paths and in the yard. Nobody climbs for a reading.
This week's facts come from the National Weather Service JetStream school and the U.S. Geological Survey Water Science School, both at .gov sites.
Safety first
Dewy grass and steps are slippery. Walk, do not run.
Never take a reading during a thunderstorm or flood.
Nothing outdoors is tasted, including dew.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
The temperature is 52 and the dew point is 51. What does that tell you?
By midday the dew is gone. What happened to it?
Which surface is most likely to stay dry at dawn?
Station Day checkTrue or false?
Dew is condensation on surfaces that cooled overnight.?
Dew disappears at midday because it soaks into the air as vapor.?
A dew point of 51 means the air holds no water.?
WHY THIS EXERCISEThe dew on your station is the whole water cycle in miniature: condense by night, evaporate by day.
Dew that disappears in the morning Sun has gone through which change? Type one word.
WHY THIS EXERCISENaming the change of state is the heart of this week's standard.
Draw your yard or street at dawn with every dewy surface marked. Then draw it again at midday.
← Friday