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800-WX-BRIEF, and a variety of internet sites that specialize
in local and regional weather.
Local Conditions
In gathering weather information for a fl ight, obtain current
and forecast conditions where fl ying, as well as alternate
airports in case landing at the intended destination is not
possible. These conditions should include wind (surface and
winds aloft), moisture, stability, and pressure.
Surface wind predictions and observations can be looked
at with a number of internet resources. The National
Weather Aviation service provides observations (METAR)
and forecasts (TAF) for areas with weather reporting
capabilities.
Winds aloft are forecast winds at higher altitudes than the
surface for locations throughout the United States. Refer
to the Pilot’s Handbook of Aeronautical Knowledge for an
understanding of the winds and temperatures aloft tables.
Winds aloft, too, are important for flight planning and
safety.
5-4
COLD AIR BELOW
WARM AIR ABOVE
CALM BELOW
Turbulence where
the strong winds mix
with the calm air
STRONG WINDS ABOVE
Figure 5-5. Typical morning inversion layer—calm cold air is below; high winds are above.
A typical situation during morning hours is cold air from the
night settling, creating calm winds at the surface with the
winds aloft (300 to 3,000 feet) at 30 knots. As the surface
begins to warm from the sun, the cold surface air starts to
warm and rise, allowing the high winds from above to mix
and lower to the surface. The wind sheer area in between the
high winds above and calm winds below is usually turbulent
and can overwhelm aircraft or pilot capabilities. Therefore,
it is a dangerous practice to look only at the wind sock for
surface winds when there could be strong winds above. Winds
aloft must be evaluated for safe fl ight. [Figure 5-5]
During initial solo fl ights, the wind should be relatively calm
to fl y safely. As experience is gained, pilot wind limitations
can be increased. It is not until the pilot has had dual training
in crosswinds, bumpy conditions, and signifi cant pilot in
command (PIC) time soloing in mild conditions that pilot
wind conditions should approach the aircraft limitations. A
safe pilot understands aircraft and personal limitations.
Moisture in the air has a signifi cant effect on weather. If
the relative humidity is high, the chance of clouds forming
at lower altitudes is more likely. Clouds forming at lower
altitudes create visibility problems that can create Instrument
Meteorological Conditions (IMC) in which the visibility is
below that required for safe fl ight. The temperature-dew point
spread is the basis for determining at what altitude moisture
condenses and clouds form. It is important to be particularly
watchful for low visibilities when the air and dew point
temperatures are within a spread of three to four degrees.
The closer these temperatures are to each other, the greater
the chance for fog or clouds forming with reduced visibility
conditions. Consider a scenario where the destination airport
currently has a temperature-dew point spread of 4 °F, and it
is evening when the atmosphere is cooling down. Since the
temperature-dew point convergence rate is 4.4° for every
thousand feet, the clouds/ceiling would be about 1,000
feet above ground level (AGL). Since it is cooling down,
the temperature-dew point spread is decreasing, lowering
the cloud level. Therefore, the 1,000 foot AGL ceiling is
lowering, creating IMC conditions that are not safe. For this
scenario, the fl ight should not be attempted.
Air temperature and humidity directly affect the performance
of the WSC wing and engine. The higher the temperature,
humidity, and actual altitude of the operating fi eld, the greater
role density altitude plays in determining how much runway
the WSC aircraft needs to get off the ground with the load
5-5
on board, and how much climb performance is required once
airborne. The WSC aircraft may have cleared the obstacle
at 8 a.m. when the weather conditions were cooler with less
humidity; at 1 p.m. with increased air temperature and higher
humidity levels, the pilot must reevaluate the performance
of that same aircraft. A full understanding of density altitude
is necessary to be a safe WSC pilot; refer to the Pilot’s
Handbook of Aeronautical Knowledge for density altitude
and weight effects on performance.
The rate of temperature decrease with increased altitude
determines the stability of the air. The stability of the air
determines the vertical air currents that develop during the
　
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