A sunny day, or clear sky during nighttime, is when there are clouds above the horizon. If one is at the top of a mountain or travel in a plane, possible clouds and fogs don't count as long as they remain below the horizon level. This also means that the sky is always sunny/clear above the highest altitude at which clouds can generate, which is about 85,340 meters (53 miles). Sunny days are bought by anticyclones.
Sunny weather can help temperatures to rise under the Sun's influence but a clear sky during the night allows temperatures to significantly drop, especially in winter, as heat is radiating from the surroundings and there is no cloud cover to keep it from escaping to space; the decrease in temperature keeps going until an hour after sunrise. Thus, sunny weather causes the most important variations in temperatures within the same air mass.
Sunny/clear weather is commonly found in deserts. Tops of mountains also enjoy plenty of sunny days since few clouds reach them; this is why the most powerful telescopes were built in mountains. However, sunny/clear weather is quite rare for the rest of the world. There is practically no sunny/clear day under the Inter-tropical Convergence Zone since it rains almost on a daily basis.
Partially cloudy weather is when some clouds are hovering above the horizon level; however, most of the sky remains clear and there is no precipitation of any kind. The clouds may temporarily drop the temperature when they hide the Sun during the day, which is particularly helpful during a heatwave.
Cloudy weather is when a significant amount of clouds is covering the sky (at least half the sky). A cloudy day may result in some weak showers, snowfalls, or even thunderstorms, although a thunderstorm temporarily overcasts the sky as it happens.
Overcast weather is when the sky is completely covered by a cloud blanket. Days are significantly dimmer and nights are completely dark without artificial sources of light. If cirrus or cirrostratus are covering the sky, the Sun or Moon may remain visible, and daytime is slightly brighter than with a stratus cover; however, the latter is far more common than the former. Depending on the type of clouds and time of the day, an overcast sky may appear white, light grey, grey, or dark blue during sunrises/sunsets.
The cloud cover prevents temperatures from either dropping or rising too sharply, decreasing the differences in temperatures between the different parts of the day. Sometimes, the temperature can remain the same the whole day.
Rain is the condensed moisture of the atmosphere above falling under the form of liquid droplets. It is an extremely important phenomenon that allows life to flourish on continents; without regular rains, the land turns into a hot desert. However, strong rains can also cause catastrophic floods, capable of destroying areas as large as an entire metropolis. In addition, rain causes other side effects such as slippery roads, longer braking distances, decreased visibility (pseudo-fog)... Episodic rains that last only a few minutes are called showers.
Rain intensity is often measured in millimeters (or inches); an mm of rain corresponds to a liter of water over a square meter. Typical rains give less than 5 mm (0.1 in) per hour; however, there are also torrential rains that can give way more than a hundred millimeters (4 in) per hour; the record of rain amount in an hour is 305 mm (12 in)!
Drizzle is similar to rain, but the droplets are very small and hardly noticeable with the naked eye. Drizzles are always weak, but they also limit visibility. Drizzle may precede or turns into a fog.
Snow is atmospheric water that froze and fell to the ground, covering it. It happens when the temperature of the air falls at 0 °C (32 °F). Snow remains as long as the temperature of the ground doesn't go beyond 0 °C (32 °F).
Snow may be appreciated by children, but it causes much worse problems than rain, as it makes roads impassable and walking difficult, may block the entrances of houses, and may even destroy cars and rooftops with the weight of the snow accumulated on it. Snowfalls also decrease visibility way worse than rain, and if accompanied by strong winds, we talk of blizzards. In mountains, large packs of snow may slide over the slope they lie in, causing an avalanche that is the deadliest event associated with snow. Also, snow melting can cause some floods: for example, a meter tall (3 feet) snow blanket melting usually turns into a 10 cm (4 in) puddle. Despite this, snow is important during the cold season as mountain stations rely on it to bring in tourists.
The height of the snow blanket is often measured in centimeters (or inches). Snow can already cause accidents at 1 cm (0.4 in), and driving may become impossible at 10 cm (4 in); walking becomes particularly difficult beyond 15 cm (6 in). However, some snowfalls can bring up to a meter (3 foot) of snow, especially in mountains.
Snow is characteristic of mountains and remains permanent at higher altitudes (“eternal snows”). Snow also happens quite frequently under continental climates during the cold season, such as in Eastern Europe and Russia. It is however much rarer in mild climates and only happens during a limited window of time, usually around the coldest month of the year, although early or late snow can also take place there if an exceptionally cold front was to sweep over said regions. It never falls under hot climates, especially around the equator.
