Meteorology 2

Wind

• Wind is represented as a vector

  • Speed is typically measured in knots (kt)

  • Direction is where the wind is blowing from

• A change in wind direction is either

  • Veering - clockwise change

  • Backing - anti-clockwise change

Friction layer

• Friction with the earth’s surface cause geostrophic (free) wind to slow down

• Slower moving air experiences less Coriolis force so direction also backs (in northern hemisphere)

• Friction varies over land and sea

  • Sea: ~20% decrease, ~10° backing

  • Land: ~50% decrease, ~30° backing

• Friction layer typically ~2000ft thick

Local winds

• Local temperature variations drive localised winds

• Sea breeze

  • During the day land warms up more quickly than water

  • Leads to an on-shore breeze of typically ~10kts below 1000ft

  • Opposite effect at night as water cools more slowly than land

• Katabatic wind (Bora)

  • Air close to surface of a slope cools at night, increasing in density, flows down under gravity

  • Can reach over 80kts in snowy conditions

• Anabatic wind

  • Opposite

Humidity

• Air can hold invisible water vapour

• Warm air can hold more than cold at the same pressure

• Air becomes saturated when it can’t hold anymore vapour

• Temperature at which this occurs is known has Dew Point

• Below this temperature visible droplets form i.e. clouds

Cloud formation

• Relationship between dew point and temperature, known as spread, gives an indication of how high clouds will form

• Every 1°C spread corresponds to 400ft increase in cloud base

• Zero spread means fog/mist

Cloud formation mechanisms

• Convection

  • Air is warmed reducing its density causing it to rise and drop in temperature

• Orographic

  • Air is force to rise as it flows over rising terrain

• Advection

  • Air flows over a cooler surface causing its temperature to drop

• Turbulent

  • Mixing of two atmospheric layers with different temperatures

• Convergent

  • Two air masses meeting forces air to rise

Cloud classification

• Cloud type

  • Cumulus (Cu) - significant vertical extents

  • Status (St) - significant horizontal extents

  • Stratocumulus (Sc) - combination of both

• Height band

  • Cirrus (Ci/Cc/Ca) - high altitude

  • Alto (Ac/As) medium altitude

• Presence of precipitation

  • Nimbostratus (Nb)

  • Cumulonimbus (Cb)

Air masses

• An air mass is a large body of air with similar temperature and humidity

• UK typically experiences

  • Tropical maritime

  • Tropical continental

  • Polar continental

  • Polar maritime

Fronts

• A front is the boundary between 2 different air masses

• Defined by the direction of motion of the masses

• Two main types

  • Warm front - where a warmer air mass is moving into a cooler one

  • Cold front - where a cooler air mass is moving into a warmer one

• Two additional types

  • Occluded front - occurs when a cold front meets a warm front

  • Stationary front - when there is no relative motion of air masses

Cumulonimbus

• Cumulonimbus clouds - thunderstorms

  • Strong winds

  • Heavy precipitation

  • Hail

  • Lightening

  • Turbulence

  • Windsheer

  • Severe icing

  • Electromagnetic interference

• Do not fly in the vicinity of them!

Visibility

• Water droplets or other contaminants reduce transparency of air

• Broadly 2 types considered

  • Flight visibility

  • Meteorological visibility

• Many factors effect flight visibility

• Haze

  • Layer of smoke or dust in the atmosphere

• Slant visibility

• Mist and fog

  • Water droplets in the atmosphere

  • Two main types

    • Advection fog

      • Warm moist air flows over colder surface

    • Radiation fog

      • Land cooling overnight with light mixing