How can water evaporate

[Yai]

Lakes, the sea and rivers apparently evaporate and create clouds which then rain, but if water boils and turns into steam at 100'c, and the sea is usually only about 8'c, how can it evaporate ?

[Ewood27]

I'm not a scientist, but in essence air can absorb a certain amount of water vapour: the warmer it is the more it can absorb. You've probably heard of "relative humidity" which is a measure of how much it has absorbed against what it could. If the air blowing across a lake has room for more water vapour it will pick some up from the water surface. Just think how a wet road dries off after rain. Boiling is simply forcing water into a state of vapour more violently and visibly.

[j2buttonsw]

The is a dynamic equilibrium between the water molecules in the liquid and gaseous state. Heating the liquid gives more water molecules in the liquid state enough energy to become molecules in the gaseous state (to overcome hydrogen bonding etc).

[anthomson]

Water molecules can obtain enough energy to become gaseous at much lower temperatures than boiling point. The higher the temperature the greater the number of molecules will escape. The significance of boiling point is that at this point the temperature of the liquid levels-off regardless of the amount of heat being applied. Even when you reach this point you still have to continue applying energy to get all of the molecules to escape.

[bernardo]

To put it another way - the water molecules at the top surface of the sea (or pond, or puddle) are able to whizz off into the air (i.e. evaporate) (i.e. dry up) because they are moving fast enough to do so. The water further down does not escape because it is being squashed (held in) by the water at the top. Boiling point (100 degrees C) is the temperature at which the molecules in the water whizz about so fast that they are able to escape despite the weight of the water on top of them. If a puddle of rain water on the pavement is at normal temperature, the top layer will whizz away (dry up) continuously (slowly) until the whole puddle has dried up. If the water in a kettle is heated to 100 degrees C, it will be so hot that the water at the bottom will be able to escape (form bubbles) which are strong enough to resisit being squashed (kept in) by the water at the top.