ahead of the storm center at all altitudes within the cloud. Because of the strong up and
downdrafts associated with a thunderstorm, there is always a possibility of severe low-
level wind shear.
The electricity generated by a thunderstorm is rarely a great hazard to an aircraft from
the standpoint of its airframe, but other lightning hazards include temporary blindness,
damage to navigational and electronic equipment, and punctures to the aircraft's skin.
Lightning occurs within the cloud, including the anvil portion, and is most frequent near
the freezing level. Lightning also occurs between adjoining clouds and between the
cloud and the ground. Although lightning frequently exits the cloud base, it may exit the
side of the cloud and strike the ground up to 12 miles away from the cloud.
Both rime and clear icing may be encountered. Clear ice accumulation in thunderstorms
above the freezing level can be so rapid that an aircraft may become incapable of
maintaining level flight.
Effect on Altimeters
Pressure usually falls rapidly with the approach of a thunderstorm. It raises sharply with
the onset of the first gusts and the arrival of the cold downdraft and heavy rain showers.
The pressure then falls back to the original pressure as the rain ends and the storm
moves on. This cycle of pressure change may occur in 15 minutes. Of greatest concern
are pressure readings that are too high. If you issue an altimeter setting to an arriving
aircraft during the peak of a storm, the aircraft could be too high on its approach.
A significant hazard associated with thunderstorm activity is the rapid change in surface
wind direction and speed immediately prior to storm passage (first gust). The strong
winds at the surface that accompany thunderstorm passage are the result of the
horizontal spreading out of downdraft currents from within the storm that occurs as
these currents approach the surface of the earth. The total wind speed is a result of the
downdraft divergence plus the forward velocity of the storm cell. Thus, the wind speeds
at the leading edge of the cell are greater than those at the trailing edge. During the
passage of a thunderstorm cell, winds shift and become strong and gusty. Also, wind
speeds occasionally exceed 55 knots.
WEATHER OBSERVATION CODES AND PHRASEOLOGY
Accurate knowledge of developing weather conditions within your airfield or ship's area
of concern is critical to flying safety. You will base many operational decisions on the
current and forecast weather situations.