Yes - If the food is uncovered the surface loses heat to the cool air inside the microwave oven. So you always want to cover food unless you want it to be browned or crisp. Be sure to cover it loosely so that you don't develop steam inside thecovered dish that can burn you when you open it.
If you cover food with something like plastic, which is watertight, then the food will retain more moisture. However, you have to be careful doing this. Food cooked in a microwave does generate steam if cooked long enough. That steam will escape, so you may need to make some small "relief" holes in a plastic covering. Otherwise the pressure may simply force open the plastic and you're back where you started.
Microwaves penetrate food to a depth of roughly 1" (on all sides). This is a property of this sort of energy that cannot easily be changed. Anything larger than this is going to be unevenly heated. If the food can be stirred, stir it about halfway through. If not, try cooking it longer but at a lower power setting, and/or letting it rest for a minute or so after cooking, so the heat can evenly distribute through the food.
You can use pierced clingfilm (not in contact with the food). You can also use a microwave safe plate or casserole lid.
The warmer the air, the more moisture it can hold. This is an exponential relationship.
Warm Air
the less moisture it can hold
Trees hold moisture that moisture evaporates forming clouds
to hold moisture
To absorb the moisture and hold it in.
relative humidity is the amount of moisture that air can hold in a certain temperature
A bottle
north and south
to hold in moisture
Relative humidity is a measure of how much moisture is in the air relative to how much the air can hold. All the matters in the case of the latter is the air temperature. So if you cool the air temperature, the air can hold less moisture. Therefore, the air now has a larger percentage of moisture relative to what it can hold.
relative humidity