Met Lesson 3 Flashcards
Air Mass Source Region Types
Sea/land: maritime/continental
By latitude: tropical/polar
4 Air Mass Source Regions
Tropical maritime (Tm) Tropical continental (Tc) Polar maritime (Pm) Polar continental (Pc)
Source Regions Affecting the Characteristics of Air Masses
Air takes on the characteristics of the surface its over
Continental air masses lose moisture due to contact with the land
Air within Polar cell = polar air
All other air cells are comprised of tropical air
A Front
The boundary between two different temperature air masses
Cold Fronts
Warm air forced to rise over heavier cold air
Cold, dense air will wedge in under the warmer air
Very little mixing due to the different densities
Weather in an Approaching Cold Front
Reducing pressure (rising air particles) Northwest winds Cumulus clouds and showers Cb if the warm air is conditionally unstable Altocumulus and altostratus cloud
Ahead of the Cold Front
Gusts and squalls
Fast moving low cloud
Turbulence
Possible squall line
Passing Cold Front
Increasing pressure
Northwest wind backing to Southwest winds
Reducing temperature
Cumulus/towering cumulus/small cumulonimbus as frontal inversion increases with height
Warm Front
Warm less dense air will gradually slope up against the colder and more dense air
Weather Associated with a Warm Front
If the warm air is stable: Cirrus and cirrostratus creating the halo effect
Cloud becomes thicker and lower with passing front
Altocumulus, altostratus, nimbostratus
Heavy and continuous rain
If the warm air is less stable more cumulus will develop
Weather Associated with a Passing Warm Front
Warm air advancing over a cold ground creates a stable environment and the cloud will clear rapidly
Fair weather
Wave Depressions
A low pressure system is formed on the tip of the cold front as the warm air is being lifted by the cold air
The cold air swings (veers) around and catches up with the warm front because the colder air is heavier
Once it catches up it produces an occluded front
Occluded Front
Fast moving cold front catches up with the slower moving warmer air
The cold air forces the warm front upwards
Embedded Cumulonimbus
Thunderstorm obscured by other types of cloud
Stationary Front
When two air mass systems become stationary and there is no resultant movement present
When some upper air disturbance occurs the stationary system becomes displaced
Visibility
The greatest horizontal distance at which a person can identify a dark object with normal eyesight
May be specified for each direction and reported in m or km
Visibility Obscurations
Reduce in-flight visibility Moisture - precipitation/fog/cloud/etc Smoke Pollution Dust/sand Sun - glare, light/darkness
Reported Visibility
Visibility from the ground reported by an accredited observer on the ground
Flight Visibility
Horizontal visibility as observed by the pilot from the cockpit when in flight
Slant Visibility
The air to ground visibility observed by the pilot from the cockpit when in flight
Night and Day Visibility
Night visibility is normally better than daylight visibility
Daylight visibility is the worst at dawn and dusk
Vertical Visibility
Air to ground visibility when above the object (ft)
Runway Visual Range
Maximum distance that the runway can be seen from the average eye level of the pilot at touchdown
Measured with electronic instruments installed next to an ILS equipped runway
Dew
Water vapour condensates onto objects in the form of dew when sufficient moisture is available during overnight cooling
Normally after a clear night with sufficient moisture
Frost
The temperature of the ground is below 0°C
Frozen dew
Disrupts laminar airflow over the wing, loss in lift and increase in drag
Cooling effects on the wing due to acceleration
Conditions Required for Icing
Visible moisture, near freezing temperatures, super cooled water droplets (subzero water in liquid state)
Freezing temperatures
Freezing airframe temperature
Resistance to freezing due to spherical shape
Any disturbance/shock to change the shape of the droplet, allows the release of latent heat and freezing
Hoar Frost
Nighttime cooling close to the ground
Deposits of ice crystals
Doesn’t require super cooled water droplets for formation
Affects of Hoar Frost on Aircraft
Negatively affects the aerodynamics of the wings
May occur when aircraft cruising in cold altitudes
Airframe cools down - cold soaking
On descent, the aircraft enters warmer air and turns to frost via deposition
Rime or Clear Ice
Usually both present, however one type tends to predominate
Type depends upon the supercooled water droplet size and the type of cloud
Larger droplets require a larger cloud as strong up draughts are required
Catch Efficiency
A thicker wing will catch more water
Thicker is better
Clear Ice
0°C to -15°C
Large drops of freezing rain
The drop flows backwards on the surface and spreads before it freezes as it loses more latent heat in warmer conditions
No air is trapped inside the ice making it transparent and difficult to see
Clouds Producing Clear Ice
Cumuliform
Nimbostratus cloud
Thick altostratus
Rime Ice
-10°C to -20°C
Small supercooled water droplets
Opaque in colour due to air pockets within the droplets
Clouds Producing Rime Ice
Altostratus
Altocumulus
Cumulus tops
Stratiform above the freezing level
Surface Areas Mostly Affected by Icing
