Aviation Weather

what is every physical process of weather accompanied by/is a result of?

heat exchanges

what does unequal heating of the Earth’s surface cause?

pressure and altimeter changes

what direction does the Coriolis force deflect wind in the Northern Hemisphere?

to the right

what are the deflections caused by the Coriolis like at the surface and why?

they are less at the surface due to slower wind speed caused by friction between the wind and the Earth’s surface and the influence of terrain. wha

what are sea breezes caused by?

cool and more dense air moving inland off the water

how are sea breezes created?

• once inland over the warmer land, the air heats up and rises

• currents push the air over the water where it cools and descends, starting the process over again

what does the development of thermals depend on?

solar heating

what is a front?

the zone transition (boundary) between two air masses of different density

-the area separating a high-pressure system and low-pressure systemand different temperatures

what is a high-pressure area?

an area of descending air

what is a low-pressure area?

an area of rising air

what color and shape is a warm front?

red semi circles

what color and shape is a cold front?

blue triangles

what color and shape is a stationary front?

alternating red semi-circles and blue triangles

what color and shape is an occluded front?

purple triangles and semi-circles

what is always present when flying across a front?

changes in weather conditions, including wind direction, temperature, and cloud formations

what is the most recognizable change when crossing a front?

change in temperature

what will typically occur after passage of a cold front?

clearing of the sky with gusty, turbulent, winds and cooling temperature

what are the three phases of a thunderstorm?

Cumulus, Mature, Dissipating

cumulus stage of a thunderstorm

The building stage of a thunderstorm characterized by the development of towering cumulus clouds as warm, moist air rises (updrafts)

mature stage of thunderstorms

time of greatest intensity when there are both warm updrafts and cool downdrafts (causing wind shear and turbulence)

-rain indicates the beginning of the stage

-rain descends through could and creates downdrafts by dragging air downwards

dissipating stage of thunderstorm

final stage of a thunderstorm when the downdrafts dominate, leading to a decrease in intensity and eventually the dissipation of the storm

what are thunderstorms created by?

cumulonimbus clouds

when do thunderstorms form?

-sufficient water vapor

-unstable lapse rate

-initial upward boost to start the process

what can thunderstorms produce?

wind shear turbulence , lightning, heavy rainfall, hail, and tornadoes

what is always found within and many miles away from the precipitation area?

adverse winds- crosswinds, gusts, and variable winds/sudden wind shifts

what should you not attempt to fly by regarding thunderstorms? why?

the anvil of the storm because there is still potential for severe and extreme clear air turbulence

what are the most severe thunderstorm conditions associated with?

squall line thunderstorms

squall line

nonfrontal narrow band of thunderstorms, usually ahead of a cold front

what should pilots anticipate with any thunderstorm, specially beneath the anvil of large cumulonimbus clouds?

hail

embedded thunderstorms

obscured thunderstorms that occur within larger weather systems, often hidden from view in very cloudy conditions

what are the two conditions required for structural icing?

1. flight through visible moisture

2. freezing temp or below

what usually causes the greatest accumulation of structural ice?

freezing rain

when do ice pellets form?

when rain droplets freeze at higher altitudes

when does frost form?

• when the collecting surface (wing) temperature is at or below the dew point of adjacent air

• and the dew point is below freezing

water vapor changes state through deposition and forms as ice crystals on the wing

what does frost do? how does this affect the plane?

disrupts smooth airflow over the airfoil by causing early airflow separation from the wind

-decreases life

-creates friction

-increases drag

what does frost make difficult/impossible?

takeoffs

-should be removed before attempting

lenticular clouds

almond- or lens shaped clouds usually found on the leeward side of a mountain range

what’s important to know about lenticular clouds?

-contain winds of 50kts or more

-appear stationary as wind blows through them

when should you expect mountain wave turbulence?

when air is stable and winds of 40kts or more blow across a mountain or ridge

what is located underneath each wave crest of air over a mountain?

a rotary circulation (rotor) where turbulence is the most severe in or below the rotor clouds

when can windshear occur?

at any altitude and be horizontal and/or vertical

-whenever adjacent air is flowing in different directions or speeds

wind shear

the rapid change in wind speed and/or direction over a short distance

when can you expect wind shear?

in a temperature inversion whenever wind speed at 2,000 to 4,000’ AGL is 25kts or higher

when is hazardous windshear expected?

