Why is sunshine so important for storm development?

Weather Blog

ROCHESTER, N.Y. (WROC) — Our sunniest months of the year are fast approaching in WNY, and with the increased sun means more warmth, and also the return of thunderstorms.

Sunshine bodes well for a nice, toasty afternoon in summer, but it can also be a key ingredient when talking about storm formation. Before we talk about why the sun is so important, let’s talk about all the ingredients that make up storms.

There are THREE key ingredients that are required for thunderstorm development:

Lift

A lifting mechanism can include a frontal boundary such as a cold or warm front, an approaching low pressure system, or a moisture front such as a dry line. Higher terrain such as hills or mountains can also help force air upwards to get storms going.

Instability

Instability is the air’s tendency to rise or sink. If a parcel, or bubble of air is pushed in one direction, it will keep going in that one direction if the air is unstable. This is good for getting clouds to form.

Picture This: If you were to take an inflated soccer ball or baseball and push it all the way down to the bottom of a swimming pool, it would immediately shoot back up as soon as you let it go. This is what an unstable atmosphere is like. When a parcel, or bubble of air gets warmer than its surrounding environment, that bubble of air starts to rise since warm air is less dense than cooler air.

Moisture

Having enough moisture present in the air is crucial for precipitation and for thunderstorm cloud development. High humidity with dew points in at least the mid 50s, 60s, and sometimes 70s is the typical criteria when temperatures are in the 60s, 70s, and 80s.

What about severe thunderstorms?

Severe storms have all the same ingredients as regular storms, but they have one extra ingredient that gives them that extra punch.

Shear

When we talk about shear in a thunderstorm, we mean wind shear. The wind field around a thunderstorm is important in determining how it strengthens into a severe storm. When you have winds that are changing both speed and direction with height, you can get the storm to strengthen as the updraft becomes tilted. When the updraft is tilted it becomes less likely for the storm to fall apart, and therefore it can strengthen in the process. Having a strong updraft (upward vertical motion in a storm) can create large hail, produce strong winds, and if there’s enough wind shear for the storm to rotate, a tornado can form.

You can remember these ingredients by remembering the acronym SLIM.

Where does sunshine fit in all of this?

Sunshine is an important fuel source for storms. When the sun is out it heats up the ground. When the ground warms, the air directly above it warms along with it. The less dense warm air rises as it becomes increasingly buoyant. You can measure this instability using Convective Available Potential Energy (CAPE). We sometimes refer to this as “storm fuel” when discussing the potential for storms or severe weather. The higher the CAPE, the more “energy” there is in the atmosphere. Pair that with enough moisture and lift, and you have yourself prime conditions for a storm.

Here’s an example of a sunny day versus a cloudy one across Western New York:

The first image would provide excellent conditions for storms to form if there was enough moisture present, and a frontal boundary was moving through that day. The second image would not be enough to form storms even if there was enough moisture and lift in the air, because the absence of the sun creates a cooler, more stable atmosphere that’s not so favorable for storms.

For information on how to prepare for strong to severe storms, check out the article below:

~Meteorologist Christine Gregory

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