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Thunderstorms over the ocean are most common during the night and early morning. They frequently occur offshore when a land breeze is blowing toward the water. The cool land breeze is heated by the warmer water surface, which results in sufficient convection to produce thunderstorms. After sunrise, heating of the land surface reverses the airflow (sea breeze). The thunderstorms then dissipate over the water, but they may re-form over the warmer land surface. As an example, the air mass weather that exists in Florida combines both types of convective thunderstorms. Circulation around a semipermanent high pressure system off the southeastern United States (Bermuda high) carries moist ocean air over the warm land surface of the Florida Peninsula. At night, thunderstorms off the Florida Coast are caused by the warm water of the Gulf Stream heating the surface air, while the upper air is cooling by radiation to space. This heating from below produces thermal convection over the water. When the sun rises, the heat balance necessary to maintain storm formation over the water is destroyed. By day, the storms appear to move inward over the land areas, but actually dissipate off the coast and re-form over the hot landmass. The heated land surface sets up an unstable lapse rate over the Peninsula and causes storm development to continue until nocturnal cooling occurs. Usually, convective type storms are randomly distributed and easily recognized.
Orographic thunderstorms will form on the windward side of a mountain if conditionally unstable air is lifted above the level of free convection. The storm activity is usually scattered along the individual peaks of the mountains. Occasionally, however, this activity may form a long unbroken line of storms similar to a squall line. The storms persist as long as the circulation causes upslope motion. From the windward side of the mountains, identification of orographic storms may sometimes be difficult because the storm clouds are obscured by other clouds (usually stratiform). Almost without exception, orographic thunderstorms enshroud mountain peaks or hills.
Minimum Factors
The minimum factors essential to the formation of a thunderstorm are conditionally unstable air with relatively high moisture content and some type of lifting action. Lifting of warm air will not necessarily cause free convection. The air may be lifted to a point where the moisture condenses and clouds form. These cloud layers, however, will be stable if the level of free convection has not been reached by the lifting. Conversely, it is possible for dry heated air to rise convectively without the formation of clouds. In this condition, turbulence might be experienced in perfectly clear weather. Cumulonimbus cloud formations require a combination of conditionally unstable air, some type of lifting actions, and high moisture content. Once a cloud has formed, the latent heat of condensation released by the change of state from vapor to liquid tends to make the air more unstable.
Some type of external lifting action is necessary to bring the warm surface air to the point where it will continue to rise freely (the level of free convection). For example, an air mass may be lifted by thermal convection, terrain, fronts, or convergence.
Summary
By no means is the information contained here a complete discussion of all the weather information and factors affecting balloon flight. There are many resources available, both
4-28
Figure 4-27. This map can be used to gather hourly AWOS and ASOS type weather information from various sites in each state.
through government and private agencies, which may be of value to the pilot in planning a flight. A pilot should take the time to explore the internet, read weather books, and gain a complete understanding of the myriad of weather information and products that are available.
The second section of this chapter will expose the reader to some of the weather reporting products available, both through FSS briefings and Internet searches. With the knowledge gained from the first half of this chapter, the pilot will be able to make a good interpretation of the reports, and determine how present and future conditions will affect the decision to fly.
How to Obtain Weather Information
An integral part of flight preparation for any pilot is checking the weather conditions expected to occur during the flight. FAA regulations place the responsibility for flight planning on the pilot. To effectively plan a flight, a pilot needs to understand what weather information is available, how to obtain it, and how it can be applied to a flight.
While weather forecasts are not 100 percent accurate, meteorologists, through careful scientific study and computer modeling, have the ability to predict the weather patterns, trends, and characteristics with increasing accuracy. Through a complex system of weather services, government agencies, and independent weather observers, pilots and other aviation professionals receive the benefit of this vast knowledge base in the form of up-to-date weather reports and forecasts. These reports and forecasts enable pilots to make informed decisions regarding weather and flight safety.
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Balloon Flying Handbook(57)
