Weather Words

January Thaw: A period of mild weather, usually following a cold spell, which often occurs in late January in the eastern regions of North America, particularly around the Great Lakes region, New England and the Canadian Maritime Provinces. The thaw is associated with a strong flow of warm air northward from the Gulf of Mexico on southerly winds on the back of a high pressure system.

Jet Stream: A relatively narrow band of high-speed winds, generally greater than 50 knots (57 mph or 93 km/h), found in the upper troposphere above regions of strong horizontal temperature contrasts such as fronts. The major jet streams are the subtropical jet and the polar jet.

Khamsin: A dry, dusty and often hot wind blowing from the Egyptian desert over Red Sea, generally from the south or southeast. In the spring, their onset is usually preceded by a heat wave lasting about three days and followed by a dust storm.

Lake Effect: The effect of any lake in modifying the weather and climate along its shore and a distance inland, which depends on the size of the lake. In some areas such as the North American Great Lakes, lake effect refers to snowfall generated by the lake on the downwind shore. See lake-effect snowfall.

Lake Effect Snowfall: Snowfalls along the lee shore of a lake or downwind some distance from the shore caused by the modification of cold, subfreezing air by the relatively warmer lake water. The intensity of the lake-effect snowfall depends upon several factors: the temperature contrast between the lake surface and the air passing over it, the over-water distance the air has traversed (the fetch), and the regional weather situation.

Latent Heat: The energy released or absorbed during a change of state. For example, the change of water vapour to liquid water releases the latent heat of condensation.

Lee Side or Leeward: The direction or side of an object (building, mountain, etc.) that faces away from the wind.

Lightning: All forms of visible electrical discharges moving through the atmosphere, usually eminating from tall cumulus and cumulonimbus clouds during thunderstorms. Lightning is often categorized for the manner in which it is visible to the observer: streak lightning, forked lightning, sheet lightning, heat lightning.

Lightning Bolt: Alternate, popular name for lightning stroke, usually when seen in streak or forked form.

Lightning Channel: The irregular path taken by a lighting discharge from its point of origin to end. Channel is formed of many small segments known as mesotorturous segments. A number of lightning strokes may follow a single lightning channel or the major portions of it. The channel may be highly ionized.

Lightning Discharge: A transfer of electrical charge along a narrow channel (the lightning channel), the result of a buildup of electrical pressure (voltage) between two distant points to levels to levels greater than the channel can resist. When the resistance breaks down, charge jumps across the space in a lightning discharge.

Lightning Flash: The complete visible flash of lightning accompanying a lightning discharge during one or a series of lightning strokes. A flash comprised of a number of strokes in rapid succession is also termed a composite flash.

Lightning Stroke: One of a series of repeated lightning discharges over a common channel.

Low Level Jet: A relatively narrow band of high-speed winds found in the lower troposphere, well below the upper troposphere where the jet stream is found.

Mediterranean Climate: Climate type characterized by mild, wet winters and warm to hot, dry summers. Originally describing the climate of the area around the Mediterranean Sea, it also describes the climate of the North American Pacific Coast, ranging from hot Mediterranean in Southern California, to warm Mediterranean on Washington and British Columbia coasts.

Mercury Barometer: The most common form of liquid barometer designed on the principle that the downward pressure of the air's weight will support a column of liquid mercury in an inverted, evacuated glass tube. The height of the column is the measure of atmospheric pressure and was originally reported as millimetres or inches of mercury (Hg). Today, the preferred unit is the Pascal, usually expressed as the kilopascal (kPa).

Meridional Flow: Large-scale atmospheric flow in which the north-south component, i.e., flow parallel to the longitude lines or meridians, is much greater than the zonal east-west)component. Compare with zonal flow.

Mesoscale: The middle of three scales used to describe the size range of atmospheric processes. The mesoscale covers the range from a few kilometres to a few tens of kilometres. Scales smaller than the mesoscale are considered microscale processes and those larger, macroscale processes. Often loosely called the local scale.

Mesotortuous Segments: "Straight" segments between five and 100 metres long separated by sharp bends which comprise the lightning channel. (Segments shorter than five metres are called microtortuous and those longer than 100 m, macrotortuous.)

Microburst: The microburst is a downburst which is confined to a small area, less than 4 km (2.5 miles) in diameter from the initial point of downdraft impact and lasting for less than 10 minutes. An intense microburst can result in damaging winds near 280 km/hr (170 mph).

Midnight Sun: A high latitude phenomenon observed around midsummer when the sun does not sink below the horizon throughout the 24 hour period; therefore the sun may be seen at midnight. At the poles, the midnight sun occurs from the Spring to Autumnal Equinoxes.

Millibar: An older metric unit of atmospheric pressure still widely used. One millibar is equal to one hectopascal.

Mizzling: A term used by Henry David Thoreau to describe weather combining thick mist and drizzle: a mizzling rain. North American usage may be peculiar to New England. Term is also found in Jane Austen's novel Northanger Abbey. (Thanks to Hal Noakes for the Austen information.)

Monsoon: Very persistent winds which may blow for months at a time from one general wind direction during one season and then a quite different, often opposing direction for another season. Originally, the name for the seasonal winds that blow across the Arabian Sea. For six months the winds blow consistently from the northeast and for six months from the southwest. The name has been extended to other areas of the world. The most well-known are the Asian monsoons which blow across India. In the winter, these cold, dry winds blow from the northwest off the Asian Plateau. In winter, they come from the southeast and bring hot, humid weather often with torrential rains. For this reason, the name has often been applied by the layperson to heavy rainfalls.

Nor'easter: Common contraction for northeaster, a northeast wind often blowing at gale or storm speeds. Also refers to a type of storm moving up the North American Atlantic coast which is first felt as a northeast wind and it moves northward.

Nimbus: The Latin word for "rain" used to describe a cloud or group of clouds from which rain is falling, e.g., nimbostratus.

Numerical Models: Computer models using the fundamental equations of hydrodynamics and thermodynamics with observed initial conditions to forecast the weather or describe the state of the atmosphere or ocean.

The Weather Doctor's Weather Glossary ©2006, Keith C. Heidorn, PhD. All Rights Reserved.
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