In winter, the phenomenon is quite rare. But in the summer we are used to thunder rumbling. Let the lightning streak our sky. And meteorologists know why thunderstorms occur more at this time of year.
A sharp noise. A muffled roll. Possibly heavy rain. Even hail. And some lightning. Those are the ingredients of a thunderstorm. And even if the thunderstorms can perfectly well burst in full winter, we generally experience them more during the summer. The season thunderstorms begin more exactly at the crossroads of spring and summer. When the lower layers of our atmosphere. Those that are in contact with the ground.
Because thunderstorms form thanks to a hot air massed close to the ground and a air cold parked at altitude. The first tends to rise. The second tends to go down. When the two masses of different temperatures meet, an instability is created. But that is not quite enough to unleash the storm.
A matter of temperature
For this, it is necessary to form cumulonimbus. These huge clouds anvil head. Flat on top and fluffy on the bottom. So you need… humidity. Moisture in the air mass hot. Of the water vapour which can then be transported by an updraft. And which in contact with colder air, at altitude, condenses into droplets and forms a cumulus. It is this cumulus which can evolve into cumulonimbus, the only cloud in which we observe the electrical phenomena associated with thunderstorms.
Because at the heart of the cumulonimbus, the movements of the air are violent. The collisions they cause between water and ice particles electrify the cloud. The positive charges concentrate at the top while the negative charges migrate towards the base. Everything is in place so that the first lightning appear.
The updraft that causes water vapor to rise is what meteorologists also call a trigger. It can be fed by the slopes of a mountain which will encourage warm air to rise high speed. By a sea breeze that brings moist air to an unstable area. By the passage of a cold front. Or by lower atmospheric layers strongly heated by the Sun. This is what we observe in summer. And that explains why thunderstorms are more numerous at this time of the year.
Strange altocumulus clouds Not all clouds are so easy to identify. The one pictured here is particularly impressive. It is probably an altocumulus castellanus undulatus opacus; some liken it to a new form of cloud proposed in 2009 in the International Cloud Atlas: the asperatus. Location: Georgia, United States. © Bradley Huchteman, Flickr, CC by-nc-nd 2.0
The pyrocumulus cloud of fire A pyrocumulus is a cloud that forms above a source of intense heat, during forest fires or volcanic eruptions for example. Like any cumulus, it occurs when the low-level air is warmer and more humid than that aloft. When the air becomes saturated, the water vapor condenses into droplets to create a cloud. Location: Arizona, United States. © Eric Neitzel, Wikimedia Commons, CC by-sa 3.0
The cumulonimbus, a thundercloud The cumulonimbus cloud results from the convection of hot and humid air particles. It differs from other shower clouds both in its vertical development and in its ability to give rise to electrical phenomena. Often characterized by an anvil shape and a dark base, it is usually a sign of worsening weather conditions. Location: Mali. © Nasa’s Marshall Space Flight Center, Flickr, CC by-nc-nd 2.0
A devastating supercell storm Supercell thunderstorms occur between updraft and downdrafts in a cloud like cumulonimbus. They are characterized by the formation of a giant cell up to 20 kilometers in diameter. Within this structure, strong winds can be accompanied by torrential rains and even hail. Location: New Mexico, United States. © Greg Lundeen, Wikimedia Commons, DP
The morning glory cloud, an impressive cloud roll The morning glory cloud is an impressive roll-shaped cloud. It is observed particularly in the north of Australia, in the Gulf of Carpentaria and has also been seen in other regions of the globe such as Canada or the United States. Location: Texas, United States. © Kenneth Cole Schneider, Flickr, CC by-nc-nd 2.0
Lenticular clouds, similar to UFOs Lenticular clouds, or altoculumulus lenticularis, are created with three ingredients: moist air, wind and a mountain. When air encounters relief, it rises where the temperature and pressure are lower. As the water can no longer maintain itself in a gaseous state, it condenses and small droplets appear: it is a cloud. Often, the lenticular cloud takes on an elongated, flat shape reminiscent of UFOs such as flying saucers. Location: California, United States. © Simon Bisson, Flickr, CC by-nc-nd 2.0
The dreaded funnel cloud When an unstable warm air mass meets a cold air mass, the consequences can be catastrophic: it can create a violent air vortex that stretches the clouds into a funnel shape. We then speak of a funnel cloud or snorkel. If the latter hits the ground, it is called a tornado and if it hits the water, it becomes a waterspout. This creates winds that can reach up to 500 km/h and which ravage everything in their path. © Justin1569, Wikimedia Commons, CC by-sa 3.0
