365 Days of Climate Awareness 34 - Thunderstorms

A thunderstorm is an intense disturbance in the atmosphere, where a strong upward current of warm, moist air produces a stack of clouds called "cumulonimbus" which can reach into the stratosphere, to a height of 20 km (12 mi, 63,300 ft). The condensation from this updraft leads to a very powerful downward convection, forming rain and enough electrical charge to create lightning and thunder.

A number of factors can cause a warm updraft, but the most common is when a large parcel of cold air encounters a parcel of warm, moist air. The boundary between these two air masses, with very different temperatures and humidities, is called a front. (Another common method is topographic, where a mountain takes the place of the cold air mass, and forces the warm air up.) The colder, denser air (or mountain) passes underneath the warmer, more moist, less dense air.

Formation and structure of a thunderstorm.

As the warm air is forced upward, it loses pressure. Per the ideal gas law, as the air loses pressure, it cools. It loses some of its capacity to store water vapor, some of which condenses into droplets, and releases its heat into the air. The heat released by the condensed water warms the air a bit and continues moving the air upward.

This is known as wet adiabatic cooling. The term "adiabatic" is from Greek. "Dia-batic" refers to crossing over, and refers to heat. "A-dia-batic" means no heat is gained or lost from the parcel of air. The lower temperature is solely a function of change in pressure, not heat loss.

Wet diabatic cooling is slower than dry diabatic cooling, because water vapor condenses and adds heat to the air. This mitigates the cooling as the air continues to rise. Very warm air with a large amount of vapor will rise quickly, and generate a lot of condensation. But the higher the air current rises, the colder the surrounding air. A strong enough updraft with enough moisture will generate not only rain droplets, but frozen rain: hail.

When the updraft has exhausted its heat energy, and is no warmer than the surrounding air, the storm stops building, and has reached maturity. Now the cooled air and condensation begins falling back to earth, creating a powerful downdraft in the mature storm phase. In this phase intense rain and wind, and sometimes hail, occur on the ground.

One mystery about these storms is how the tremendous electrical charges are built up which lead to lightning. The prevailing theory is that the continually colliding particles of water in the air exchange electrons, and the lighter, positively charged ice crystals move upward in the cloud, while the heavier, wetter, negatively charged particles move toward the bottom.

So the storm cloud is polarized, with a positively charged (lacking electrons) top and a negatively charged (with an excess of electrons) bottom. The negatively charged raindrops fall to earth and imbue their charge to the ground. Eventually the huge disparity in charge, which can reach 120,000 volts, equalizes itself, delivering a current of up to 200,000 amperes. When a lightning bolt forms, and electrons pass from the ground up to the cloud and create a superheated path as they travel.

Earlier theory held that the superheating of the air by lightning left a vacuum, whose collapse caused thunder. Current theory holds that the superheating of the air creates a shock wave which itself is the thunder.

Most of us have heard the old story about counting seconds from the lightning strike until the thunderclap to determine the distance of the strike. Sound moves at about 1100 ft/sec in air, so every five seconds the thunder travels a little over a mile. So, once you've counted the seconds between the flash of lightning and the thunderclap, divide the number of seconds by five to get a rough count of miles.

Tomorrow: air and water masses.

Be well!