Hail

hard balls or pellets of ice usually associated with CUMULONIMBUS cloud. The hail (stones) form by rain drops being taken into higher, colder regions. Upon falling the hailstone grows by the build-up of layers due to condensation of moist air on the cold hail.

Taken from Dictionary of Science

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Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 mm or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Hail is possible with most thunderstorms as it is produced by cumulonimbi (thunderclouds),[1] usually at the leading edge of a severe storm system. Hail is possible within 2 nautical miles (3.7 km) of its parent thunderstorm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm (similar to tornadoes) and lowered heights of the freezing level. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics. Hail formation is preferred during the summer months in the afternoon and evening hours of the day. Hailstorms normally last 3–15 minutes.

Unlike ice pellets, hail stones are layered and can be irregular and clumped together. Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 millimetre (0.039 in) thick, which are deposited upon the hail stone as it cycles through the cloud multiple times, suspended aloft by air with strong upward motion until its weight overcomes the updraft and falls to the ground. There are methods available to detect hail-producing thunderstorms using weather satellites and radar imagery. Hail stones generally fall at faster rates as they grow in size, though complicating factors such as melting, friction with air, wind, and interaction with rain and other hail stones can slow down their descent through Earth's atmosphere. Severe weather warnings are issued for hail when the stones reach a damaging size, as it can cause serious damage to man-made structures, and most commonly, farmers' crops. In the United States, the National Weather Service issues severe thunderstorm warnings for hail 1" or greater in diameter. This threshold, effective January 2010, marked an increase over the previous threshold of 3/4" hail. The change was made for two main reasons: a) public complacency and, b) recent research suggesting that damage does not occur until a hailstone reaches 1" in diameter.

Taken from Wikipedia

Mackerel Sky

a cloud pattern made up of wavy CIRRO- or ALTOCUMULUS with holes, suggesting the markings of a mackerel.

Taken from Dictionary of Science

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A mackerel sky is an indicator of moisture (the cloud) and instability (the cirrus-cumulus form) at intermediate levels (2400–6100 m, 8000-20,000 ft). If the lower atmosphere is stable and no moist air moves in, the weather will most likely remain dry. However, moisture at lower levels combined with surface temperature instability can lead to rainshowers or thunderstorms should the rising moist air reach this layer. In the winter it is often said to precede snowstorms and flurries. Mackerel skies are spoken of in the popular bywords, "Mackerel in the sky, three days dry," "Mackerel sky, mackerel sky. Never long wet and never long dry," and the nautical weather rhyme, "Mare's tails and mackerel scales / Make tall ships carry low sails." The phrase 'mackerel sky' came from the fact that it looks similar to the markings of an adult king mackerel.

Taken from Wikipedia

Warm Front

the edge of a mass of warm air advancing and rising over cold air. Cloud develops in the rising air with heavy NIMBOSTRATUS forming as the front passes. There is usually heavy rain associated with the front. The temperature rises after the front with the rain clearing and often there is a change in wind direction.

Taken from Dictionary of Science