Stormy skies hover over the Lincoln Memorial. Photo courtesy of CWG’s Ian Livingston

When it comes to the summertime, the dominant weather around the D.C. area is  characterized by two words: hot and humid (even if we have a break from it coming up).  While these conditions feel oppressive after a while, one positive about the heat and humidity is it can produce some spectacular cloud formations!

This article provides visual examples of some neat cloud types common for the summer season.  Summer clouds are often referred to as “convective” clouds, meaning they form when warm and moist buoyant air rises (like a convection oven) creating tall, often puffy, thunderstorm-producing clouds.

The “beanstalk cumulus” – vertical cumulonimbus

A vertical cumulus cloud over Dulles International Airport. Photo by Kathryn Prociv

These cloud types are your most common summertime convective cloud.  They are the cumulus clouds that grow vertically, or straight up, and can be as tall as 50,000 feet (or higher!).  These clouds produce your ordinary, or “popcorn,” thunderstorm that lasts for about an hour then rains itself out.  You have likely also heard of these types of clouds as “cumulonimbus clouds,” or simply “thunderheads.”

“The leaning tower of cumulus” – tilted cumulonimbus

A leaning cumulonimbus cloud approaching Ocean City, Maryland. Photo by Kathryn Prociv

Like the vertical cumulus cloud, this forms when warm moist air rises from the surface and eventually cools and condenses into a cloud.  There is one difference between the vertical cumulus cloud and this leaning cumulus, and that is the tilted updraft.  When you see cumulus clouds growing tilted as above, that means there are strong winds aloft and the atmosphere is primed to produce not only ordinary thunderstorms, but also severe thunderstorms (longer lived thunderstorms and those capable of damaging wind, hail, and tornadoes)!

By observing the tilted cloud, you are witnessing the upper-level winds blowing the updraft away from the downdraft, so the storm cannot “rain itself out.”  In other words, the rain-cooled downdraft is separated far enough from the updraft that it will not choke off the warm, moist air fueling the storm.

“The cumulonimbus traffic jam” – cumulus congestus

Congested cumulus clouds off in the distance near Blacksburg, Virginia. Photo by Kathryn Prociv

We’re all very familiar with traffic in the D.C. area, and clouds can experience the same thing!  The name “cumulus congestus” describes this cloud type perfectly; multiple cumulus clouds form and become congested or bunched together in the atmosphere.  Warm temperatures and high humidity in the atmosphere produce unstable air conducive for cloud development and numerous thunderstorms.

Each individual storm within a cumulus congestus formation typically contains an individual updraft and goes through an individual life-cycle.  When multiple cumulonimbus clouds form close together, they are called cumulus congestus and can eventually mature into multi-cell clusters of storms.  When you see these types of clouds, there are likely numerous storms nearby!

“The shelves of the sky” –  shelf cloud

A shelf cloud approaching Centreville, Virginia. Photo by Kathryn Prociv

Shelf clouds are one of the most menacing convective cloud types you will ever witness.  They are large clouds, often engulfing the entire sky, and signify the leading edge of a strong or severe thunderstorm.  The cold gust front of air that surges out ahead of the parent storm forces the air ahead of it to rise quickly and condense into this horizontal cloud.  Shelf clouds got the name simply because they have visible layers which may resemble “shelves.”

Though incredibly ominous-looking, these types of clouds actually pose a very low tornado threat and typically occur with linear thunderstorm complexes such as squall lines and derechos.  However, don’t let your guards down as they often portend an imminent threat for damaging winds.  There is a saying for these types of clouds, “the worst is first!”  The worst of the weather associated with this cloud (and storm) occurs right as the shelf cloud passes overhead, including the strongest winds, heaviest rain, etc.

“Looks like a cow udder?” – mammatus clouds

Sunlit mammatus clouds after a storm in Blacksburg, Virginia. Photo by Kathryn Prociv

Mammatus clouds (pronounced me-mat-es) are some of the most unusual clouds you could ever witness.  While very common with storms out in the Great Plains, they can be quite rare for the Mid-Atlantic region.  These clouds form under the anvils of strong thunderstorms, and ones that are often severe or even tornadic.  The bag or pouch-like formations are the result of strong downdrafts within the cloud characterized by cold and dense air that wants to fall toward the surface.  A good rule of thumb is if you see these types of clouds associated with a storm nearby, it means business!

(I wanted to stick with the genre of using only pictures I’ve taken in Virginia and around the D.C. area, but do a quick search of “mammatus clouds” and you will see some dramatic examples of these spectacular cloud formations from other areas of the country!)

Related post: Mammatus clouds bubble up in Washington, D.C.’s evening sky (PHOTOS)

“The hat cloud” – pileus cloud

A pileus cloud above a mushrooming cumulonimbus cloud over the Chesapeake Bay. Photo by Kathryn Prociv

Pileus clouds (pronounced pi-le-us) are fun little clouds that form on top of towering cumulonimbus clouds and have been described as looking like a “hat” on the cloud; they are also commonly referred to as “cap clouds.”  These small clouds form when strong and moist upper-level winds are diverted up and over the top of a cumulonimbus cloud.  This lifting mechanism causes a rapid cooling of the air resulting in condensation and the formation of this small, smooth, and rounded cloud.

These clouds are often very short-lived.  One can form and dissipate in a matter of just a few minutes, so if you want to see one of these you have to be quick!


For weather enthusiasts like myself, there is nothing more exciting than spotting interesting or unusual cloud formations.  Now that you’ve seen some examples of some exciting convective cloud types common during the summer months, which ones have you seen?

For even more cloud action, check out CWG Ian Livington’s post on scary looking clouds!