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Are there more or fewer tornadoes today?
Alice Bourque of Saskatoon asks: are tornadoes happening more often now than years ago, and will their numbers continue to increase?

There were three times the number of tornadoes in 1990 than in 1953. But that number does not account for all the tornadoes. Anywhere from 1/3 to 1/2 are never reported. But the recent increase has little to do with changes in stormy weather or global warming. What has changed dramatically is that we are reporting more of these tornadoes now than ever before. There are many more eye witness accounts by people armed with camcorders and cellular telephones. Greater numbers also reflect improved technology such as better radar systems and weather satellites. Today population is more wide spread. And police, highway crews, farmers, fishermen and weather volunteers are paying much more attention to severe weather, and the general public is more alert to the dangers of severe weather, especially tornadoes. So as our awareness of severe summer weather increases, so likely will the tornado count. There is also a physical link between warmer weather and tornado formation. It is reasonable to speculate that tornado frequency might well increase with climate warming.

What are cyclones?
Are cyclones the same as tornadoes and hurricanes?
For many years the word cyclone was widely misunderstood to mean only a twister or tornado - probably because a cyclone is what Dorothy in the Wizard of Oz called the Kansas storm. Cyclone is a generic term that describes all classes of storms from local thunderstorms and tiny dust devils to monstrous hurricanes and typhoons - the greatest storms on earth. It comes from the Greek word, kyklon, meaning cycle, circle or coil of a snake and refers to all circular wind systems. All cyclones whether they be tornadoes or typhoons involve winds which converge in toward the centre of the storm, around a centre of relatively low pressure. That's why cyclone and low are used interchangeably. Apart from that similarity, the family of cyclones is different in many aspects - where they form, their size and effects. Even though the word cyclone is very loosely applied to any strong wind today, the name is usually reserved for huge, intense tropical low-pressure systems: those high wind and heavy rain storms called hurricanes in the Eastern Pacific and the Atlantic, typhoons in the Western Pacific, and cyclones in the Arabian Sea, and Indian Ocean around Australia.

Popular Hurricane Names
Matt Doherty, a grade 7 student from Tecumseh, Ontario asks me which name has been used most often in naming Atlantic hurricanes. First some background.

Different naming rules are use in different ocean areas. For example, storms in the Northern Pacific Ocean region are not given names - only numbers. In the Atlantic Ocean, the practice of naming tropical storms using female names was started in 1953. Male names were first used in 1979. Today a United Nations weather committee rotates six separate lists with alternating female and male names in alphabetical order. They chose names that are common in French, English or Spanish, are easy to pronounce and have fewer than 10 letters. Names of violent, very destructive storms such as Andrew and Hugo are retired from the list.

As to which name has been used most often, Anna has been used 8 times over the years, but with various spellings. However, there's only one way to spell Arlene, and it has been used seven times.

Some other hurricane name facts:

— first female name was Alice; first male hurricane was Bob
— first female name retired was Hazel in 1954; first male name retired was David in 1979.

What are the Horse Latitudes?
Elizabeth Wood of Hamilton has a very short question: what is meant by the horse latitudes?
The Earth is circled by bands of winds that generally blow in the same direction. North of the equator there is a zone of steady winds blowing from the northeast called the trade winds; and in our latitudes, the prevailing winds are called the westerlies. Between the trades and the westerlies, there is a large subtropical high pressure area centred over the oceans at about 30° north latitude [near Bermuda]. Since the air is generally descending to the Earth in these zones, the air is warming, there is little cloud, and it's very dry. In fact, most of the world's great deserts - Gobi, Sahara, Arabian - are found along this latitude belt. These latitudes also experience long periods of weak, or sometimes completely dead wind. Before the days of steam, Spanish sailing vessels, carrying horses to the New World were occasionally stalled for weeks by the calms under a blistering sun in these low latitudes. When a ship's food and water supplies ran short, crews threw the starving horses overboard. The corpses of these horses floating on the calm waters led to the name horse latitudes.

What is an Alberta Clipper?
Mrs. Luise Schwab of Winnipeg questions why Americans call snowstorms from Canada Alberta clippers. The term Alberta clipper is much more popular in the United States than in Canada. I think it's part of that delight people have in blaming others for foul weather, like we blame Siberians for cold air and Americans for hot humid air.
An Alberta clipper refers to fast-moving winter storms that breed just east of the Rockies and sweep south and eastward across southern Canada and the Upper Midwest States.

Alberta clippers are usually weaker than most winter storms. Because they move so quickly, they don't drop much snow. But behind them there is often a surge of frigid Arctic air, producing near-zero visibility in blowing and drifting snow - real blizzard conditions.

As to its origin, I suspect it has something to do with its high speed like clipper sailing ships, which at one time were the fastest vessels on the seas. You'd be surprised how many terms are coined by meteorologists on midnight shift...this one probably came from a native of Halifax stationed in Edmonton but who longed for a return to the sea.

