Weather Flashcards
1
Q
Lapse Rate
A
- the rate at which temperature decreases with altitude
- standard temp lapse rate is 2 degrees C per 1000ft (3.5F)
- dry adiabatic lapse rate is 1.1- 2.8C per 1000ft
- standard pressure lapse rate is 1” per 1000ft
2
Q
High and low pressure systems
A
Air flows from high pressure (more dense) to low pressure (less dense) which creates wind
3
Q
High pressure systems
A
- high pressure is more dense so the air moves down, once it hits the ground and has no where to go it spreads outward
- high pressure relative to surrounding air
- air flows clockwise, down and out (screw in wood)
- ridge: elongated area of high pressure
- produces generally favorable weather
- could also produce strong winds because high pressure is trying to equalize with low pressure surrounding it
4
Q
Low pressure systems
A
- low pressure surrounded on all sides by high pressure
- the low pressure is squeezed by high pressure, the less dense air rises
- airflow counterclockwise, up, and in
- trough: elongated area of low pressure
- associated with bad weather
1. Col: neutral area between 2 highs and/or 2 lows
2. Pressure gradient force: air that moves from high pressure to low pressure, perpendicular to the isobars
5
Q
Temperature inversions
A
- Temperature increase w/altitude- exists in layers
- below is a smooth stable layer of air
- above is unstable
- restricted visibility below inversion
- turbulence above inversion due to unstable air
- 2 types:
1. Surface based temp inversions - night: cools ground quickly, cool, clear, calm nights
- morning: heats up air overlying ground first (farm areas)
2. Frontal temperature inversions - cool air forced under warm air (definition of cold front)
- warm air spreads over cool air (definition of warm front)
6
Q
Types of clouds: 3 things necessary for formation
A
- Condensation nuclei (dust, salt, pollution, particles, etc)
- Moisture
- Small temp/dew point spread
7
Q
Low clouds
A
- Surface to 6500’ AGL
- composed of small super cooled water droplets (water cooled below freezing temps w/o freezing)
- types:
1. Stratus- grey uniform sheet like cloud with relatively low bases in stable air near surface due to cooling from below
2. Nimbostratus- grey or dark massive cloud that can be several thousand feet thick and contain large quantities of moisture
3. Stratocumulus- white puffy clouds that form when stable air is lifted
4. FOG- base of usually 50ft off the ground
RIME ICE:
(-15C to -20C)
8
Q
Middle clouds
A
- 6500’ to 20000’
- composed of large super cooled water droplets
- Types:
1. Altostratus: flat dense clouds that cover a wide area, minimal turbulence and can contain moderate icing
2. Altocumulus: patchy clouds of uniform appearance, light turbulence and icing
MIXED ICE
9
Q
High clouds
A
- 20,000 to tropopause (60,000’ AGL)
- composed almost entirely of ice crystals
- Types:
1. Cirrus: composed of ice crystals that usually form above 30,000ft
2. Cirrostratus: thousands of feet thick, moisture content is very low and they pose no icing hazard
3. Cirrocumulus: white patchy clouds, look like cotton, form from shallow convective currents at high altitudes may produce light turbulence - No icing- the moisture is already frozen
10
Q
Extensive vertical development
A
- any altitude (sfc to 60,000ft)
- composed of all small and large super cooled water droplets, and ice
- very unstable/turbulent
- Types:
1. Cumulus: puffy white clouds with flat bottoms and dome shaped tops. Indicate a shallow layer of unstable air. Expect turbulence but little icing and precipitation
2. Towering cumulus- similar to cumulus except with more vertical development. Contain moderate to heavy turbulence with icing, often develop into thunderstorms
3. Cumulonimbus: large, vertically developed rain clouds. Containing large amounts of moisture, turbulence, icing, and lightning - first cloud layer can be calculated by:
