Meteorology Exam Notes

These are my personal revision note that I used when I took the meteorology exam, they may therefore not be 100% in either accuracy or breadth, please always defer to you instructor or their recommended text.


Notes:

Attributes of air: Weight, Pressure, temperature and moisture. 

Density The weight of the molecules in a given mass or parcel of air. Parcel = Mass or volume of air. Density does NOT equal Pressure.

Density decreases

with a decrease in pressure

with an increase in temperature

with an increase in water vapour, as humidity increases density decreases

with an increase in temperature 

Source: metoffice.gov.uk

 
Pressure

Is the weight of a column of air from the surface up.
Unit of measurement mb (millibar)  but ICAO (International Civil Aviation Organization) use hPa ( hector pascal) 1mb = 1hPa.
USA use inHg (Inches of mercury)

atm = Standard atmospheric pressure (or 1 atm)

Standard pressure = 29.92 inHg or 1 atm or 1013.25 hPa or 1013.25mb

Pressure decreases with height. 1hPA or 1mb every 30ft or 9m, after 20K -30K ft it is 1hPA every 50 – 60ft.

Pressure is measured with a barometer of which there are 3 main types: Mercury, Aneroid and precision Aneroid.

Altimeter is an Aneroid Barometer.

QFE = Pressure at the airfield, termed Height (think F for field)
QNH = Pressure at sea level, termed altitude (thin N for nautical!)

(Note F for Field and N for Nautical are not correct meaning just how I remember them)

Regional QNH = altimeter setting Region or ASR is the altitude above mean sea level within a certain defined region. Within United Kingdom airspace, these are known as Altimeter Setting Regions (ASRs).

Standard pressure = 1013.25mb is used to when flying at FLxx, e.g. FL45 the height will vary with pressure.

Microlights without a pressure setting, set the altimeter to QNH at the departure point. On arrival reduce by the AMSL of the destination as per your map.

Pressure systems

Isobars=lines joining places of equal MSL pressure on a weather chart.
High = High Pressure also known as Anticyclone.
Low = Low pressure also known as Depression.
Ridge = a wedge or tongue of high pressure from an anticyclone.
Trough = a wedge or tongue of high pressure from a depression.
Col = a neutral area between opposing systems.

Wind

Movement of air from one place to another e.g. from high to low pressure. Differences in temperature create differences in pressure. For example, local winds along lake and ocean shores are the result of the temperature differences between land and water, which cause a pressure difference and therefore wind. These pressure differences drive winds in a never-ending attempt to reach equilibrium.

Wind Direction
Given in degrees from the direction it blows.
Clockwise changes are called Veering anti clockwise are called Backing.
Above 2,000ft around high pressure anticyclones blow clockwise, around depressions they are blow anti clockwise. These winds are known as Geostrophic winds.

NOTE: in the southern hemisphere the directions are reversed.

Friction from the Earth’s surface means at low levels the wind blows from high to low pressure at approximately 30° to the isobars over land and 10° over the sea.

With a strong wind on your port or left side you are flying towards low pressure centre and the altimeter will read high.

Wind on your back and your left arm held out to the side, it will be pointing towards the area of low pressure.

Katabatic wind – colder air from tops of mountains roll down.
Anabatic  Wind – warm air from valleys rise.

Wind speed given in degrees/knots e.g. 270/22

Wind Shear – change in direction and or speed.

Low level turbulence – caused by air passing over obstacles or uneven heating of air from the ground.

Hill Lift – wind blowing towards a hill is forced up it, thus having a vertical and horizontal component, the vertical component  gives lift, best lift 1/3 up the hill.

Leeward side – (Lee from old English for shelter) the vertical component is downwards if caught in a leeward downdraft head away from the hill; don’t try to climb as you may stall.

Foehn or Föhn wind – dry warm down slop wind leeward side.

Wave effect known as standing wave may repeat from hills.

Rotor effect – sudden or sharp drops on a leeward side can cause rotor effect resulting in spins, loss of control etc.

Wake turbulence – disturbed air left behind an aeroplane. Landing behind and aeroplane taking off, land as near the beginning of the runway as possible. Max distance and time between.
Landing behind another aeroplane landing, land beyond its touchdown point if possible, leave 2 min minimum.

Temperature

Not a direct result of the sun! Sun heats the earth, the earth heats the air. The higher you go the colder it becomes as you move away from the heat source. Clouds act as insulators reducing warming but also reducing heat loss.

Measured in degrees C in UK and F in the USA.

Lapse Rate - Rate at which temperature drops with height.

Inversion – rate at which temperature increases with height, e.g. warm air moves over cold. Traps Fog and haze in.

Clouds and weather – only form in the Troposphere.

