What is weather? The sun heats the earth unevenly, creating air masses (or regions of air) of varying density, and causing air to circulate around the globe. This heating, along with other factors, also develops areas of relatively high and low pressure. Air tends to flow from areas of high pressure to areas of low pressure, which we experience as wind.
High or low pressure is typically depicted on weather maps with the letters H and L. Generally, good weather is associated with highs, while poor weather is often found in the lows. If you watch the weather forecasts on television or download weather maps from the Internet you can compare where the Highs and Lows are positioned to where the rain or snow is falling - and often make a connection.
Wind causes air masses to move, and they encounter other air masses that have different characteristics. The boundary between two air masses is called a front. Weather along fronts may be hazardous to pilots because of the clouds, precipitation, and turbulent air (turbulence) that is can produce. A cold front is where a cold air mass displaces warmer air. A warm front is where warm air displaces colder air. Stationary fronts have no movement.
Temperature, wind, and ambient air pressure change as a front passes by.
Pilots watch the areas of high and low pressure and the movement of fronts to determine what the weather will be like for an upcoming flight. In particular, they look at the clouds, precipitation, wind, and convective activity associated with the weather patterns to make the decision to fly or not.
You know convective activity if you've ever witnessed a thunderstorm. Heating of the earth's surface also causes clouds to build if enough moisture is present in the air. When conditions are ripe, clouds tower in thunderstorms, sending heavy rain, hail strong gusty winds, and sometimes tornadoes, into the area covered by the storm. Thunderstorms are dangerous for airplanes because they combine a number of hazards into one area.
Precipitation, in the form of rain, snow or sleet, can determine whether or not a pilot is able to proceed. Heavy rain can reduce visibility. Snow can block engine air intakes, affecting the engine performance, water droplets adhere to the airplane in the form of ice, changing both the weight and the shape of the wings and other components, decreasing the airplane's ability to produce lift. Wind affects the airplane's speed over the ground, and also may create turbulence or wind shear. Turbulence caused by gusty winds can range from uncomfortable bumps to severe jolts that render the airplane uncontrollable.