Weather Flashcards
Reference pubs
AIM
FIH
Atmosphere composition
78% Nitrogen
21% Oxygen
1% other
Troposphere
-Adjacent to surface (28k-55k AGL) (36k’ US)
-temp dec w altitude
-winds inc w altitude
-nearly all weather occurs here
Pressure
Ambient static pressure is measured by weight of a column of air above Earth’s surface
Always dec w altitude
2 units for pressure
- Inches of Mercury (“ Hg)
- millibars (mb)
Standard pressure lapse rate
1” Hg lost per 1,000 ft alt
Pressure gradient force
As pressure is exerted on Earth’s surface it moves outward in either ascending or descending manner
Air moves from areas of high pressure to low pressure
PGF (high pressure)
Descending air
PGF (low pressure)
Ascending air
How far are isobars separated
4 mb pressure
Standards at sea level
29.92” Hg
15 C (59 F)
Temp lapse rate
2 C/1,000 ‘
Sea level pressure
SLP is the pressure read directly at sea level or calculated from known station pressure
Station pressure
SP is atmospheric pressure read directly at an airfield or specific altitude
Will always be less than equivalent sea level pressure when station is above sea level
SLP= SP+FE
Types of altitudes
Indicated
True (MSL)
Absolute (AGL)
Density
Pressure
Indicated altitude
Simply what altitude indicates
True altitude (MSL)
Height above mean sea level. This is standard altitude used
Absolute altitude (AGL)
Height above terrain below you
Density altitude
Not displayed on altimeter, calculated to determine the altitude the airplane “thinks” it’s at. Used for performance purposes
Pressure altitude
Altitude displayed when 29.92 is set in the Kollsman window (standard altitude setting in class A airspace 18k ft and above)
What does the altimeter measure
Measures the difference between the static pressure and pressure set in Kollsman window
Calibrated for 1,000 ft change for every 1” Hg diff
Indicated altitude will never match the true altitude whenever the setting in the Kollsman window is incorrect
How does pressure changes affect the altimeter
Change in pressure of 1” Hg will change the altimeter by 1,000 ft
Important to update altimeter settings throughout flight
“High to low - look out below”
“Low to high - plenty of sky”
High to low
MSL = assigned alt (-) alt error
AGL = MSL (-) field elevation
Indicated alt on deck = field elevation (+) alt error
Low to high
MSL = assigned alt (+) alt error
AGL = MSL (-) field elevation
Indicated alt on deck = field elevation (-) alt error
Effects of temp deviations on altimeters
If colder than standard atmosphere, altitude will indicate higher altitude
If warmer than standard atmosphere, alt will indicate lower alt
Air masses
Large body of air with relatively same temp and moisture across a horizontal plane
Named for temp relative to surface
Temp, location, moisture for naming
What causes circulation and how does it affect the air
Constant heating and cooling of the Earth’s surface leads to ascending and descending air
Dew point
Amount of moisture in air
Air is saturated when temp = dew point
Cold air holds less moisture
Dew point spread
As spread dec, moisture will begin to condense and become visible
Visible moisture forms frost, fog, clouds
Effects of temp deviations on altimeters
If colder than standard atmosphere, altimeter will indicate higher altitude (MSL lower)
If warmer than standard atmosphere, altimeter will indicate lower altitude (MSL higher)
What’s an air mass and what’re they named for
Large body of air w relatively same temp and moisture across a horizontal plane
Named for temp relative to surface,
temp, location, moisture for naming
Stability vs type of air mass
Cold air mass = unstable
Warm air mass = stable
What is weather the result of
Heat exchange
Compounded by spinning of earth (Coriolis effect)
What is relative humidity
Relative humidity is not direct measurement of the actual amount of water vapor/moisture, only the actual amount in the air compared to the amount that could be present if saturated
The closer you get to 100% more likely you’ll see visible moisture
Warmer air can hold more moisture than cold air
What are two key elements in pre flight planning
Temperature and dewpoint
Temperature inversion
Temperature normally decreases as you gain altitude. However sometimes temperature can increase as you gain altitude
What is pressure gradient force
Driving force of all wind
Descending air result in high pressure and flows outward away from high-pressure center
Ascending air result in low pressure and flows inward toward the low pressure center
“Out of the highs and into the lows”
Winds don’t travel the same direction pressure does due to Coriolis effect
What are gradient winds
Found above 2000 AGL and flow parallel to how the isobars are depicted on a surface chart
Clockwise around high-pressure centers
Counter clockwise around low pressure centers
What are surface winds
Heavily influenced by surface friction and found below 2000 AGL
Do not flow exactly parallel to the isobars due to surface friction
Surface winds around low pressure center will tend to turn towards center
Surface winds around high pressure center tend to turn away from center
What is a jet stream
Average height is 30,000 feet
generally flows west to east
Average speed is 100 to 150 kts
100 to 400 miles wide
1000 to 3000 miles long
3000 to 7000 feet thick
What is Seabreeze
Cool dense air over water moves over warm land during day
Air warms rises and returns over ocean
Usually 1500 feet to 3000 feet AGL and 15 to 20 kts
What is land breeze
Land cools faster than water so now water is warmer causing the cycle to reverse as result
What’s a front
Discontinuity between contrasting air masses
 specific discontinuities used to locate  classify fronts are:
Temperature, dew point, wind, pressure
“Touch downs win playoffs”