Layers - high to low, Stratosphere, Tropopause (line not belt), Troposphere, Earth.

Adiabatic Lapse rate – Adiabatic cooling effect, ascending air decrease in pressure causes a decrees in temperature. E.g. fridge decrees pressure cools while an increase gives of heat.

DALR – Dry Adiabatic Lapse Rate, 3°C per 1,000ft, dry air may contain moisture, but not yet condensed into cloud.

SALR - Wet or saturated Adiabatic Lapse Rate, 1.5°C per 1,000ft moister content has condensed into cloud up to 6,000ft then increases the same as DALR. The gas condensing uses up heat hence half the DALR rate.

Sea Breeze – land heats air more quickly and air rises, cool air from over the sea flows in to replace it, on sure breeze.

Relative humidity - % of the maximum amount of water vapour a given mass of air can hold, varies with temperature.

Cold air less water vapour.
100% humidity = saturated.

Dew point = Temperature air become saturated.
Cooling past the dew point causes condensation which is the formation of clouds, if at ground level it is fog or dew.

Air density – low density means less lift and ASI (Air Speed Indicator) reads low.

Air pressure less than Standard (1013.25mb) and temperature above 15°C and the aeroplane needs longer to take off and is slower to climb.

Clouds

Cumulus Nimbus = Heavy Rain (storms).

Stratus = Drizzle.

Cumulus clouds = unstable air.

UK Low – Surface to 6,500ft

UK Medium – 6,500ft to 20,000ft

UK High – 20,000ft and above
Height is to the base of the clouds.

 Cloud types, you need to what weather is associated with each type!

This information is in the Microlight handbook and at the following links:

http://www.metoffice.gov.uk/media/pdf/r/i/Cloud_types_for_observers.pdf

http://downloads.bbc.co.uk/tv/greatbritishweather/cloudspottingguide.pdf

 

Air Mass Types

Polar Maritime = Cold Moist
Polar Continental = Cold Dry
Tropical Maritime = Warm Moist
Tropical Continental = Warm Dry

Front = term to describe boundary separating two masses of air e.g.  cold and warm meeting.

Occlusion = Process by which an occluded front forms, e.g. cold front moves under a warm front, can causes Cumulonimbus clouds.

Hail = in Cumulonimbus is caused by being taken very high in the clouds updraft where most of the cloud is in the form of "supercooled" water, a supercooled water drop needs something on which to freeze, or it remains liquid. Ice crystals, frozen raindrops, dust, and salt from the ocean are also present in the cloud. On collision, supercooled water will freeze onto any of these hosts, creating new hailstones or enlarging those that already exist.

Snow = When condensation takes place below freezing, Vapour to Ice without becoming liquid e.g. rain.

Fog

Fog is Stratus at ground level.
For is when visibility is below 1Km (officially in the UK 200yards).

2 Main types of fog:

Radiation Fog – Usually occurs in the winter. Normally after sunset  when there is a clear night and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fog requires a light breeze to trigger it off, but will not form if it’s windy! Radiation fogs usually dissipate soon after sunrise as the ground warms.

Advection Fog – Advection meaning transport or wind, is when a warn moist air mass passes over a cooler surface. This is common out at sea.

Thunder – Cumulonimbus clouds (meaning heap)

Distance approximately 1 mile for every 5 seconds after the flash.

Depression – Low pressure, bad weather, rain, wind, low cloud, wind veers with fronts.

Trough – Similar to depression, but not long lasting.

Col -  Neutral weather, area between high and low pressure. If high lapse rate and humidity could cause thunder.

Anticyclone – High pressure, good weather, cloud less in summer, dry. If thick cloud exists know as anticyclonic gloom.

Ridge – period of settled weather between two poor spells.

Icing

Water into ice known as sublimation in metrology.

Must be a nuclei for ice to form above -37°C e.g. dust.

Liquid below 0°C is known as supper cooled. Turns into ice by sticking to an object

Supercooled droplets in Cumulonimbus get liquefied by a process known as Glaciation.

Hoar Frost – formed by sublimation on to cold surfaces below 0°C. Must be removed before flight.

Rime Ice – flying through clouds builds as an opaque layer on leading edge. Rime ice forms when liquid water droplets in the air freeze onto a surface.

Clear or Glazed Ice – like rime ice, but travels backwards as it freezes covering more of the surface and accretion / layers are built up.

Rain Ice – Sub-zero temperature rain, rain becomes super cooled and turns into ice on contact with the airframe, similar to clear ice.

Airmet – Airmen's Meteorological Information, Plan language concise description of weather.

Metar – Meteorological Aerodrome Report, formatted / coded report of weather information.

TAF - Terminal aerodrome forecast formatted / coded weather forecast.