Slide 1
Escape from the surface
Evaporation occurs when molecules leave a liquid for its vapour. It can happen at temperatures lower than boiling, at the liquid’s surface.
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A puddle can disappear on a cool day without ever boiling. Boiling additionally involves vapour bubbles forming within the liquid, while surface evaporation requires no such bubbles.
Slide 2
Escaping takes energy
Molecules need energy to overcome attractions in the liquid. Evaporation draws energy from the liquid and its surroundings.
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The remaining liquid can cool when faster-moving molecules preferentially escape. If heat arrives from the surroundings at the same rate, evaporation can continue without a large measured fall in liquid temperature.
Slide 3
The air can slow escape
Humid air contains more water vapour and reduces the net evaporation rate under otherwise similar conditions. Dry air allows faster net loss.
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Molecules also return from vapour to liquid. In a closed container, evaporation and condensation can reach equal rates; the liquid then maintains a dynamic balance rather than stopping molecular exchange.
Slide 4
Moving air changes the boundary
Airflow can carry vapour away from a wet surface. Replacing that humid boundary layer with drier air often speeds evaporation.
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Surface area also matters: spreading the same liquid into a thin layer exposes more of it to air. Temperature, humidity and airflow remain important, so surface area alone cannot predict a drying time.
Slide 5
Water carries energy elsewhere
Water vapour transports energy away from an evaporating surface. Condensation later releases energy where the vapour becomes liquid again.
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This phase-change transport links distant parts of the water cycle. Energy taken up at a wet surface can be released higher in the atmosphere, contributing to the energy exchanges involved in weather.
Sources
- Evaporation and the Water Cycle
US Geological Survey
- The Transfer of Heat Energy
NOAA