WEATHER
General Structure of the Atmosphere
2.1 Characteristics of the troposphere, tropopause, and stratosphere &
2.2 The flight characteristics associated with the troposphere,
tropopause, and stratosphere.
Troposphere: the layer adjacent to the earth’s surface; varying in
height from 55,000’ at the equator to 28,000’ at the polls.
Atmosphere becomes less dense with altitude and roughly 50% of it, by
weight lies below 18,000’ and 90% below 53,000’.
 The temperature decreases with altitude.
 An abrupt change in the lapse rate signals the start of the tropopause.
 Large amounts of moisture and condensation nuclei are found.
 Most weather systems are within this boundary
Tropopause: the transition zone between the troposphere and stratosphere
 Temperature is constant with altitude.
 Winds normally increase in speed with altitude below the tropopause.
 Contrails form and persist since it is normally the coldest areas within the
lower atmosphere at –57 C
 Water vapor and large scale vertical currents do no exist to any extent
Stratosphere: has increasing temperature with altitude due to the ozone layer
which plays a major part in heating the air
 Flying in the stratosphere is generally smooth with excellent visibility.
 The air is thin and offers very little resistance to flight.
 Above these are the Mesosphere and the Thermosphere
2.3 Identify the six weather elements that a pilot may encounter
1. Temperature
2. Atmospheric Pressure
3. Wind
4. Humidity
5. Clouds
6. Precipitation
Memorize as TAW PRECH
2.4 Identify the six primary hazards of flight
1. Turbulence
2. Thunderstorms
3. Wind Shear
4. Icing

5. Low Ceilings
6. Low Visibility
Atmospheric Temperature and Pressure
2.5 Define specific heat and how it effects the warming of the earth
Is the amount of heat required to raise the temperature of one gram of a
substance one degree Celsius. The specific heat of a substance is constant and can
be thought of as its heat capacity.
During the day a land surface will heat faster and to a much greater extent than
will a water surface. It will also cool at night much faster and to a greater extent
than water. The specific heat of water is about four times that of most land
surfaces
2.6 State the primary source for all weather
The sun heats the earth during the day. Insolation is the total radiation reaching
the earth’s surface and it is the primary source for all weather phenomena on
earth.
2.7 Define Lapse Rate and
2.8 State the average lapse rate in degrees Celsius
Lapse rate is the change in atmospheric temperature with the increasing altitude.
It is 2 C / 1000’
2.9 List and define lapse rates: steep, shallow, isothermal, and inversion relative to
the standard lapse rate
STEEP 3 C / 1000’
SHALLOW 1.5-3 C/1000’
ISOTHERMAL no change with altitude
INVERSION temperature increases with altitude
2.10 Define atmospheric pressure
The pressure exerted on a surface by the atmosphere due to the weight of a
column of air directly above that surface. It always decreases with altitude. The
higher you are the faster it decreases.
2.11 State the standard units of pressure measurement
Inches of Mercury and millibars. It varies from 28 in-HG to 31 in-HG.
In the lower altitude, 1000’ increases in altitude will result in a pressure decrease
of 1 in-HG (34 mb)
2.12 Differentiate between sea level pressure and station pressure
SLP is the pressure at mean seal level (MSL) measured directly at sea level or
calculated if the station is not at sea level
Station Pressure is the atmospheric pressure at the airfield or station
MSL is used as a reference level for all surface pressure observations
2.13 Define the Standard Atmosphere to include temperature and pressure
29.92 in-HG at 59 F
1013.2 mb at 15 C
2.14 List the major items found on the surface pressure/analysis chart
 Isobars – lines of equal barometric pressure
 Highs- where the pressure in the center is higher than surrounding areas
 Lows – where the pressure in the center is lower than surrounding areas
 Ridge – is an extension of high pressure area
 Trough – is an extension of low pressure area
2.15 Explain Pressure Gradient
The rate of pressure change in a direction perpendicular to the isobars and can be
steep (strong) or shallow (weak) depending on the distance the isobars are apart.
It is the initiating force for all winds!
2.16 Define Indicated Altitude, Calibrated Altitude, Mean Sea Level, Altitude,
Above Level Ground Altitude, Pressure Altitude, Density Altitude
 Indicated Altitude – the altitude read on the altimeter when the current
local altimeter setting is displayed in the Kohlsman window
 Calibrated Altitude – the indicated altitude corrected for instrumental
error
 Mean Sea Level, Altitude – the actual distance above mean sea level and is
found by correcting calibrated altitude for temperature deviations from the
standard atmosphere
 Above Level Ground Altitude – the aircraft’s height above the terrain
directly beneath the aircraft and is measured in feat above ground level
 Pressure Altitude – the height above the standard datum plane where
barometric pressure is 29.92 in-Hg
 Density Altitude – the altitude in the standard atmosphere that has the
same density as the local air. It is found by correcting the pressure altitude
for non standard temperature deviations (gives an idea of engines
performance)
2.17 Describe the effects of pressure changes on aircraft altimeters
A change of .1 in-Hg will change the altimeter reading by 100’
If going from H to L, the aircraft will be lower than altimeter indicates.
