what is humidity and pressure with regard to weather?

Question

i often hear these terms and never understood how they influenced weather or the effects they have inside the home

Answer 1

Pressure is the pressure that a column of air exerts on a point on earth. Since it is measured with a barometer, it is often referred to as barometric pressure (BP). BP is generally less over warm areas, since warm air rises, and greater over cool areas. Also, as alititude increases, BP goes down according to Boyles' Law.
Standard BP is 29.92 inches of mercury (inHg) or 1013.2 millibars (mb) or 14.9 psi at 60°F / 15°C at sea level. It will decrease by about .1 inHg or 100 mb for each 100 feet increase in elevation or 3°F / 2°C decrease in temperature.


As BP decreases, air cannot hold as much evaporated moisture, so it generally becomes less stable. In other words, reduced BP usually means a greater chance of rain or snow. Also, something called the Coriolus Effect causes rising air to spin, which results in cyclonic activity. Strong enough and fast enough spinning causes a very strong cyclone, which is also known as a tornado or a hurricane.

Humidity refers to the amount of evaporated water vapor in the air. The more humid, the hotter (or colder) the air feels to bare skin. Warmer air can hold more evaporated water, so that the term "relative humidity" is usually used. When the air is fully saturated for the given temperature, the relative humidity is said to be 100%. Since warm air holds more water, there is actually more evaporated water by weight in a cubic foot of warm air than in a cubic foot of cold air. Because of this, warm wet air (like over the tropics) has much more mass and hence much more energy that cool air or warm dry air. That is one reason there are so many more severe storms in the tropics than there are over more temperate areas.

Hope this helps!

Answer 2

Humid air does not have more mass than dry air. Humid air is less dense than dry air as water vapour is less dense than air. Air does not "hold water vapour" any more than it "holds" oxygen or carbon dioxide. The Bad Meteorology site gives an excellent explanation of this. You will find it here:
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html

Air pressure is the weight of air pressing down on anything on the earth. There are bands of different pressures around the globe. Warm, moist, less dense air rises near the equator and moves away towards the poles high in the troposphere. As air is moving away there is less air above any point on the surface and so the pressure falls. The air blowing away from the equator sinks back to earth at about 30°S or N. As air is being added, there is more air over a point on the surface and so the pressure rises. There is another belt of low pressure about 50°S and N and another area of high pressure at the poles.

Air blows from higher to lower pressures at the surface so air moves into a low and out from a high. The air in a low is rising so if there is sufficient moisture, clouds will form. Clouds will not form in descending air so there is little cloud associated with high pressure systems.