High pressure systems inhibit cloud formation and therefore usually
bring sunny days. However, why is that high pressure usually brings
low humidity as well? Shouldn't the water vapor in the air stay closer
to the ground, therefore making it more humid?

Hello Douglascarey,

I could not find anything that was truly definitive (thus, this is a
comment, not an answer), but it appears there are a combination of
effects.

[1] High pressure has air dropping and spiraling out from the center.
[2] High pressure is also a result of less humidity (less water vapor
displacing N2 and O2 molecules)

So the air near the ground in a high pressure system will actually
have less humidity than the surrounding air - therefore it feels less
humid. At least that is how I read the sites that describe weather in
laymans terms.

  --Maniac

Maniac-ga has identified the important effect in his point #1, but
didn't really explain why air "air dropping and spiraling out" from
the center of a high pressure system has lower humidity.

Remember that humidity is normally expressed in terms of *relative*
humidity.  The relative humidity is the actual partial vapour pressure
of water to the saturation vapour pressure at that temperature.  The
partial pressure is directly proportional to the concentration of
waver vapor. The saturation vapor pressure is the maximum amount of
water a parcel of air can hold without having the water condense into
liquid water.  The saturation vapor pressure is a strongly increasing
function of temperature.  At 0 degrees C, the saturation partial
pressure is only about 0.0006 atmospheres.  At room temperature, it's
about 0.03 atmospheres, and at 100 C (the 1-atmosphere boiling point),
of course, it's equal to 1 atmosphere.

In the troposphere (the layer of the atmosphere nearest the surface of
the Earth, and where most "weather" occurs) the air temperature drops
by about 6.5 degrees C for every km gain in elevation.  As a parcel of
air falls from higher in the atmosphere toward the Earth's surface in
the center of a high pressure system, it warms, but the total amount
of water present in the parcel basically remains constant.  As it
warms, it's capacity to carry water vapor increases (i.e., the
saturation vapor pressure of water goes up), but because the total
amount of water is constant, the relative humidity decreases, making
for nice comfortable weather at the surface!

Rel. Humidity (%)= [Actual H2O partial pressure]/[Saturation H2O
partial pressure(function of T)] * 100

As the denominator increases due to increasing temperature, the ratio
decreases.

That completely explains it. Thank you very much to both Maniac and hfshaw.

Doug Carey