Before we get started – we need to remember two principles from Physics 101:
1. Warm/moist air is less dense than cool/dry air (always true)
2. ’Usually’ the air at the surface is warmer and gets cooler the further you go up in elevation (this isn’t always the case in the atmosphere and will be important in future discussions)
KEY MESSAGE: Warm/moist air will rise if it is less dense than the cooler air above it. Think: hot air balloon.
Thunderstorm Ingredients
1. Moisture – Warmer / moist air is less dense and will be easier to rise
When moist air rises (just like a hot steamy shower) it will eventually reach a point of cooler air above it and condense. These are clouds. If there is enough moisture, the air can condense into clouds but keep rising until it reaches a stable point in the atmosphere. A lot of our moist air comes from the Gulf of Mexico, so winds from the south will aid in good moisture “return” feeding more warm/moist air into the environment.
How it’s measured: Moisture is measured by dew point temperature as well, as relatively humidity, at various levels of the atmosphere).
2. Instability – Amount of potential ‘buoyancy’ the atmosphere has for air to rise up freely
If a chunk of warm/moist air was lifted above it’s current location, it would continue to rise on its own as long as it was warmer than the surrounding environment. Again, think of a hot air balloon… with the flame heating the air, it rises. If you stop heating, once the air in the balloon cools to match the surrounding air temperature, the balloon will stall and stop rising, and eventually sink. Another analogy is to think of submerging a ball in a pool. Air is less dense than the pool water, so when released the ball rises on it’s own until it reaches the surface where it matches the density of air above the pool.
How it’s measured: Instability is measured at different points in the atmosphere and can help determine where the greatest/fastest rise of air might occur.
CAPE (Collective Available Potential Energy) – is the difference between the what the temperature of forced risen air ‘should be’ as it cools vs. the actual measurement of temperature as you go up in the atmosphere.
CIN (Convective Inhibition) is the opposite, a measure of ‘stability’. This is negative buoyancy and will prevent prevent air from rising.
“Cap” – is a term used to explain a pocket of warmer air aloft (an “inversion” from ‘usual’) that prevents air below from rising. A little Cap can be good for building up pressure, but a lot of Cap will prevent air from rising altogether. The Cap can be “broken” a couple of different ways, but the most common is by daytime heating the surface up even warmer than that pocket of air..and then everything above it is cool again.
3. Lift – A forcing mechanism that can gets things started by lifting air
These can be a number of events, but the most common are fronts (cold or warm) and boundaries (different air masses moving – e.g. dry → moist). Think of a cold front as a sharp wedge lifting the warmer air up out ahead of it. A warm front can accomplish the same thing, but less dramatic because of the more shallow slope.
How it’s measured: While there are ways to measure these, they aren’t as important for basic understanding. Just knowing they are there is a good start. For example, seeing a cold front advance, and the timing for it, will covers the basics. However, if you’re digging in more advanced, looking at “Vorticity”, “Vertical Velocity”, “Warm Air Advection”, “Cold Air Advection”, “Frontogenesis”, etc. are all ways to see where these forcing fronts/boundaries and how they are affecting the atmosphere and how.
4. Wind Shear – Difference in wind speed/direction as you go up in elevation
Wind Shear plays an important role on Storm Mode (type of storm). Thunderstorms can happen without wind shear, but severe thunderstorms require wind shear for a number of reasons. For example, the differences in winds at elevations help tilt/curve the storm so the condensing air (rain) doesn’t fall on itself and smother the rising air (updraft). This allows the thunderstorm updraft to sustain itself for longer and for rotation of air coming in to develop a mesocyclone (and potentially tornadogenesis). The best wind shear for supercells usually starts out southerly at the surface (also aids moisture “return”) and then turns clockwise (’veering’) as it goes up… resulting in westerly winds as you get mid-way through the atmosphere.
This is why classic right-moving supercells, typically take on a shape where upper levels fan out to the northeast/east (anvil). It also sustains the updraft, so the risk of hail goes up…as well as the potential for tornadoes. If winds did not curve with height, isolated supercells would not be able to form. Instead, scattered or linear storms (e.g. bow echo) might form. These still have the potential for severe winds, but the risk of hail goes down. And, the risk of tornadoes goes way down.
How it’s measured: There are a couple parameters that help us understand wind shear. More important parameters to pay attention to are “Bulk Shear” (difference in winds) and “SRH [Storm Relative Helicity]” (difference in wind but also factoring how likely that wind is to cause cyclonic rotation). Both are measured for different ranges of the atmosphere e.g. 0-1km, 0-3km, 0-6km which play different roles in storm.