$_{Figure 4-6.}$  A cold front underrunning warm, moist, unstable air. Clouds are cumuliform with possible showers or thunderstorms near  the surface position of the front. Convective clouds often develop in the warm air ahead of the front. The warm, wet ground behind the  front generates low-level convection and fair-weather cumulus in the cold air. Fast-Moving Cold Front Warm Front Fast-moving cold fronts are pushed by intense pressure  systems far behind the actual front. The friction between the  ground and the cold front retards the movement of the front  and creates a steeper frontal surface. This results in a very  narrow band of weather concentrated along the leading edge  of the front.  [Figure 4-7]  If the warm air being overtaken by  the cold front is relatively stable, overcast skies and rain may  occur for some distance ahead of the front. If the warm air  is unstable, scattered thunderstorms and rain showers may  form. A continuous line of thunderstorms, or squall line,  may form along or ahead of the front. Squall lines present  a serious hazard to pilots as squall type thunderstorms are  intense and move quickly. Behind a fast-moving cold front,  the skies usually clear rapidly and the front leaves behind  gusty, turbulent winds and colder temperatures. A warm front is actually the trailing edge of a retreating  mass of cold air. A warm front occurs when a warm mass  of air advances and replaces a body of colder air. Warm  fronts move slowly, typically 10 to 25 mph. The slope of  the advancing front slides over the top of the cooler air and  gradually pushes it out of the area. Warm fronts contain warm  air that often has very high humidity. As the warm air is lifted,  the temperature drops and condensation occurs. Prior to the  passage of a warm front, cirriform or stratiform clouds, along  with fog, can be expected to form along the frontal boundary.  [Figure 4-8]  In the summer months, cumulonimbus clouds  (thunderstorms) are likely to develop. Light to moderate  precipitation is probable, usually in the form of rain, sleet,  snow, or drizzle, punctuated by poor visibility. The wind  blows from the south-southeast, and the outside temperature  4-8 $_{Figure 4-5.}$  A cold front underrunning warm, moist, stable air. Clouds are stratified and precipitation continuous. Precipitation induces  stratus in the cold air. St. Louis Indianapolis 200 miles Columbus 400 miles Pittsburgh 600 miles COLD AIR WARM AIR NIMBOSTRATUS ALTOSTRATUS CIRROSTRATUS CIRRUS St. Louis Indianapolis 200 miles Columbus 400 miles Pittsburgh 600 miles WARM AIR C O L D A I R CUMULONIMBUS