Page 273 - NS-2 Textbook

268 NAUTICAL SCIENCES

COLD FRONTAL SURFACE

COLD AIR
COLDEST AIR
! l':It---O(:CLUDED FRONTAL SURFACE

COLD AIR

COLDEST AIR

WARM-FRONT OCCLUSION
Two types of occluded fronts. In the upper drawing of a cold-front occlusion, the cold air behind the (old front is colder than the cold air
ahead of the warm front, pushing the warm front aloft. In the warm-front occlusion on the bottom, the cold air ahead of the warm front is
colder than the cold air behind the cold front, forcing the cold front aloft. From Kotsch Weather for the Mariner, 133

friction, these downdrafts take the place of updrafts and A thunderstorm is most turbulent in the area of
spread out. The entire lower portion of the cloud be- heaviest precipitation. Icing will often occur just above
comes downdraft, and the high winds in the upper alti- the freezing level, making this a very hazardous area for
tudes flatten the top of the cloud into an anvil shape. an"craft.
Rain is now falling heavily on the ground, but the storm The leading gust of wind, sometimes called a mi-
will dissipate in a short time. croburst, is one of a thtmderstorm's dangers. This gust oc-
curs just prior to the passage of the storm. The strong
winds at the surface are the result of the horizontal
WEATHER PHENOMENA WITH
spreading-out of the storm's downdraft currents as they
THUNDERSTORMS
approach the surface of the Earth. The speed of the first
Rain is found in every thunderstorm. Some of this rain gust usually is the highest recorded during any thunder-
will form in regions below freezing level. Hail will form storm and can blow in any direction--€ven in opposition
if the updrafts carry melted or partially melted raindrops to the sm-face wind that is "pushing" the storm. Such con-
into the higher, colder altitudes. Snow and ice crystals ditions can result in u,vind shear/' "which is very danger-
also may be in any thunderstornl, ·winter or sumnlel~ ous to aircraft in the process of taking off or landing.
though in summer they will fileH into rain vvhen nearing Surging air currents in the thunderhead cloud create
the Earth. static electricity, the source of lightning. This process is