Background
The greenhouse effect
It used to be thought that a greenhouse worked because the shorter wavelengths of electromagnetic radiation could pass through the glass and the longer wavelengths could not. In this model, light in the visible and near infrared parts of the spectrum stream into the greenhouse from the sun and heat up the soil, whereas the soil re-radiates in the far infrared because the soil is not as hot as the Sun; this radiation cannot pass through the glass.
This model of a greenhouse is now thought to be over-complicated. Nevertheless, the term 'greenhouse effect' has been adopted to describe the mechanism by which the Earth's atmosphere is thought to be getting warmer. Carbon dioxide, and other 'greenhouse gases' such as methane, are building up in the atmosphere. These gases play the same role as the glass in the greenhouse described above. They let through the shorter wavelengths from the very hot Sun, but block the longer wavelengths re-radiated by the Earth and stop them from escaping. This causes the Earth to warm up.
Interestingly, the current thinking about how an actual greenhouse works is far simpler than the 'greenhouse effect'. The sun's rays heat the soil and other surfaces in the greenhouse. The air next to the soil is heated and rises; it is replaced by cool air which is itself heated. A convection current is set up; the air circulates and is continually reheated by the warm soil. Thus the air temperature rises. Cooling by evaporation
There are two ways of thinking about why a liquid cools when there is evaporation from its surface. The first is based on the idea that temperature relates to the average kinetic energy of molecules. The second uses the concept of binding energies.
The average kinetic energy of the molecules in a liquid is related to the liquid's temperature. The higher the average kinetic energy, the higher the temperature. Within a liquid, the molecules have a range of kinetic energies - some above average and some below. At the surface of a liquid it is only the faster molecules (with higher than average kinetic energy) that break free of the surface. If there is no draught, they will tend to return into the liquid. However, if they are carried away on a draught, the average kinetic energy of the molecules that are left in the liquid is lower than before. This means that the temperature of the liquid falls.
By way of analogy, imagine a prison in which the prisoners have a range of heights (like the kinetic energies of the molecules). If the prison fence is 2 metres high, allowing anyone over 1.5 metres to jump over it, then all the tall prisoners will escape. The average height of the prison population will drop.
Binding energy is the energy needed to break a bond. The molecules in a liquid are only loosely bonded together, but energy is needed to break the bonds. This could be supplied by a heater used to boil the liquid. But if only a few molecules are to escape, their binding energy may be supplied by the remaining liquid or by the surface that they are in contact with (in the case of sweating, a person's skin). This means that energy is being transferred away from that surface. Therefore it feels cool - just as, if you touched a cold drink bottle, energy would be transferred from your hand to the bottle, making your hand feel cool.
