Re: why is the rate of evaporation high in tropical rainforests

This is a bit of a false dichotomy. Humidity is the amount of water vapour (gaseous water) in a volume of air. There are several ways of measuring it. The one that we see most commonly is "relative humidity" which tells us the percent compared to 100% humidity for that temperature and pressure.

So, a humidity of 90% means that the air is 90% of the way towards being saturated. And at 100%, saturation occurs and it should result in "condensation". Note that condensation is not necessarily precipitation. A fog or mist can occur, which removes water vapour from the air and into the form of droplets, without it actually "raining".

The rate of evaporation, on the other hand, is a measure of the rate at which water vapour is added to the air. It is dependent, primarily, on the difference in temperature between the ground (or other water bearing construct such as leaves) and the air. It is possible to have water evapoarating from the ground at the same time that it is raining. The "net evaporation rate" - in this case - might be "negative". That is, the rain might be adding more water to the ground than is evaporating but the process of evaporation continues.

Maybe a good image is to think of a bathtub. The amount of water in the tub is the humidity. The amount that you turn on the tap is the rate of evaporation. The stronger the stream, the faster the tub fills. But the amount of water in the tub doesn't necessarily affect the amount coming out of the tap.

Add one more layer to this analogy. Most bathtubs are equipped with a safety overflow - a small opening that begins to drain the water if it gets to a certain height. The point is to prevent the tub from overflowing. Imagine, then, that this safety overflow represents the point at which we reack 100% humidity. The system will then lose water at the same rate it is gaining it. The tap is still on, though, but the difference between the rate the water enters via the tap and the rate it drains away through the safety overflow is our "net evaporation" rate.

Of course, the amount of vegetation in the rainforest assures that there is lots of moisture in the air - lots of constructs from which evaporation occurs. This means that the humidity reaches 100% on almost a daily basis (typically towards the late afternoon) and condensation happens (usually in the form of rainfall). Because of this drain, there is a daily cycle of rain and a high rainfall. A lot of the driving force, though, is the relative temperature of the air and the soil ensuring a high rate of evaporation.

As to whether or not it always rains in a rainforest only, well, I am sure that it has rained where you live recently. Do you live in a rain forest? But rain will occur in a rain forest almost daily whereas elsewhere, it depends on larger meteorological equilibria. The rain forest, because of the over arching and protective layer of the forest canopy, is a microcosm of the atmosphere on the whole. Hence, rainforest rain tends to stay in the rainforest.

Hope this helps.