Humid air is less dense than dry air and will consequently be less
effective at cooling a radiator.

Rain is a completely different story.

It's simple physical chemistry.

If you wish to be convinced I invite you to do a Google search for "air
density" or read the following article that was cut and pasted from the
USA article was found at:

        http://www.usatoday.com/weather/wdensity.htm

        Humidity and air density

                    Most people who haven't studied physics or chemistry
find it hard to believe
                    that humid air is lighter, or less dense, than dry
air. How can the air become
                    lighter if we add water vapor to it?

                    Scientists have known this for a long time. The
first was Isaac Newton, who
                    stated that humid air is less dense than dry air in
1717 in his book, Optics.
                    But, other scientists didn't generally understand
this until later in that century.

                    To see why humid air is less dense than dry air, we
need to turn to one of the
                    laws of nature the Italian physicist Amadeo Avogadro
discovered in the early
                    1800s. In simple terms, he found that a fixed volume
of gas, say one cubic
                    meter, at the same temperature and pressure, would
always have the same
                    number of molecules no matter what gas is in the
container. Most beginning
                    chemistry books explain how this works.

                    Imagine a cubic foot of perfectly dry air. It
contains about 78% nitrogen
                    molecules, which each have an atomic weight of 28.
Another 21% of the air is
                    oxygen, with each molecule having an atomic weight
of 32. The final one
                    percent is a mixture of other gases, which we won't
worry about. Molecules
                    are free to move in and out of our cubic foot of
air. What Avogadro
                    discovered leads us to conclude that if we added
water vapor molecules to
                    our cubic foot of air, some of the nitrogen and
oxygen molecules would leave
                    � remember, the total number of molecules in our
cubic foot of air stays the
                    same. The water molecules that replace nitrogen or
oxygen have an atomic
                    weight of 18. This is lighter than both nitrogen and
oxygen. In other words,
                    replacing nitrogen and oxygen with water vapor
decreases the weight of the
                    air in the cubic foot; that is, it's density
decreases.

                    Wait a minute, you might say, "I know water's
heavier than air." True, liquid
                    water is heavier, or more dense, than air. But, the
water that makes the air
                    humid isn't liquid. It's water vapor, which is a gas
that is lighter than nitrogen
                    or oxygen.

                    Compared to the differences made by temperature and
air pressure, humidity
                    has a small effect on the air's density. But, humid
air is lighter than dry air at
                    the same temperature and pressure.