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 12
1997: An F1 tornado affected downtown Miami, Florida, causing minor damage.
May 13
1980: A strong tornado devastated downtown Kalamazoo, Michigan.
May 18
1902: A tornado killed 114 in Goliad, Texas.
May 19
1780: An unusual darkness, lasting from noon until after sunset, fell over parts of New England.
May 20
1916: A tornado struck the town of Codell, Kansas. Incredibly, the same town would be hit by two more tornadoes on May 20, 1917 and May 20, 1918.
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