A4-11. The pressure tendency with the passage of a slow moving cold front is indicated
by a steady or unsteady fall prior to frontal passage, followed by weak rises after
A4-12. The slope of a fast moving cold front is usually 1:40 to 1:80 miles.
A4-13. Prefrontal squall lines form about 50 to 300 miles in advance of fast-moving cold
A4-14. The average speed of a warm front is usually between 10 and 20 knots.
A4-15. The cloud types in advance of a warm front, in order, are cirrus, cirrostratus,
altostratus, nimbostratus, and stratus.
A4-16. The difference between warm and cold occlusions is that warm occlusions form
when the air in advance of the warm front is colder than the air to the rear of the
cold front. A cold occlusion forms when the cold air in advance of a warm front is
warmer than the cold air to the rear of the cold front.
A4-17. The most violent weather associated with an occlusion occurs near the apex or
tip of the occlusion.
A4-18. When a stationary front moves, the speed is normally less than 5 knots.
A4-19. The weather associated with an unstable stationary front depends on the frontal
slope. Severe thunderstorms and heavy rain showers usually occur with steep
slopes. Broad or extensive areas of showers, fog, and reduced visibility occur
with shallow slopes.
A4-20. The modifications of fronts are caused by movement and orographic effects.
A4-21. When a cold front moves off the eastern coast of the United States, it intensifies
and waves develop along the frontal boundary.
A5-1. Rain is precipitation that reaches the ground as water droplets and the droplet
size measures .5 mm or greater. Drizzle is very small and appears to float with
the air currents and the droplet size measures less than .5 mm.
A5-2. The altitude range of cloud occurrence in the tropics is from sea level to 60,000
A5-3. The altitude range of middle clouds, in the temperate regions, is from 6,500 to
A5-4. Sea fog occurs when the wind brings moist, warm air over a colder ocean
current. Steam fog is caused by saturation of the air through the evaporation of
water, when cold air moves over warm water.
A5-5. Blowing spray occurs when the water droplets are lifted in such quantities that
they reduce visibility to six miles or less at eye level.
A5-6. Haze appears as a bluish tinge when viewed against a dark background and a
dirty yellow or orange tinge when viewed against a bright background. Smoke
appears as a reddish tinge when viewed against the solar disk during sunrise and
A5-7. Dust devils are usually observed on clear, hot afternoons in desert regions.
A5-8. The two sources of light are natural and artificial.