Rocket walks out of the cool lab into a warm, humid morning and his glasses fog over instantly.
"I am blind!" he laughs, wiping them on his shirt.
Raven points at the windows of a parked car, gray with fog on the inside. "What do you notice? Same thing, bigger scale."
Nova hovers at Rocket's eye level. "What was cold, and what was the warm, moist air touching?" she asks.
"My glasses were cold from the lab," Rocket says. "The warm air hit them and condensed."
Raven bends to look at the grass. Every blade carries a bead of water. "Dew," she says. "The ground cooled all night."
"So condensation happens everywhere," Rocket says. "Not just on my can."
The U.S. Geological Survey lists everyday condensation. Glasses fog when you step from a cool room into humid air. Water coats the inside of a window on a cold day.
Car windows fog on the inside after people have been breathing out moist air.
Ground fog is condensation too. Fog forms when humid air meets a colder surface, often the ground, and cools to its dew point.
Unlike clouds higher up, fog does not need rising air. It can form on a still night as the ground cools.
Evaporation removes heat, which is why it cools things. Wiping your arm with a quickly evaporating liquid makes it feel cold.
In hot, dry climates, an evaporative cooler can lower the air temperature by 20 degrees Fahrenheit while raising the humidity.
It works by pulling dry outdoor air through a wet pad. The water evaporates, and the air comes out cooler.
Evaporative coolers work best in dry regions. In humid regions, the air already holds so much vapor that little more can evaporate.
| Everyday sight | Change of state | What cooled or warmed |
|---|---|---|
| Fogged glasses | Condensation | Cold lenses met warm, moist air |
| Dew on grass | Condensation | The ground cooled overnight to the dew point |
| Fog in a valley at dawn | Condensation | Humid air cooled on the cold ground |
| Wet hair feels cold | Evaporation | Evaporating water took heat from your head |
| An evaporative cooler | Evaporation | Water in the pad evaporated and cooled the air |
| Week review | True or false? |
|---|---|
| Water vapor is invisible and always present in the air. | ? |
| Evaporation takes energy, and condensation releases it. | ? |
| The water cycle is driven by the Sun and gravity. | ? |
| A higher dew point means drier air. | ? |
| Fog needs rising air to form. | ? |
Strong week, weather watcher. Tomorrow is Station Day: a family dew hunt and a humidity log.