Kevin Myatt is The Roanoke Times' weather columnist.

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For a few weeks now, the local weather pattern has definitely been summerlike.

We've had warm to downright hot days with lots of humidity. Some days we've heard thunder, other days have been sunny.

We will have many similar hot and humid days this summer. Several thunderstorms will develop on one such afternoon. A few isolated ones will pop up on another. On yet another, hardly any clouds form, much less storms.

What is the difference between the thunderstorm activity on these nearly identical summer days?

While many different things could be at play, the most important is often the amount of "capping" high in the atmosphere.

For puffy cumulus clouds to rise upward into the mighty cumulonimbus towers that produce thunderstorms, warm, moist air near the surface has to be able to rise into cooler, drier air aloft. This is called "instability."

In summer, however, there is often a layer of air that develops high up that is as warm or even warmer than that at the surface.

This is called a "cap." When a cap is present, thunderstorm development is restrained.

In a situation with no cap or a weak cap, numerous thunderstorms may shoot up fairly early in the day. These are the kind of summer days when it seems that by 2 p.m. there is thunder in many different directions and off-and-on downpours.

A moderate cap can limit the afternoon's development to a few scattered storms, which may be quite intense as all the energy goes into a few storms rather than being spread out among many storms. Not everyone hears thunder, but some folks get heavy rain, hail and high winds.

A strong cap -- a thick layer of warm air aloft -- can prevent storms, or sometimes even clouds, from developing. When we are under strong, hot, high-pressure systems in the summer known as "heat domes," our air stagnates as hot air sits on top of hot air with little opportunity for the air at the surface to rise into thunderstorms.

But sometimes, even a strong cap can break if there is enough forcing at the surface. Forcing refers to any feature that physically lifts air upward.

Cold fronts are a common source of such forcing. A strong cold front plowing into a hot, humid air mass at the surface can sometimes shove that air upward right through a cap into the colder air above it. The result is often powerful storms.

The mountains themselves do some subtle forcing in our area, causing storms to occur on days when they wouldn't if our terrain was flat. Also, if there are thunderstorms one afternoon, those storms can send out cooler winds that act like miniature cold fronts. These "outflow boundaries" can help force surface air upward into new storms the next day.

When there is a strong cap during typical summer heat and humidity, it's like putting a lid on a boiling pot. Maybe that lid is enough to hold in the steam, but if it's not, it will erupt explosively. This is why it is not uncommon for some of our hottest days to end with severe thunderstorms.

So every hot, humid summer day has the possibility of big storms -- if the cap breaks.