Cloud Families

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In meteorology we divide clouds in 4 cloud families, which are at different heights of the troposphere: High level clouds (altitudes of 5-13 km), medium level clouds (2-7 km), low level clouds (0-2 km) and clouds with large vertical extending (0-13 km).
These cloud families are divided in 10 cloud genus:

  • Ci Cirrus Fibrous, threadlike, white feather clouds of ice crystals.
  • Cs Cirrostratus Milky, translucent cloud veil of ice crystals, which sometimes causes halo appearances around moon and sun.
  • Cc Cirrocumulus Fleecy clouds; Cloud banks of small, white flakes.
  • Ac Altocumulus Grey cloud bundles, sheds or rollers, compound like rough fleecy clouds, which are often arranged in banks.
  • As Altostratus Dense, gray layer clouds, often even and opaque, which only lets the sun shine through a little bit.
  • Sc Stratocumulus Cloud plaices, rollers or banks compound dark grey layer clouds.
  • St Stratus Evenly grey, low layer clouds, which cause fog or fine precipitation and are sometimes frazzled.
  • Cu Cumulus Heap cloud with a flat basis, in the middle or lower level, whose vertical development is reminiscent of towers, cauliflower or cotton.
  • Cb Cumulonimbus In the middle or lower level developing thundercloud, which mostly rise into the upper level.
  • Ns Nimbostratus Rain cloud. Grey, dark layer cloud with indistinct outlines.

Microbursts And The Dangers To Aircraft

  • (1) Finals The aircraft is on final approach at a slow speed and low altitude.
  • (2) Headwind Suddenly the aircraft experiences a sharp spike in indicated airspeed as the aircraft meets the oncoming rush of air from the leading edge of the microburst. If the pilots aren't aware of what is happening, they'll reduce the throttles to slow the aircraft down.
  • (3) Downdraft As the pilot is descending the aircraft back to its normal descent angle, the aircraft will experience a sudden surge of downdraft and the winds will push directly down on the aircraft, causing it to rapidly lose altitude. The pilots will increase the throttle and try to pull the aircraft back up to a safe altitude.
  • (4) Tailwind In the last stage, the plane will experience a strong tailwind, greatly reducing its airspeed, the tailwind decreases lift and makes it difficult to pull the aircraft up. Because of its low speed, the aircraft is unable to attain sufficient lift and since it is very close to the ground the chances are high of the aircraft crashing.

All commercial aircraft and many commercial airports in the United States and around the world now have wind shear detection systems to alert aircraft to the dangers of microbursts. Thanks to better training and major advances in technology, the last commercial airplane crash in the U.S. attributed to a microburst was in 1994.

What Are Microbursts?

Relatively recent meteorological studies have confirmed the existence of microburst phenomenon around these dangerous weather phenomena are small scale intense downdrafts which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types and categories of aircraft, especially at low altitudes. Due to their small size, short life span, and the fact that they can occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems. The life cycle of a microburst is seldom longer than 15 minutes. An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground. Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types. Note, however, that microbursts commonly occur within the heavy rain portion of thunderstorms, and in much weaker, benign appearing convective cells. that have little or no precipitation reaching the ground.

Characteristics of microbursts include:

1. Size. The microburst downdraft is typically less than 1 mile in diameter as it descends from the cloud base to about 1,000-3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter.

2. Intensity. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground.

3. Visual Signs. Microbursts can be found almost anywhere that there is convective activity. They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga. When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence.

4. Duration. An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approximately 2-4 minutes. Sometimes microbursts are concentrated into a line structure, and under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.

Microburst wind shear may create a severe hazard for aircraft within 1,000 feet of the ground, particularly during the approach to landing and landing and take-off phases. The impact of a microburst on aircraft can be sever as the aircraft may encounter a headwind (performance increasing) followed by a downdraft and tailwind (both performance decreasing), possibly resulting in terrain impact.

