Weather is an all-encompassing term used to describe all of the many and varied phenomena that can occur in the atmosphere of a planet. The term is normally taken to mean the activity of these phenomena over short periods of time, usually no more than a few days in length. Average atmospheric conditions over significantly longer periods are known as climate. Usage of the two terms often overlaps as the concepts are obviously very closely related.
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.
Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes stationary, and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.
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Recent and ongoing weather
This week in weather history...
May 6
1965: A series of violent tornadoes affected the Twin Cities area of Minnesota.
1975: An F4 tornado struck Omaha, Nebraska, causing more than $1 billion (1975 USD) in damage.
May 7
1840: The Great Natchez Tornado struck Natchez, Mississippi, killing at least 317 people.
May 10
1933: A tornado killed almost every resident of the small town of Beatty Swamps, Tennessee. Because of the destruction, the town was abandoned permanently.
May 11
1953: An F5 tornado destroyed areas of downtown Waco, Texas, killing 114 people.
1970: Seventeen years later, another F5 tornado struck downtown Lubbock, Texas, killing 26 people. It was the only F5 tornado in history to strike a skyscraper, which had its steel infrastructure twisted by the storm.
May 12
1997: An F1 tornado affected downtown Miami, Florida, causing minor damage.
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