If going from L to H, the aircraft will be higher than altimeter indicates.
2.18 State the effects temperature deviations from the standard lapse rate on
aircraft altimeters
If the air is colder than standard atmosphere (H to L temps), the aircraft will be
lower than altimeter indicates, and if the air is warmer than standard (L to H), the
aircraft will be higher than the altimeter reading. For every 11 C that the
temperature varies, the altimeter will be in error by about 4%.
2.19 Calculate MSL altitude, AGL altitude, and the altimeter error resulting from a
change in atmospheric pressure
IF you fly at 5000’ from a station that is 500’ and 30.2 in-HG to a station that is
800’ and 30.0 in-HG… do the following
1. Moving from high to low pressure so look out below!
2. 30.2 – 30.0 = .2 in-Hg
3. .2 * (1" = 1000’) = 200’
4. Actual MSL will be 5000’-200’ = 4800’
5. The airport would indicate at the current settings on the altimeter as 800’ +
200’ = 1000’
6. And the AGL is 4800 – 800 or 4000’
2.20 Identify the factors in Wind Circulation
The recurring movement of air relative tot he earth’s surface. It is created
primarily by the large temperature difference between the tropics and the polar
regions, and complicated by uneven heating of the land and water areas by the sun.
The circulation is caused by:
1. Irregular distribution of oceans and continents
2. The relative effectiveness of differing surfaces in transferring heat
to the atmosphere
3. Irregular terrain
4. Daily variations in temperature
5. The changes of seasons
6. Other factors
2.21 Identify the forces affecting wind direction
Isobars determine the direction of wind. Wind wants to move from High to Low
pressure systems. As the distance increase, the forces are affected by the
Coriolis Force, friction, gravity and the pressure gradient force.
2.22 Explain Coriolis Force and its apparent effect on wind
If a pocket of air moves northward from a lower latitude to a higher latitude,
because of its greater speed, it will move to the right of the point directly north
of its initial starting point. This affects any migrating pocket of air moving North
or South. A pocket of air moving south would encounter lag because it is entering a
plane with a greater rim speed than the plane it left. Because of the lag, it would
encounter the effect of a relative deflection to the right.
As wind speed increases, so does the strength of the Coriolis force.
2.23 In accordance to the Tri-Cellular Theory describe the location of the semipermanent high and low pressure centers
The atmosphere is a large heat engine. It takes heat from the sun and converts
the air about the surface causing wind. With the addition of the Coriolis Force, a
single hemispheric cell breaks down into three cells: the tropical, the polar, and the
less distinctive temperate or midlatitude cell.
2.24 State the three major wind belts in the northern hemispher that result from
the Tri-Cellular Theory
 Polar Easterlies – the cold dense high pressure system forms on the North
Pole. This air spreads from the pole towards the equator and is deflected to
the right resulting in a shallow layer of wind and form a belt of low pressure
at 60 N
 Prevailing Westerlies – the norther moving component of wing from the 30
N Lat is deflected to the right. It is one of westerly winds throughout the
troposphere.
 NE Tradewinds – The southerly moving component is deflected west
becoming the very persistent NE widns from 30 N to the Equator
Read 2.3-9
2.25 Explain and identify gradient winds with respect to the isobars around high
and low pressure systems in the Northern Hemisphere
Air flows clockwise around High Pressure systems (anti-cyclonic) and counterclockwise around Low Pressure systems (cyclonic)
See figure 2.3-6
2.26 Explain and identify the surface wind direction with respect to the gradient
winds in a high and low pressure system in the Northern Hemisphere
Surface friction reduces the speed of the wind. Within 2000’ of the ground, the
wind is force to go across the isobars from high pressure to low pressure.
2.27 State the direction of the wind flow associated with high pressure and low
pressure systems
2.28 Define Buys Ballot’s Law and describe its effect on an aircraft flying towards
the center of a high or low pressure system
If the wind is at your back, the area of lower pressure will be to your left. When
standing on the earth’s surface the low will be slightly forward of directly left
because the winds flow across the isobar
If you are facing toward a low, the wind will blow from your left to your right. So if
you’re flying towards a low, you will drift you your right!
2.29 Describe a Jet Stream
A narrow band of strong winds found most often in the vicinity of the tropopause.
These winds average about 100-150 knots and may reach up to 250 knots. It must
be 50 knots or greater to be a jet stream and core area must extend over
considerable length.
2.30 Describe Land and Sea Breezes
Breezes created by the constant temperature surface of water’s body and the
changing temperature of the land. During the day, the land is hot… causing uplift
and pulling colder air off the ocean (Sea Breeze). During the night, the land is
cooler causing an uplift of air in the ocean and the air moving from land to sea
(Land Breeze)
2.31 Describe Valley and Mountain Winds
During the day, mountain slopes heat through conduction. The air becomes warm
and rises up to the colder slopes with higher altitude causing valley winds. At night,
the air in contact with the slop is cooled and becomes denser than surrounding air
and sinks causing Mountain winds.