Cirrocumulus, thin white clouds White and fine clouds, cirrocumulus clouds are mainly made up of ice crystals. They are located at an altitude of between 5,000 and 10,000 meters. Location: Algeria. © Pir6mon, Wikimedia Commons, CC by-sa 3.0
The noctilucent clouds, the highest clouds Noctilucent clouds, literally “glow in the night”, form 80 km high, in the mesosphere. They are mainly observed in summer, when the Sun is setting. They are indeed clouds of ice, which cannot be seen in the middle of the day. To become visible, they must be able to reflect the light emitted by the Sun, from below. Location: Netherlands. © Hrald, Wikimedia Commons, CC by-sa 3.0
The fog, this cloud that touches the ground Fog is a cloud whose base touches the ground. For it to form, the air humidity must be high enough, the wind must be neither too strong nor too weak and there must be enough condensation nuclei in the air. If all these conditions are met, very small water droplets are kept suspended in the atmosphere, which reduces visibility on the ground. However, there are several processes by which water vapor condenses near the earth’s surface and therefore several types of fog. Location: Mississippi, United States. © Roger Smith, Flickr, CC by-nc-nd 2.0
Pearly clouds, destroyers of the ozone layer Formed in the upper part of the stratosphere at altitudes varying between 15 and 25 km, pearly clouds, or polar stratospheric clouds, take their name from their iridescence which is reminiscent of that of mother-of-pearl. They are made up of ice crystals and sometimes nitric acid, which gives them an orange color. These clouds contribute to the depletion of stratospheric ozone at the poles: the transformation of nitrogen dioxide into nitric acid prevents the nitrogen dioxide from trapping chlorine, responsible for the destruction of the ozone layer. Location: Norway. © Kjetil Lenes, Wikimedia Commons, CC by-sa 3.0
The cumulus fractus, under the base of other clouds Located in the lower part of the atmosphere, cumulus clouds are distinguished into four types. The cumulus fractus correspond to the first stage of the formation of a cloud or on the contrary to the decomposition of a larger cloud. Very light and scattered, they are often found under the base of other larger clouds. Location: Pennsylvania, United States. © Nicholas A. Tonelli, Wikimedia Commons, CC by 2.0
orange mammas The mamma, or mammatus (“udder” in Latin), designates a particular and rare phenomenon that occurs at the base of certain clouds if certain very particular conditions are met. Resembling small spheres, mammas form when the unstable part of a cloud hovers over a layer of dry air. The droplets and ice crystals contained in the cloud then descend towards the base while evaporating. However, some regions that are wetter and cooler than others continue to descend, destabilizing the cloud base and creating the mammas. Generally, their color is the same as that of the cloud that gives them birth, but they can also take on a completely different color, which gives the sky an unreal appearance. © Jürgen Mangelsdorf, Flickr, CC by-nc-nd 2.0
The orographic cloud, on top of the mountains When the air encounters an obstacle, a mountain for example, it must rise and sees its pressure reduce, which causes a cooling of the water vapour. The humidity then increases and a cloud, called an orographic cloud, forms. Location: Switzerland. © Andrew Bossi, Wikimedia Commons, CC by-sa 2.5
The mushroom cloud, a type of pyrocumulus As their name suggests, mushroom clouds resemble a mushroom. It is a type of pyrocumulus, that is to say a cloud that is created above a source of intense heat. They form when a large mass of hot gas suddenly appears at ground level creating a strong instability. The Hiroshima mushroom cloud is the best known, but other more limited explosions can lead to this type of phenomenon. Location: Kansas, United States. © M Stewart, Flickr, CC by-nc-nd 2.0
The cumulus, a budding cloud With its budding appearance, the cumulus is often compared to a cauliflower. This cloud usually does not prevent the Sun from shining because it is often scattered in the sky. Sunlit parts are bright white but the base is relatively dark. © Woody, Flickr, CC by-nc-nd 2.0
Clouds over the Pacific Ocean Despite these thick clouds over the Pacific Ocean, the Sun still manages to make its way into the sky… © christopher walling, Flickr, CC by-nc-nd 2.0
Threatening clouds in Crete Crete, rather accustomed to the Sun, can also welcome disturbing clouds like those in this photograph. Location: Crete. © Theophilos Papadopoulos, Flickr, CC by-nc-nd 2.0
Clouds over the Irish Sea Clouds carry billions of tons of fresh water from the oceans across the sky. Location: Irish Sea. © Adrian Kingsley-Hughes, Flickr, CC by-nc-nd 2.0
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