Wing leading edges Windscreens Tail section Engine/propellers/engine intakes Antennae
Affects of Icing on an Aircraft
Increase in drag Increase in weight Decrease in lift Decrease in thrust Blocked pitot tubes Decreased visibility May restrict control surfaces Reduced breaking action on the runway
Icing Due to Warm Fronts
Rain falling from the warmer air through the colder air may become severe clear ice
Flying into a lowering cloud base due to a warm front may lure you into severe icing conditions
Differences from Fog and Cloud
Height at which they occur Formation process (fog from conduction)
Fog and Mist
Natural visibility reducing phenomenas just above the ground level
Fog
Horizontal visibility < 1000m
Mist
Horizontal visibility => 1000m
Conditions Required for Radiation Fog
Light winds < 5kts
Clear skies at night (max terrestrial radiation)
High relative humidity (more humid = more likely for dew point to be reached)
The Clearing of Radiation Fog
Winds > 5kts
An increase in solar heating
A decrease in humidity
The Effects of Solar Heating on Radiation Fog
Initially after sunrise the fog thickens due to the increased mixing by solar heating (also the coldest time of the day)
Thin and evaporates from below as the ground surface heats up
Advection Fog
Warm humid air passing horizontally over a cold surface
May form at any time of day or night
May persist for days
Eg. Warm humid air from tropical oceans over polar surfaces
Dissipation of Advection Fog
Wind > 15kts
Reduced humidity due to deposition of water droplets on vegetation
Change in wind direction
Fog Due to Mixing
Radiation for = 2 - 5kts (light wind)
Small or no difference between OAT and dew point
Advection fog = 10 - 15kts (stronger wind
Frontal Fog
Cloud forms on the frontal boundary of the warm front
Warm rain causes the colder air below to become saturated
Steam Fog
Cold moist air flows over warmer water
Evaporation from the water surface causes the air to become saturated
Dangers of Fog and Haze
Slant visibility obscurations
Poor visibility
Requirements for the Formation of Thunderstorms
- An unstable atmosphere (ELR > 3°C/1000ft) or conditionally unstable
- An abundance of moisture
- Lifting mechanism (convection, orographic lifting, frontal, convergence)
Stages of a Thunderstorm
- Cumulus/growing/developing stage
- Mature stage
- Dissipating stage
Cumulus Stage
Strong up draughts
No precipitation
Mature Stage
First gust Lightning Wind change 180° = runway change Strong up and down draughts 20 - 30 mins Rain showers begin
Dissipating Stage
Continuous precipitation until the cloud is empty (rains out)
Storm moves away in the direction of the anvil
Thunderstorm Hazards
Lightning strike (compass swing) Static electricity Gust/dust storms Turbulence and windshear Temporary affects on night vision due to lightning flashes Poor visibility Severe airframe icing Aquaplaning Hail Excessive cockpit noise
Down Draughts Due to Cloud
Strong underneath the clouds
Virga
Down draughts due to an air mass becoming heavier and more dense than the warmer air
Microbursts
Often associated with cumulonimbus clouds
Very strong downburst with a diameter of 4km
Avoid cumulonimbus by 20nm
Airflow spreads out near the ground
Severe windshear
Tropical Cyclones
Small intense low pressure systems over warm ocean
Generally occur in the Southern hemisphere summer: November to April
Formation Requirements for Tropical Cyclones
Narrow band of latitude in the Southern Hemisphere (5° - 15° South)
In this area the oceans are warm and the coriolis force is strong enough for rotation
When the cyclone is too close to the equator it has less of a spin as the coriolis force is weaker
The 4 Stages of A Tropical Cylone
- Formative
- Immature
- Mature
- Decaying
Formative Stage of a Tropical Cyclone
Eye forming
Humid air pulled into the low and forced to rise via convergence
Air cools adiabatically: dew point reached and cloud forms
Condensation releases latent heat, causing faster rising due to an increase in temperature
< 1000hPa
Immature Stage of a Tropical cyclone
Strong winds
Pressure gradient steepens as pressure beneath becomes lower
Sucks more air in and the cycle continues
Winds light and variable inside the eye
Strong winds > 120kts around the eye with CB’s
Mature Stage of a Tropical Thunderstorm
Surface pressure about 950hPa remaining constant
Strongest wind in the left forward quadrant due to steepest pressure gradient
Nimbostratus with spiral bands of cumulus and cumulonimbus, cirrus on top
Heavy rain clouds around the eye
Cloud-free eye as the temp is above dew point due to the release of latent heat
Decaying Stage of a Tropical Thunderstorm
Rain depression
Die-out or become rain depressions once they move inland or move towards the colder pole (water temp < 26°C) or beyond 15°S
Due to reduced up draughts and water vapour supply
Colder and drier air over the ground (increased surface friction)
Widespread rain may continue for several days
Tornados in North America
Great plains are exposed to cold and warm air
Massive convergence with sharply inclined isobars
Rotating twist due to differing winds that becomes a spiral
Tornado
Massive super cell thunderstorm
<300m in diameter
Wind speeds up to 200kts
Central pressure rotating funnel 50hPa lower than the surrounding air: buildings explode as the tornado passes over
Funnel Cloud
Doesn’t touch the ground
Waterspouts
Touching the water surface and sucking water up
