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Questions for Claire

Archives: March

Friday, March 20th, we had our first thunderstorm of the year in Grand Forks B.C. (unusual for mid-March, during such relatively cold temperatures). What conditions would have to be present for a thunderstorm to hit the valley in the evening?

Monday, March 23, 2009 | 11:28 AM PT

Question submitted by Jay
(Grand Forks, BC)

Ok, Jay, first let's denounce the misconception that thunderstorms only occur when it's "warm". As Einstein would say "it's all relative!" Thunderstorms can occur whenever a pocket of RELATIVELY cooler air is introduced to an airmass - at a RELATIVELY warmer temperature - and "lift" or a upward forcing trigger is applied to that airmass. On the date in question, a nasty squally cold front swept through the region.. introducing an area of cooler temperatures aloft to an area that was relatively cool, but much wamer than the air moving into the region. That together with the lift provided by the cold front, set the thunderstorms off.

Also Grand Forks - topographically speaking - is in a bit of a low land. The mountains surrounding the area often provide lift to incoming weather systems.. thereby generating their own thunderstorms (in situ so to speak) relatively easily.

Hope this answers your great question.

I am not clear on "high pressure" and "low pressure" and their consequenses. Please explain.

Monday, March 16, 2009 | 11:56 AM PT

Question submitted by Tim Hoang
(Surrey, BC)

Wow, Tim, this is a huge question!

Basically the atmosphere can be thought of as a fluid. As this fluid ebbs and flows around the world, we get swells and troughs forming, and these swells and troughs bring certain types of weather regimes with them. That's the "Coles Notes" version, here's the science:

A low pressure area, or "low", is a region where the atmospheric pressure is lower in relation to the surrounding area. Low pressure systems form under areas of upper level divergence on the east side of upper troughs, or due to localized heating caused by greater insolation or active thunderstorm activity. Those that form due to organized thunderstorm activity over the water which acquire a well-defined circulation are called tropical cyclones.

Lows are frequently associated with atmospheric lift. This lift will generally produce cloud cover through adiabatic cooling, once the air becomes saturated as it rises. Thus, low pressure typically brings cloudy or overcast skies, which may minimize diurnal temperature extremes in both summer and winter. Since the clouds reflect sunlight, incoming shortwave solar radiation is less which causes lower temperatures during the day. At night, the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. The stronger the area of low pressure, the stronger the winds that are experienced in its vicinity.

A red letter "L" is used to denote an area of low pressure on a weather map.

Conversely, a high-pressure area (also called a "high" and denoted with a blue "H" on weather maps) is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward due to the higher density air near their center and friction with land. Due to the coriolis force, winds flow clockwise around high-pressure systems in the northern hemisphere and counter-clockwise in the southern hemisphere. Regions of high-pressure are alternatively referred to as anticyclones. High-pressure areas are generally associated with cooler, drier air as well as clearing skies due to their formation within areas of atmospheric subsidence, or areas of large scale air descent. The strongest high-pressure areas are associated with arctic air masses during the winter, which modify and weaken once they move over relatively warmer water bodies. The area of high pressure associated with the descending branch of the Hadley cell, known as the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most the world's deserts. Arctic high-pressure systems weaken with height, while subtropical ridges strengthen with height.

Hope this gives you an idea of why we track these areas on weather maps.

I have noticed over time that there is often increased weather activity after a full moon. Given the moons effect on tide levels there must be at least a small effect. What is the link between the moon and weather?

Friday, March 6, 2009 | 12:35 PM PT

Question submitted by Mark Jennings
(Castlegar BC)

What you may have noticed was nothing more than a coincidence. There is virtually no effect on the weather from the moon.