(Temp-dp)x1000= cloud height
11
Q
Fog is formed by
A
- adding moisture to the air
- reducing temperature to dew point
12
Q
Steam fog
A
- forms when cold dry air moves over warm water
- the warm water evaporates and rises up (water is added to the overlying cold air and it saturates it)
- Ex. Lakes in the morning
13
Q
Upslope fog
A
- moist air is lifted by rising terrain
- cools with altitude and condenses
- required a 15kts+ wind
- air is adiabatically to its dew point
14
Q
Radiation fog
A
- forms on cool, calm, clear humid nights
- as surface is cooled by radiation, the overlying air is also cooled to its dew point
- morning warm up “burns off” the fog
15
Q
Advection fog ( often called coastal fog)
A
- warm moist air moves over a cooler surface
- sfc cools air to DO and condenses
- most often occurs when wind transports air from warmer water to cooler land
- requires wind to develop (15kts+)
16
Q
Precipitation fog
A
- warm rain falls into a cool pool of air and it saturates it
- adds moisture to the cooler air
17
Q
5 types of fog
A
- Steam fog
- Upslope fog
- Radiation fog
- Advection fog
- Precipitation fog
18
Q
Temperature/ dew point
A
- when temp and DP are equal, a parcel of air can no longer hold anymore water and visible moisture occurs
- the temp at which air can no longer hold any more moisture (saturated)
- the DP is where a parcel of air becomes fully saturated
- if the temp=DP, 100% relative humidity resulting in fog, rain, ect…
- relative humidity: amount of moisture in air to what it can hold
- effected by humidity
- hot air can hold more moisture then cold air
19
Q
Stable air
A
(Cool dry air, ie. Arctic winter)
- poor visibility
- stratus clouds (low level 0 to 6,500)
- little to no turbulence
- rime ice (-15 to -20C)
- steady precipitation
- high pressure
20
Q
Unstable air
A
(Warm, moist air, ie. Tropical, T-storms) - good visibility - cumulus clouds (exist at any altitude) - rough turbulence - clear ice (0 to -10C) - showery precipitation - low pressure Stability: the resistance to upward moving air (vertical air)
21
Q
Thunderstorms 3 conditions necessary for formation
A
- High moisture content
- Lifting action (topographic, frontal, thermals)
- Unstable air
22
Q
3 stages of the life cycle of a thunderstorm
A
- Cumulus stage
- mostly updrafts
- building of clouds
- rising air cools to dew point
- heat released by condensation of moisture helps sustain upward movement of air - Mature stage
- moisture becomes too large and heavy to be supported by the updrafts and begins to fall
- consists of updrafts and downdrafts
- most intense stage
- anvil top to the clouds
- downward motion of moisture creates downward movement of air
- air spreads outward at surface producing temp drop and pressure increase
- surface winds become strong gusty and turbulent - Dissipating stage
- downdrafts increase to overcome updrafts
- the storm begins to weaken
- as the lifting capacity is diminished, so is the strength and existence of the storm
- it starts to kill itself
23
Q
Standard temp and pressure
A
Unequal heating of the earths surface causes changes in pressure
- 29.92” Hg (in Mercury) at sea level
- 15 degrees C or 57F
- 1013.9 MB
- 14.7 PSI
24
Q
Types of thunderstorms
A
- Single cell
- single thunderstorm
- usually lasts an hour - Multi cell
- cluster of thunderstorms
- different stages of development - Super cell
- extremely large single thunderstorm
- 2 hour duration - Embedded
- a thunderstorm obscured by a massive cloud layer - Squall line
- line of thunderstorms
- embedded or hidden in the clouds
- most dangerous and up to 700 miles wide
25
Q
Thunderstorm hazards
A
Stay clear of thunderstorms by at least 20 NM because the turbulence and hail can extend out to 20 NM
- hazards of thunderstorms:
1. Turbulence
2. Hail
3. Microbursts
