Thunderstorms form when there is instability and lift in the atmosphere. Wind shear helps sustain storms for longer periods of time.
Storms form when warm, moist air is lifted upwards towards the cooler portions of the mid and upper atmosphere. As air rises, the clouds you know as storm clouds form.
Instability, Explained
Instability.
We talk about it a lot. We talk about as if you know what it is. Sorry about that.
So let’s explain. First a definition from the officially official experts:
The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather. The greater the instability, the greater the potential for severe thunderstorms.
-NWS Glossary
The basics of instability is that warm air in the atmosphere tends to rise, so when especially cold air is above warm and humid air, expect instability.
What is Wind Shear?
We oftentimes talk about wind shear as if you already know what it is, but you may find yourself asking what is wind shear when you hear the term.
Simply put, wind shear is the change of direction and speed with winds with heigh within the atmosphere.
Phew.
Here’s the official definition from the NWS:
The rate at which wind velocity changes from point to point in a given direction (as, vertically). The shear can be speed shear (where speed changes between the two points, but not direction), direction shear (where direction changes between the two points, but not speed) or a combination of the two.
-NWS Glossary
So what about lift?
Severe storms need three main ingredients: wind shear, instability, and atmospheric lift.
Atmospheric lift is important to get air parcels in an unstable atmosphere to begin lifting upwards and eventually condensing into clouds. Lift is necessary to overcome a capping inversion on the most dynamic days as well.
Usually, lift is associated with an upper level storm system aloft and with surface boundaries where winds converge down low. You can achieve atmospheric lift in other ways as well.
Here’s a primer on the basics of how lift helps thunderstorms form.
Some Ways to Achieve Atmospheric Lift
Low Level Convergence: Occurs when winds and air masses converge along surface boundaries like drylines or fronts.
Warm Air Advection: When warmer and less dense air meets and rises above cooler and denser air, you can get enough lift for storms to form.
Orographic Lifting: This form of atmospheric lift is caused by winds meeting upward sloping terrain. Air meeting upwards sloping terrain is lifted upwards, which can create storms.
Upper Level Divergence: As winds change direction in the upper atmosphere a vacuum effect occurs. This causes air to lift upwards underneath.
Lift is Just One Ingredient, But Its Important
Chasers often overlook lift as an important ingredient to forecast thunderstorm formation.
Often, chasers sit out under a blazing sun wondering why there aren’t cumulus clouds bubbling upwards. Sometimes, despite big model numbers, no thunderstorms form.
Conditional instability is the problem. An atmosphere with no way to lift air parcels through a capping inversion isn’t actually that unstable for your practical usage as a storm chaser.
Thus, it is important to always remind those who chase that you need lift to create storms.
Adequate atmospheric lift often comes from surface convergence down low and some form of divergence up high on storm days.
The balance of too much or too little lift along with capping is delicate. Often, this balance makes or breaks chase days.
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