Thunderstorms types review
Three requirements
- Conditionally unstable environment
- Adequate low-level moisture for air to be saturated
- Trigger (surface heating, lifting by terrain or cold front)
Single cell (also called "ordinary" or "air mass")
Occurs when there is very little vertical winds shear (difference in winds at different altitudes), go through life cycle in short period (~1 hour)
Cumulus stage
Grows vertically in gradual but rapid steps, saturating air up to greater heights, allowing updraft to reach higher
Latent heat of condensation provides more energy for air to rise
Precipitation particles grow but do not yet fall, no lightning/thunder
Entrainment of drier air from outside cloud occurs, causing some evaporation which cools air and leads to formation of denser and colder air up in cloud
Mature stage
Begins with the start of the downdraft - that colder, denser air starting to fall through the cloud
Precipitation begins falling (which also helps pull the downdraft toward the ground)
May take on anvil shape as ice crystals at top of cloud spread horizontally since the updraft does not penetrate up into stable stratosphere
Lightning and thunder at this point as storm is at most intense
When downdraft reaches surface this cold air below is called the cold pool, and it spreads horizontally across the surface with its leading edge (called the gust front) acting just like a mini-cold front with gusty winds and change in temperature
Dissipating stage
After about half an hour of the mature stage the storm begins to die off
Downdraft comes to dominate and eliminates the updraft from feeding the cloud
The cold pool has spread out beneath the cloud so that the updraft no longer enters the bottom of the cloud
Precipitation becomes very light and the cloud often precipitates and evaporates away until only the ice crystal anvil remains
Multicell
Occurs when the cold pool and gust front from one storm provide the lifting trigger to generate new storms
Severe thunderstorms
Unlike single cell these form where there is strong vertical wind shear
Shear has effect of keeping the downdraft from cutting off the updraft, often by tilting the storm so that it remain over the edge of the cold pool thus allowing it to continue to receive the rising warm, moist air that feeds it
Updraft can be very intense, even overshooting slightly into the stable air topping the troposphere
Hail grows large as ice particles have water droplets freeze onto them with the more intense updraft able to hold the hail particles in the cloud longer
The bigger they are, the harder they fall - stronger downdraft in severe thunderstorms, very strong winds at surface when downdraft comes to ground
Supercell thunderstorms
Single gigantic cell that forms with slightly weaker vertical wind shear than that most favorable for multicell
Rotation induced in supercell resulting in balanced updrafts and downdrafts that allow the storm to persist much longer than a single cell
Updrafts may reach 100mph, producing large hail, but precipitation occurs on the rear flank of the storm away from the updraft which is too great to allow precipitation to fall
Though not all do, this is the type that produces tornadoes