A: In order to tackle this question, let's learn a little bit about the motion of air within the troposphere.
Typically, air is mainly composed of molecular nitrogen, N2, (78% of the atmosphere) and molecular oxygen, O2 (21% of the atmosphere). When air is more humid than normal (has more water vapor per unit volume), it is actually less dense than when there is less water vapor. The reason being is that the molecular weight of water vapor, H2O, is about 18, while the molecular weight of N2 is 28 and O2 is 32. More water vapor molecules in a given volume of air take the place of some of the N2 and O2 molecules and make a given volume of air less heavy (less dense). That’s why the moist air heads for the ceiling of your bathroom and is easily sucked out with your ceiling fan when you take a shower.
Now, a little bit about the atmosphere. Since air molecules have mass, they are acted upon by gravity and have weight. The majority of air molecules are clustered near the surface of the earth and become fewer and fewer the higher you go. We say that the air gets thinner (try taking a deep breath atop Mt. Everest), but really what is happening is that air pressure is lower at higher elevations. The pressure at sea-level is around 1000 mb (1013.2 mb is standard sea-level pressure) while the pressure at 18,000 feet is about half that (500 mb).
Now, let’s get back in the shower and follow that warm, moist air that got sucked out of your bathroom and exhausted out of the vent on your roof. If that warm, moist air is less dense than the surrounding air, it will continue to rise. As it does and moves into areas with fewer air molecules (lower pressure) the air parcel expands. Thermodynamically, since it takes work to expand, the air molecules use energy to do this work and this lowers the temperature – that is why rising air cools (and conversely, why sinking air warms). This is the same principle behind your cooling propane tank example. When air cools to the dew point temperature, net condensation of water vapor can occur. Generally when this occurs above the freezing level in clouds, water vapor condenses onto existing liquid cloud droplets as well as ice crystals, causing them to grow. This condensation also releases energy (latent heat) which can warm other parcels of air, making them less dense than their surroundings and allowing the thunderstorm to grow even higher into the atmosphere.
Despite the cooling of a rising parcel, it will continue to rise as long as it remains less dense than the surrounding environment. That is why such strong updrafts can occur on warm, humid days when there is cold air aloft. It’s like stacking large heavy boxes on top of little light boxes. It’s an unstable atmosphere that wants to overturn. Updrafts form that carry warm, moist air high into the atmosphere. Generally, an updraft speed of 24 to 34 mph is required to support the formation of hailstones.
The long and the short is that it is not the pressure drop directly that causes the formation of hailstones, but rather the cooling induced by the expansion of air parcels as they move into regions of lower pressure. -- Bob Swanson
This question was submitted by Richard Kraemer.
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