Scaffolded notes
Build the idea
Humidity describes water vapor
Relative humidity compares the amount of water vapor present with the maximum amount possible at that temperature. Warm air can be associated with a larger maximum amount than cool air. Relative humidity can rise because vapor is added or because air cools.
Dew point is a moisture clue
The dew point is the temperature to which air must cool to become saturated, assuming its water-vapor amount stays constant. When air temperature and dew point are close, relative humidity is high and condensation is more likely.
Clouds require cooling and particles
Air commonly rises and expands as surrounding pressure decreases. Expansion cools the air. At saturation, water vapor condenses on tiny particles called condensation nuclei, forming cloud droplets or ice crystals.
Stop and explain
Check your thinking
1. Which is more humid: 24 C air with a 23 C dew point or 24 C air with a 10 C dew point?
Possible response: The 23 C dew-point case; temperature and dew point are closer.
2. Why does rising air often cool?
Possible response: Lower surrounding pressure allows it to expand, using energy and lowering temperature.
Interactive challenge
Try the science
Which change most directly raises relative humidity if water vapor stays constant?
Before you leave
Exit ticket
List the steps from rising moist air to a visible cloud.