4. Updrafts/downdrafts
5. Tornados
6. Icing
7. Reduced visibility (clouds, precip, haze)
26
Q
Wind shear
A
- A sudden drastic change in wind velocity and or direction
- Can occur at any altitude
- Most dangerous at low level (final approach)
Occurs in: - frontal zones: opposing wind direction between air masses
- low level temperature inversions: cold still air is covered by fast moving warm air
- clear air turbulence (CAT)
- wake turbulence
27
Q
Microburst
A
- most dangerous type of wind shear
- can produce vertical and horizontal wind shear
- small scale intense downdraft that spreads out in all directions when reaching the surface
- less then 1 NM wide
- last no longer then 15 minutes
- 6,000ft p/minute downdrafts
- 100kt wind sheer possible (50kts in one direction and 50 in another)
28
Q
To protect against wind shear threats, many airports have LLWAS
A
- low level wind shear alert system
- provides warning against possible wind shear condition
- takes measurements from multiple locations around airport
29
Q
Turbulence 5 types
A
Anything from light bumps to extreme jolts
- Mechanical
- Frontal
- Convective
- Wake turbulence
- Clear air turbulence
30
Q
Mechanical turbulence
A
- obstructions or terrain that disrupt airflow
31
Q
Frontal turbulence
A
- up/down drafts created by frontal impact
32
Q
Convective (thermals) turbulence
A
- hot pockets of rising air
- ground heats up unevenly, creates thermals
33
Q
Wake turbulence
A
- created off airfoils
- must be avoided for safe flight
- quartering tail wind keeps vortices over runway longer
- heavy, clean, and slow creates the strongest vortices
- stay 3X the diameter of the rotor away when taxiing
34
Q
Clear air turbulence (CAT)
A
- typically at higher altitudes (above 15,000’)
- air layers slide over each other
- faster moving top layer creates waves
35
Q
Virga
A
- precipitation that evaporates before hitting the ground
- creates downdrafts
- cool air sinks faster creating downdrafts
- latent heat of vaporization
- heavy downdrafts up to 6000ft p/min
36
Q
Structural icing
A
- must have visible moisture
- surface of aircraft must be at or below 0 degrees C
- OAT may be 5 to -20 degrees Celsius
- 5C aircraft (especially rotors) may be colder due to reduced pressure of faster moving air, lowering temperature at point of contact (aerodynamic cooling)
- Venturi effect- velocity of air increases
- Bernoulli principle- as velocity increases, pressure decreases
- boyles gas law- as pressure decreases, temp decreases
37
Q
4 types of structural ice
A
- Rime ice
- Clear ice
- Mixed ice
- Frost
38
Q
Rime ice
A
(-15 to -20 degrees C) temp of AC surface
- small super cooled water droplets that freeze instantaneously upon impact with the surface
- opaque appearance due to air being trapped in the water droplets as they freeze
- brittle
- changes aerodynamics of whatever it forms on (ie airfoils)
- stratus clouds
39
Q
Clear ice
A
(0 to -10 degrees C)
- large super cooled water droplets freeze slowly
- big rain drops hit surface, blow back as they freeze
- worst heavy, hard to remove, break off in sheets
- rapidly and significantly alters shape of the airfoil
- cumulus clouds or freezing rain
40
Q
Mixed ice
A
(-10 to -15 degrees C)
- combination of rime and clear ice
- varying size of water droplets that strike and freeze at different rates
- cumulus and or stratus clouds
41
Q
Frost
A
- forms when water vapor changes directly to ice on sfc that is below freezing (gas–>solid)
- occurs on the ground
- changes aerodynamics: affects ability to produce lift
42
Q
Hazards of structural icing
A
- Reduces lift
- Reduces visibility- forms on windscreen
- Reduces reception- ice forms on antennas
- Adds weight/disrupts balance/disrupts airflow
- Shedding- ice sheds and contacts critical components
- Instruments- pitot blockage
- Dynamic rollover- skids freeze to ground
