What does a thunderstorm need to form? It needs moisture, since moisture in the air is the atmosphere's fuel. (Remember the calculation from last week?) It needs instability because the warmer the air is in contrast with the environment, the more moisture the air can hold and the more explosively it can release that energy. Thunderstorms also need lift, so the parcel gets to the LFC.

How do thunderstorms form? They need a continuous supply of moisture, or when the air is lifted, a cloud can form, but it will quickly dissipate as it mixes with the drier, cooler air surrounding it. Instability can be enhanced by warmer surface temperatures, so thunderstorms form more often in the late afternoon/early evening. Instability can be even more enhanced by temperature inversions.

Stages of thunderstorm development:

Surges of warm, moist air rise upward, making a cloud taller and taller and taller, forming updrafts in the cloud. This stage is called the cumulus stage.As precipitation particles accumulate and start to fall, the falling matter cause a drag on the air, and a downdraft is created. When cooler air surrounding the cloud is mixed into it, it is pulled into and adds to the downdraft. This is called entrainment. Entrainment will intensify the speed of the downdraft because the air pulled in is cooler and drier (more dense) than the cloud's air.

The next stage is called the mature stage. Precipitation reaches the ground, and the downdraft spreads out on the ground. As updrafts reach the "warm"; base of the stratosphere, an anvil forms. This icy part of the cloud spreads out horizontally.

As the downdraft is strengthened by entrainment, more and more cooler air is pulled in from the air surrounding the cloud, leading to more condensation and falling of precipitation. The entrained air is also drier air, which will decrease the "fuel" available in the cloud. This is the dissipation stage.

That's the basics for a single thunderstorm cloud. BUT: there can be more than one downdraft in a cloud, there can be more than one updraft in a cloud, and there can be more than one cloud in a thunderstorm, and these clouds can be organized!

Kinds of organized thunderstorm clouds ("multi-cell" thunderstorms)

squall line. These are typically 500 km long, 100 km wide. They often form near the cold front of an extra-tropical cyclone. They last 2 hours to 4 days, usually on the order of 10 hours. The description of this kind of storm organization is based on radar echoes.

MCC (mesoscale convective complex). These are nearly circular/oval-ish masses of clouds, and the organization can produce storms for about 12 hours. The maximum size of an MCC is usually around midnight, and they usually dissipate around sunrise. MCCs account for a lot of rain in the upper Midwest/plains in the summer. They form over the Rockies or on plains near the Rockies. The description of MCCs is based on satellite imagery.

Stuff that can happen in thunderstorms:

lightning. Each flash you see is made of several strokes. (That's why lightning appears to flicker.) Let's consider the processes occurring in one type of lightning, cloud to ground lightning. First, negative charges at the cloud's base attract positive charges at the ground below. Then a step leader charts a path from the cloud to the ground by ionizing the air in the path. A return stroke contains current moving the negative charge from the cloud to the ground. A dart leader then re-ionizes the path, followed by more return strokes, until the charge is drained from the region of cloud it's drawing the charge from.

thunder. Air is superheated by lightning (to 8000 to 33000 ^°C!) and expands REALLY fast. This expansion is so fast it produces sound waves. Thunder travels about 5 seconds per mile, so if you hear thunder 10 seconds after seeing a lightning flash, the lightning is about 2 miles away.

hail. Water molecules/precipitation particles can be lifted in the updraft to very cold (freezing) temperatures. They freeze, start falling, and can get coated with water. If the particles are picked up by the updraft again, they can be lifted again and re-frozen. This process can go on and on several times, with the hail getting new coats of water which then freeze, making the hail grow. When the hail is too heavy to be sustained in the air by the updraft, it falls to earth.

tornados. The pressure at the center of a tornado is very low, and air rushes towards the center, and swirls around and upward. As the air pulled into the vortex is cooled adiabatically, a funnel cloud will show once water vapor is condensed. The tornado darkens as it picks up debris. Tornado formation occurs most often/always when there are air masses with VERY contrasting qualities (temp., moisture content). A high instability is needed also. However, just because there may be a place with high instability and clashing air masses, a tornado may be LIKELY. There are so many environmental factors in tornado formation that we can only say that a tornado is likely in an area, not that it will definitely form.

****more links coming soon!****
the Storm Prediction Center
this page from the University of Illinois at Urbana-Champaign has a really snazzy way of showing lots of information!
all about lightning
the National Severe Storms Laboratory
how gust fronts can trigger thunderstorm formation
the Tornado Project
the online tornado museum