-areas of low-level temperature inversions

-frontal zones

-clear air turbulence

when do you expect fog and/or low clouds to form?

when the temperature is within 5° of the dew point and the spread is decreasing

dew point

the temperature at which the air is 100% humid- saturated with water vapor, causing condensation to occur

what does air temperature determine regarding water vapor?

how much vapor the air can hold

when does water vapor become visible?

When it condenses into tiny water droplets, forming clouds, fog, or dew

evaporation

conversion of liquid to water vapor

sublimation

conversion of solid (ice) to gas (water vapor)

deposition

conversion of gas (water vapor) to solid (frost)

radiation fog (shallow fog)

a type of fog formed when heat radiates from the ground into the atmosphere, cooling the air to its dew point and causing moisture in the air to condense into tiny water droplets

when is radiation fog most likely to occur?

when there is clear sky, little/no wind, and small temperature/dew point spread

advection fog

a type of fog formed when warm, moist air moves over a cooler surface, causing the air to cool and moisture to condense into water droplets

upslope fog

a type of fog that forms when warm moist air is lifted up a slope, cooling as it ascends and causing condensation

precipitation-induced fog

a type of fog that forms when warm rain falls through cool air and evaporation from the precipitation saturates the cool air and forms fog

what is precipitation-induced fog associated with and is in proximity of?

fronts ; icing, turbulence, and thunderstorms

steam fog

a type of fog that forms in winter when cool air moves from land areas over warm water, causing steam to rise and condense into fog

what is steam fog composed of?

entirely of water droplets that often freeze quickly

what can occur in steam fog?

icing and low level turbulence

what are the four cloud families?

Cirrus, Cumulus, Stratus, Nimbus

what are the cloud families based on?

height

what clouds is the greatest turbulence found in?

Cumulonimbus

what are towering cumulus and what do they indicate?

early stages of cumulonimbus and indicate convective turbulence

what are the ingredients for cumulonimbus formation?

lifting action, unstable air, and moisture

what does nimbus mean?

rain cloud

what is the cooling rate and dewpoint rate of decrease when a convective current rises?

cooling: 5.4°/1,000’

dewpoint: 1°/1,000’

thus they converge at 4.4°F/1,000’

when do clouds form?

when dew point/temperature spread is 0°

how do you calculate the base of cumulus clouds?

surface temp/dew point spread / 4.4° + base of cloud in AGL

cirriform clouds

-high-level clouds (form about 20,000’)

-composed of ice crystals

-thin and white, create colors when sun is low on horizon

-occur in fair weather

-point in direction of air movement at their elevation

nimbus clouds

-rain clouds

-form between 7,000’ and 15,000’

-as clouds thicken and rian falls, bases of the clouds lower toward ground

cumuliform clouds

-fluffy white cottonball clouds

-show the vertical motion/thermal uplift of air taking place in atmosphere

-tops of clouds reach over 60,000’

what does a flat base of a cumuliform cloud indicate? what determines their height?

level at which condensation and cloud formation begins

-height determined by humidity of the rising air (the more humid, lower the cloud)

stratus clouds

• layer or blanket

• featureless low layer that can cover entire sky like a blanket of gray and dull weather

• cloud bases usually few hundred feet above ground

what are stratus clouds sometimes called? why?

fog- sometimes reach ground level when over hills or mountains

• as fog lifts off the ground due to daytime heating, the fog forms a lay of stratus clouds

characteristics of stable air

• stratiform clouds

• smooth air

• fair-to-poor visibility in haze/smoke

• continuous precipitation

• cool

• dry

characteristics of unstable air

• cumuliform clouds

• turbulent air

• good visibility

• showery precipitation

• warm

• humid

what happens when air is warmed from below?

rises and causes instability

lapse rate

the decrease in temp with increase in altitude

-air cools more the higher you go

as lapse rate increases….

air is more unstable

what can be used to determine the stability of air masses?

ambient lapse rate and types of clouds and precipitation

what does moist, stable air moving up a mountain slope produce?

stratus type clouds as it cools

what forms when unstable air rises?

turbulence and clouds with extensive vertical development

-such as cumulonimbus clouds.

what is usually an indication of a warm front? what does it result from?

steady precipitation preceding a front resulting from warm air being cooled from the bottom by colder air

what does warm air cooled from the bottom by colder air result in?

stabler air with stratiform clouds and little/no turbulence

temperature inversion

occurs when temperature increases as altitude increases

what do temperature inversions result in?

stabler layer of air

where does a temperature inversion usually develop?

near the ground on clear, cool nights when the wind is light

what causes a temperature inversion?

terrestrial radiation

what is usually found beneath a low-level temperature inversion?

smooth air with restricted visibility