Hail is typically formed in the strong upward motion of a thunderstorm, as the updraft physically supports the ice particle as it accretes or gathers ice on its surface. Without the strong updraft which typically does not occur in the winter months, the ice particle simply falls to the ground without growing large enough to become hail.
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Radar works by particles in the air reflecting the beam back to the source and is a ‘line-of-sight’ technology. If there are mountains between the radar source and the observer, the radar beam has to be tilted upwards at an angle to ‘see’ the observer’s weather. Any weather occurring below this tilted beam will be missed.
This is less important in the summer months because the clouds and storms tend to be taller. In the winter, the satellite infra-red view is far more instructive.
This is a direct result of an outlfow boundary undercutting the storm and physically pushing the rain and hail particles around at the lowest levels.
A downburst is created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. There may or may not be rain reaching the ground. It is a threat to aviation since it can suddenly change relative air speed of an airplane as it encounters a localized headwinds or tailwinds, suddenly increasing or decreasing lift.
These are patterns in the upper atmosphere that are directly related to the weather near them. The upper atmosphere is dominated by several jet streams which are distinct rivers of air, and these are prone to bend and close off, simliar to the way a river of water flows around rocks. Troughs are associated with rising air and storms, and are curved counter-clockwise or cyclonically in the northern hemisphere. Ridges are associated with descending air and nice weather, and are curved clockwise or anti-cycloncally in the northern hemisphere.
El Niño is the warming of the ocean surface off the western coast of South America that occurs every 2 to 7 years. La Niña is simply its counterpart and occurs when these surface waters cool.
The direct cause of this oscillation is the weakening of the west to east trade winds near the equator. Normally, these trade winds push the warm surface water of the ocean towards the west. Cold sub-surface ocean water takes its place in the eastern Pacific in a process called upwelling. When the trade winds weaken or sometimes even reverse, the upwelling decreases or stops, allowing warmer ocean surface temperatures to appear.
Because this occurs over such a large expanse of the tropical Pacific, it influences global weather patterns through a process called teleconnection. Ridges of high pressure form over the warm waters and deflect upper level winds which then affect the atmosphere not only downstream of the ridge but also northward and southward.
Why is this so difficult to predict? Changes to the global circulation affect the trade winds, which then affects El Niño, which then again affects the global circulation. It is this feedback loop that is difficult to predict.