PT4 CHP13 Weather Theory Flashcards
Why is it important to understand various weather phenomenon?
- It will help you avoid dangerous situations
- Help you understand the factors that affect aircraft performance
What are the two factors that affect the weather?
Pressure and temperature
Which layer of the atmosphere does most the majority of weather occur?
Troposphere (0 to 20,000 feet)
What occurs in the troposphere?
The majority of weather
In the troposphere, what is the average temperature decrease and at what rate?
-2°C every 1,000 feet
What causes circulation of air masses (air movement)?
The uneven heating of the Earth’s surface
Why does warm air rise?
It is less dense
Why does cold air sink?
It is more dense
What type of air rises and what type of air sinks?
Warm air rise (less dense), cold air sinks (more dense)
Due to warm air being less dense and cold being more dense, what does each air temperature do?
- Warm air, rise
- Cold air, sinks
What is the Coriolis Force?
As the Earth rotates on its axis, air is deflected upward and outward.
- Greatest at the poles and zero at the equator
How much does an air mass 1 sq. in. by 1 sq. in. at sea level?
14.7 PSI
What occurs when there is a change in pressure or change in temperature?
There will be a movement if air
In what direction(s) can air movement be?
Horizontal and vertical
What is horizontal air movement known as?
Wind
What is vertical air movement known as?
Convective current
How is wind created?
The changes in pressure on the surface of the Earth
- air molecules slide down from high pressure to low pressure, like a person going down a ski slope
- depending on how steep the slope is will determine the wind velocity
The changes in pressure on the surface of the Earth creates what?
Wind
What is are isobars?
Areas of equal pressure lines, which help predict wind direction and speed
Areas of equal pressure lines, which help predict wind direction and speed are known as what?
Isobars
What are isobars usually expressed in?
Millibars
What do widely spaced isobars create?
Shallow pressure gradient
What is a result of a shallow pressure gradient?
light winds
What do closely spaced isobars create?
steep pressure gradient
What is a result of a steep pressure gradient?
strong winds
In the Northern Hemisphere, an area of high pressures does what to the wind in which direction?
Pushes the wind down in a clockwise direction
In the Northern Hemisphere, an area of low pressures does what to the wind in which direction?
Suck every thing up from in a counterclockwise direction
Are connective current influence by the type of surface?
Yes
- Rocks, sand, barren land, concrete (parking lots) give off large amount of heat
- Water, trees, and other vegetation tend to slowly absorb heat
What surfaces tend to slowly absorb heat?
Water, trees, and other vegetation
- Cool air sinks
What surfaces give off large amounts of heat?
Rocks, sand, barren land, concrete (parking lots)
- Warm air rises
Describe the circulation of air during the day that creates a sea breeze
- The water will take longer to heat up
- The land will reflect more heat than the water
- Over the land, during the day, hot air will rise and be attracted to the cold air over the water
- This creates the airflow known as sea breeze
Describe the circulation of air at night that creates an opposite flow
- The ground will cool off quicker than the water
- The water will retain its temperature for longer, which will result in warmer temperatures than the land, reverse flow
What is low level wind shear?
- Sudden, drastic change in wind speed and/or direction over a very small area
- Occurs at any altitude but dangerous when occurring at low altitude
- Associated with passing frontal systems, thunderstorms, temperature inversions and strong upper level winds
- Microbursts are a form of LLWS that can last 5-15 minutes and create downdraft up to 6,000 feet/min with wind changes of 30-90 knots
A sudden, drastic change in wind speed and/or direction over a very small area is known as what?
Low level wind shear
At what altitude does low level wind shear occur?
Occurs at any altitude but dangerous when occurring at low altitude
Low level wind shear is associated with what?
Associated with passing frontal systems, thunderstorms, temperature inversions and strong upper level winds
Describe the process of low level wind shear can affect the aircraft.
- On takeoff, the aircraft will be hit with a headwind, which will provide better performance
- At the middle of the LLWS, the downdraft will push the aircraft down to an area that has a tailwind
- At this point, the pilot will have a tendency to pitch the aircraft up closer to the critical angle of attack; this may result in a stall
How is stability (of an air mass) defined?
The air mass’ ability to resist vertical motion
How is stability (of an air mass) determined?
Moisture, temperature and lapse rate
What type of air mass is very stable?
Cool, dry air
What type of air mass is very unstable?
Warm, humid air (think of Florida in the summer)
What is the lapse rate of the stability of an air mass?
The speed at which the air mass will cool off
- heating from below decreases stability
- cooling from below increases stability
Define the term adiabatic.
the process by which an air mass cools as it is raised into the atmosphere
Define the term lapse rate.
the rate at which an air mass cools as it is lifted
Define the term adiabatic lapse rate
how quickly or slow an air mass will cool off as it is lifted
Define the term sublimation
transition of a substance from solid to gas (without liquid phase) add or removes moisture in the air
How does sublimation affect moisture in the air?
It will add or remove moisture in the air
Define the term evaporation
transition from liquid to vapor
(adds moisture to air)
How does evaporation affect moisture in the air?
It adds moisture to the air
Define the term condensation
transition from vapor to liquid
(removes moisture from air)
How does condensation affect moisture in the air?
removes moisture from air
What is the dry adiabatic lapse rate?
- An unsaturated parcel cools at a rate of 3°C/1,000 feet
- ## The dew point cools at a rate of 0.5°C/1,000 feet
If the dry adiabatic lapse temperature and dew point temperature are the same, what occurs?
- This will result in 100% saturation
- As the air mass is fully saturated (100%), then the dry adiabatic lapse rate will convert to a moist adiabatic lapse rate
What is the moist adiabatic lapse rate?
- An saturated parcel cools at a rate of 1.2°C to 3°C/1,000 feet
- The new dew point cools at the same rate as the saturated parcel
When the moist adiabatic temperature and dew point temperature match and cool at the same rate, how will that affect the saturation of the parcel?
The temperature and dew point are equal and will result in 100% saturation, which create visible moisture (clouds)
When is an air mass considered to be stable?
A stable air mass makes vertical development difficult/resists vertical motion
When is an air mass considered to be unstable?
An unstable air mass has turbulent air and convective activity (Florida in the summer)
What is absolute stability?