A thunderstorm is a type of weather characterized by lightning. There are produced by the largest and tallest clouds that can spawn, the cumulonimbus. Thunderstorms are also accompanied by strong gusts of wind, heavy rainfalls, and sometimes hail and tornadoes, which are the most destructive meteorological event. A thunderstorm, regardless of its intensity, is always a dangerous phenomenon that should be taken seriously as lightning can cause great bodily harm, and sometimes even kill, anyone unfortunate enough to be hit with.
Thunderstorms can happen everywhere but are extremely rare around the poles. In mild climates, they happen especially during late spring, summer, and early autumn. They are very frequent in mountains during the whole year, as well as in tropical and equatorial climates.
Tornadoes are rotating air columns spinning at a very high speed. They are born under supercell thunderstorms, which are a specific kind of cumulonimbus that is spinning on itself. Tornadoes act as a sort of “natural vacuum cleaner”, aspiring then projecting everything it can take on its path. Tornadoes swallow dust and debris that make them visible the day and, depending on the materials swallowed, make them light grey, grey, or even brown. The night, tornadoes are normally invisible making them much more dangerous and threatening, but lightning can reveal their existence and position. Tornadoes make also a lot of sounds, especially the strongest ones, which can help identify them with little to no visibility.
Tornadoes are ranked by the Enhanced Fujita scale, which takes into consideration the damages caused by the tornado, to determine its speed. Most tornadoes do not produce winds faster than 200 km/h (124 mph), although these winds can already kill an unprotected person and destroy a house.
The 1999 Bridge Creek tornado produced the strongest winds officially recorded on Earth, at 512 km/h (318 mph), which is about 40% of the speed of sound. However, the 2013 El Reno tornado, which was the largest tornado that ever occurred (with a diameter of 4.2 km (2.6 miles)) may have produced winds as strong as 541 km/h (336 mph), which is approximately 44% of the speed of sound. Both of these tornadoes destroyed everything in their paths, although the El Reno tornado, fortunately, didn't cross urbanized areas.
Since tornadoes are extremely dangerous, it is important to be on the lookout for one if supercells are predicted for your area. Tornadoes can occur pretty much everywhere in the world, but they mostly appear within mild climates.
Thundersnows are extremely rare variants of thunderstorms, in which snow falls instead of rain or hail. Thundersnows tend to be very weak compared to common thunderstorms, and also last shorter, although lightning can still pose a deadly threat.
Fog is literally a cloud at ground level, which raises ambient humidity to its maximum, and considerably decreases visibility. The threshold of visibility at which we talk of fog is a kilometer (0.6 miles), but some fogs can decrease visibility up to 20 meters (65.6 foot). If visibility is greater than 1 km (0.6 miles), we talk of a mist. Fogs can happen anywhere, even in deserts, but happen more frequently in cold, humid climates. Depending on the moment of the day, they sport various shades of grey, varying from light grey at solar noon, to dark grey at the sunset.
Fogs make biking and driving particularly dangerous, especially if the visibility is less than 100 meters (328 feet). In some countries, speed limits are reduced when a fog happens. For instance, in France, the speed limit is reduced to 50 km/h (31 mph) if a fog decreases visibility to less than 50 meters (164 foot). Regardless of the speed limits, many drivers choose to drive fast which can cause severe accidents, especially on highways. In some countries, meteorological warnings are emitted if a fog is predicted to take place.
Cyclones, hurricanes, and typhoons design the same phenomenon, for different parts of the world. They are structured and large systems of clouds under which winds blow at more than 119 km/h (74 mph). They possess a zone of calm, sunny weather, called the eye, in which pressure is at its minimum. Winds are the strongest and most devastating around the eye.
Hurricanes form over hot spots of water in the ocean (which must be at 26.5 °C (80 °F) minimum; the higher, the stronger will the hurricane be), and also cannot form near the equator (precisely at less than 5° of latitude of the equator), since they must use the Coriolis' force to organize into a round-shape storm. They eventually either converge to terrestrial lands in which they cause damages but lose their strength, or more rarely disappears in the ocean if the water becomes too cold to sustain them.
Hurricanes can cause devastating damages over large areas (unlike a tornado), especially in small islands and coastal areas. In addition to the winds, hurricanes also deliver strong rains, and coastal areas are impacted by hurricane surges, which are an elevation of the sea level as pressure drops under a hurricane. The elevation can climb up to 6 meters (20 foot), which is sometimes deadlier than the hurricane's winds themselves, as seen with Katrina.
Sandstorms are large surges of sand and dust carried away by strong winds, which take the form of a dull brown wall seen from far away. Sandstorms are normally not lethal but the amount of sand makes any outdoor activity very difficult, and breathing may become unbearable with the number of particles in the air. Sandstorms usually happen in hot deserts, but exceptional weather patterns can allow sandstorms to sweep over non-desertic regions.
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