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Storms strengthen by a combination of several factors.
1) Abundant moisture
Moisture in the air is actually a measurement of how much H2O (water) is in the air. During the winter, under colder temperatures, the air can only hold about 1% H2O (99% Nitrogen, Oxygen etc...). But, in the summer months, under warmer (hotter) conditions, the air can hold as much as 4% H2O (or about 400% more water than in the winter). Moisture is in the form of gas (H2O in gas state), liquid (water we drink)and frozen (snow & ice). Storms (thunderstorms in the summer) have a lot more water to drop to the ground. Water as H2O releases heat when it goes from gas state to liquid or frozen. So, when the air rises in the summer thunderstorm, even though it gets colder the higher up it goes, the air actually is warmer than its surroundings because of the release of latent heat (H2O going from gas to liquid state). This is important, because it is a primary reason a thunderstorm generates so much energy in the form of lightning, microburst winds, heavy flash flooding rains and even tornadoes.
I discussed the importance of warmer air in the summer time. On the hotter days, the sun heats the ground and the air becomes buoyant or less dense than its surround air (depending on where the sun is the strongest). This air rises until the water in the gas state turns to liquid...then the release of latent heat gives the air parcel an added kick upwards. The higher the air goes, the greater the concentration of moisture and the more likely the droplets that form (when the moisture goes from gas to liquid) bounce into each other and make larger drops. High up (above 15,000 to 18,000 feet) the air is below freezing year round. Here we can have super cooled air (way below freezing) and snow. H2O here might go from gas to frozen and form snow flakes. These flakes bump into each other and make bigger flakes, or they might touch drops of rain and melt. Sometimes, the drops freeze into ice and the ice goes up and touches super cooled droplets and the ice gets bigger and bigger. If there is enough storm energy, the ice may fall and rise again to get even larger (forming hail). Once enough rain, snow and hail forms high up in the thunderstorm, they become heavier than the rising air and they begin to fall to the ground. Friction will cause the surrounding air to fall with the precipitation and as it reaches closer to the ground, it may come in the form of a microburst wind.
3) Electric Charges
Also, some drops are positively charged, while the ground may be negatively charged...and like walking on carpet and touching something metallic and you get a small shock...the atmosphere becomes significantly charged in opposite sides and as it equalizes, lightning may form going from ground to cloud, cloud to ground, cloud to cloud or cloud to air. The energy in one average thunderstorm could be harnessed to run Salt Lake City for 9 hours. There are 2,000 thunderstorms forming at any one time around Earth...making about 48,000 of these thunderstorms per day. Can you imagine how much energy we could gather from Earth to use in our power grid if we could harness just a small percentage of the energy from a thunderstorm?
Ultimately, it is the sun that produces the energy to make thunderstorms, or winter storms, hurricanes and other weather related storms. Without the sun, this planet would be a frozen wasteland. From the sun, the chemical reactions have resulted in air that is gravitationally held close to the planet's surface. The air contains Nitrogen, Oxygen, Water and other compounds. As the Earth turns, the friction of this air over the spinning earth causes the air to move. The different amounts of energy from the sun hitting strongest at the Equator and much less at the Poles, causes a differential in heat that is significant. This temperature difference pushes the air from Equator to the Poles and from the Poles back to the Equator. This air movement, the amount of heat from the sun hitting the oceans and land, causes significant differences in how storms form and how they move. A Meteorologist uses all of the information available to predict these movements and formations of storms.
Dan Pope AMS Certified Broadcast Meteorologist KSL-TV