Weather Glossary

  • Avalanche A dangerous slide of snow down a mountain.
  • Below Freezing (preposition/ adjective) temperature less than 0 degrees Celsius/(32F).
  • Blizzard A storm with strong winds and intense snow.
  • Breeze Light wind.
  • Boiling Hot Common expressions for describing a very hot day.
  • Clear Blue skies with little or no clound cover.
  • Cloud Cloudy White or grey water masses in the sky.
  • Cold Spell Period of weather which is colder than normal.
  • Drizzle Very fine light rain.
  • Drought A long period with no precipitation.
  • Flood Excess, overflow of rain water common with very heavy rain.
  • Flurries Of Snow Light Snow Fall
  • Fog Foggy Thick water vapour that restricts vision.
  • Forecast Formal system of predicting weather in the future.
  • Frost Ice crystals on a frozen surface
  • Frost Bite A skin condition caused by over exposure to the cold
  • Hail Small pebbles/balls of ice that fall during a storm.
  • Heat Stroke A flu-like condition one can acquire after excessive exposure to the sun.
  • Heat Wave A period of unusually hot weather.
  • Humidity Refers to the amount of moisture in the air.
  • Hurricane Very strong winds associated with tropical storms.
  • Lightning Electrical discharge between a cloud and the ground.
  • Mild Warm pleasant temperature perhaps warmer than average.
  • Partly Cloudy Weather conditions with both sun and clouds.
  • Precipitation Rain or snow that falls on an area.
  • Raindrop A single measure of rain.
  • Rainbow An arc of colours found in the sky after rainfall and particularly when the sun shines
  • Seasons Time of year characterized by certain weather Winter, Spring, Summer, Autumn (Fall USA).
  • Shower Quick and quite often light rainstorm
  • Seasons Time of year characterized by certain weather Winter, Spring, Summer, Autumn (Fall USA).
  • Scorching Hot Exceptionally hot day.
  • Sleet Rain that freezes as it falls.
  • Slush Snow on the ground which melts due to a higher temperature or rainfall
  • Smog Heavy dark cloud cover caused by pollution.
  • Snow Frozen water that warms slightly as it falls to earth
  • Sunburn Medical condition caused by over exposure to the sun.
  • Tornado/Cyclone Violently spinning wind storm.
  • Wind/ Windy Blowing Air
  • Wind Chill Factor When the wind makes the air feel colder than the actual temperature.

Idiomatic expressions related to weather

  • Any port in a storm This means that in an emergency any solution will do, even one that would normally be unacceptable.
  • Blue skies A overly enthusiastic outlook or disposition. The sales team had blue skies projections for their deals, although not many of those deals were signed.
  • Bolt from the blue If something happens unexpectedly and suddenly, it is a bolt from the blue.
  • Brighten up the day If something brightens up your day, something happens that makes you feel positive and happy all day long.
  • Boiling Hot Common expressions for describing a very hot day.
  • A Breeze Something that is easy to do.
  • Calm before the storm A calm time immediately before period of violent activity or argument is the calm before the storm.
  • Chase Rainbows If someone chases rainbows, they try to do something that they will never achieve.
  • Cloud Nine If you are on cloud nine, you are extremely happy. ('cloud seven' is a less common alternative)
  • Cloud Of Suspician If a cloud of suspicion hangs over an individual, it means that they are not believed or are distrusted.
  • Cloud on the horizon If you can see a problem ahead, you can call it a cloud on the horizon.
  • No Cloud Of A Doubt Means there is no doubt about something.
  • Cold light of day If you see things in the cold light of day, you see them as they really are, not as you might want them to be.
  • Colder than a witch's tit(Breast) If it is colder than a witch's tit, it is extremely cold outside.
  • Come rain or shine If I say I'll be at a place come rain or shine, I mean that I can be relied on to turn up; nothing, not even the vagaries of British weather, will deter me or stop me from being there.
  • Down in the doldrums If somebody's down in the doldrums, they are depressed and lacking energy.
  • Every cloud has a silver lining People sometimes say that every cloud has a silver lining to comfort somebody who's having problems. They mean that it is always possible to get something positive out of a situation, no matter how unpleasant, difficult or even painful it might seem.
  • Fairweather friend A fairweather friend is the type who is always there when times are good but forgets about you when things get difficult or problems crop up.
  • Get wind of If you get wind of something, you hear or learn about it, especially if it was meant to be secret.
  • Greased lightning If something or someone moves like greased lightning, they move very fast indeed.
  • Head is in the clouds If a person has their head in the clouds, they have unrealistic, impractical ideas.
  • In a fog If you're in a fog, you are confused, dazed or unaware.
  • It never rains but it pours 'It never rains but it pours' means that when things go wrong, they go very wrong.
  • Know which way the wind blows This means that you should know how things are developing and be prepared for the future.
  • Quiet before the storm When you know that something is about to go horribly wrong, but hasn't just yet, then you are in the quiet before the storm.
  • Rain on your parade If someone rains on your parade, they ruin your pleasure or your plans.
  • Rainy day If you save something, especially money, for a rainy day, you save it for some possible problem or trouble in the future.
  • Sail close to the wind If you sail close to the wind, you take risks to do something, going close to the limit of what is allowed or acceptable.
  • Seven sheets to the wind If someone is seven sheets to the wind, they are very drunk.
  • Silly season The silly season is midsummer when Parliament is closed and nothing much is happening that is newsworthy, which reduces the press to reporting trivial and stupid stories
  • Stem the tide If people try to stem the tide, they are trying to stop something unpleasant from getting worse, usually when they don't succeed.
  • Storm in a teacup If someone exaggerates a problem or makes a small problem seem far greater than it really is, then they are making a storm in a teacup.
  • Take a raincheck If you take a rain check, you decline an offer now, suggesting you will accept it later. ('Raincheck' is also used.)
  • Take by storm To take by storm means to captivate- eg. A new play that took New York City by storm.
  • Throw caution to the wind When people throw caution to the wind, they take a great risk.
  • Under the weather If you are feeling a bit ill, sad or lack energy, you are under the weather.
  • Weather a storm If you weather a storm, you get through a crisis or hard times.
  • When it rains, it pours This idiom means that when things go wrong, a lot of things go wrong at the same time.