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Flashcards in Weather Deck (42)
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What is fog?

Fog is a cloud at ground level through which the visibility is less than 1000mt with a relative humidity near 100%


What is mist?

Mist is similar to fog however it has a visibility greater than 1000mt and a relative humidity lower than 100%


When does fog form?

When air is cooled below its dew point temperature causing saturation and condensation


What processes may produce fog?

- Radiation


How does radiation produce fog?

Air close to the ground may be cooled due to radiation. This effect is strongest on cool nights with moist air and an inversion. A light wind up to 6kts will deepen fog as the cool air is mixed through greater depth


When is fog intensity greatest?

Usually around dawn when surface temperature is the lowest


What is advection?

Warm moist air cools as it flows over a cool surface


When does fog disappear?

When either the air temperature is raised above dew point or wind mixes drier air with the fog causing evaporation


What clouds are thunderstorms?



What conditions must exist for thunderstorms to develop?

- Unstable conditions
-Abundant supply of water vapour
-A lifting mechanism to trigger convection


3 stages of thunderstorm?

1. Cumulus
2. Mature


When are updrafts present in thunderstorm?

Only as the cloud builds (Cumulus stage)
The cloud is warmer than the environment so the vertical development continues


What occurs in the mature stage of a thunderstorm?

-Cloud has grown to a great height (some cases over 40,000ft)
-It is now a cumulonimbus cloud with anvil shaped top composed of cirrus cloud and ice crystals
-Precipitation falls generating strong downdrafts (3000ft/min)
Wind shear and severe turbulence


The whispy cirrus anvil is blown flat on top by the wind, what does this indicate?

Indicates direction of storm movement


Downdrafts from thunderstorm may cause...

Gusts and a temperature drop ahead of the storm. Downdrafts undercutting the warm air at the front of the cloud base may form a roll of cloud which can detach and move ahead of storm


What occurs in the dissipating stage of a thunderstorm?

Updrafts die out leaving downdraughts
Rain gradually decreases and cloud breaks up


How long does a typical thunderstorm last?

60 minutes


How are thunderstorms named?

By the mechanism that triggered them


What is a frontal or squall line thunderstorm?

Formed as a result of instability at the boundary of two air masses


What is the most hazardous thunderstorm in aviation?

Frontal or Squall line thunderstorm
They are often closely spaced, fast moving with a low base


What can be associated with a frontal or squall line thunderstorm?

Roll clouds


What is an air mass thunderstorm?

Tend to be more isolated and form as a result of some other trigger


How is a orographic thunderstorm formed?

Formed due to orographic uplift of moist unstable air


How is a cold stream thunderstorm formed?

A cold air stream flows over a warm surface. This generates instability in the lower layers. Thunderstorms can develop if the instability extends through a deep enough layer


How are night equatorial thunderstorms formed?

Tropical regions cloud tops cool due to radiation at night. This causes unstable conditions and a thunderstorm can develop


What are shear thunderstorms named after?

Wind shear effect which can increase storm severity


How does a shear thunderstorm form?

If the vertical development is strong enough the shear may spread the storm out horizontally, reducing the friction between up draughts and downdraughts. This allows higher velocities in the storm. Greater turbulence can result and this is why shear thunderstorms are so dangerous


Hazards associated with thunderstorms?

- Heavy rain
-Electrical interference
-Severe turbulence


What is the turbulence in thunderstorms caused by?

Vertical wind shear between up draughts and downdraughts


Where is icing generally concentrated in thunderstorm?

Around the middle of storm particularly around freezing level