Can you tell me what it means when the barometric pressure is rising or falling?
Rising (falling) barometric pressure generally indicates that an area of high (low) pressure is approaching. Because high pressure areas are generally associated with fair weather, and low pressure with clouds and precipitation, one can expect those types of weather conditions depending on what the barometer is indicating.

Most barometers have "fair" or "sunny" written next to the high pressure readings, and "stormy" or something similar indicated near the low pressure readings. If you have a barometer, look at what happens the next time a low pressure system approaches your area - you should see it drop.

What is the difference between a thundershower and a thunderstorm?
Essentially there is no difference. As meteorologists we classify these storms as a "thunderstorm" or "severe thunderstorm" and the classification of the severe thunderstorm varies from region to region, and from province to province. I believe the thundershower term is used by some of the media or the general public to describe a "weak" thunderstorm.

Can you explain the difference between a snow squall and blizzard?
There are numerous differences between a blizzard and a snow squall. A blizzard generally meets these conditions:
— Winds must be sustained at 40 km/h.
— Visibility is less than 1 km
— The wind chill value is greater than 1600 watts per meter squared, or the temperature is at least -25 degrees Celsius
— The above conditions must be met for a minimum of four hours
— You do no have to have snow falling during this event

For a snow squall the main difference is the duration of the event and the fact that you need snow to accompany the winds. A squall is generally defined as the sudden onset of snow and strong winds of at least 22 knots or around 39 kilometers per hour for approximately 1 minute. This is the reason why you can have a few clouds in the sky with fairly light wind one minute and a brief shower and period of strong winds shortly after.

Could you please explain how snow can fall in Mexico, which is due south of us in Southern Alberta while we have +5 to +11 degree Celsius temperatures and NO SNOW? Does the cold air not go from the Arctic to us in Canada, then on to the United States, then onto Mexico?
Cold air is very dense so it is heavier than warm air. Very cold air outbreaks tend to spill southward from the Canadian plains to as far south as Mexico more often than you think. Since cold air is dense and heavy, it tends to spill southward, well east of the Rocky Mountain chain, where elevations are low. You could compare this effect to water flowing to the lowest possible elevation when running its course.

Could you please tell me where, when and how often hurricanes happen in Canada?
Hurricanes that develop in the tropics and move up the eastern American coastline mainly affect the Maritimes. Usually by the time they get to the Maritimes, these systems have weakened into tropical storms or intense low pressure areas with strong winds and heavy rains. It is rare for hurricanes to have maintained hurricane strength when they reach the Maritimes but it does happen and the peak time is between September to October.

Hurricanes brush by the Maritimes once every 4 years or so.

I heard that weather moves from west to east, such as from western Canada to eastern Canada (maybe 4-5 days later) and then another 4-5 days to England and France. Is this true at all?
Yes. About 90% of the time weather does move from west to east. However, sometimes our weather moves from the Northwest to the Southeast or from the American Southwest into the Canadian Northeast. The main reason for these variations is the position of the jet stream.

How are twisters formed?
Twisters or tornadoes are still one of the elusive creatures of the atmosphere. Although our understanding of tornadoes has improved over the years, we still do not have a complete understanding of how tornadoes form. There are a few very good theories, but it will take many more years of research and study before we have a more complete understanding of this phenomenon.

Here's what we know so far: most tornadoes form in severe or intense thunderstorms. As warm, moist unstable air rushes into a thunderstorm, it collides with cold, dry air rushing down from upper regions of the storm cell. Strong surface winds and jet stream winds above the storm work together to intensify the storm and cause the base of the storm cell to begin spinning. Tornadoes are born in this "spinning" region at the thunderstorm's base. As long as the storm continues to show some spinning at the base, there is a high risk of tornado formation.

Why do some tornadoes form a 'straight' funnel between the earth and the base of the cloud and some a more curvy 's-shaped' funnel that undulates and slithers? What determines the diameter of the actual funnel, as some tornadoes seem narrow and some seem to have a massive funnel?
The reasons why tornadoes vary in shape and diameter is two-fold: first of all, every tornado forms from a severe thunderstorm and even though scientists do not have a complete understanding of how they form, videos and camera shots show us that all tornadoes have a life cycle. A tornado starts off as a small wedge shaped funnel from the base of a severe thunderstorm and work it's way down to the ground. Once it touched the ground and starts to draw in all kind of debris, the funnel becomes more visible, widens and looks like a 'straight' up and down funnel. As it proceeds further into the life cycle, it begins to change its shape and 'slithers' until it dissipates. The pictures or videos of tornadoes that you see are actually the different stages of a particular storm.

As for the diameter, the massive wedge shaped tornadoes are formed in exceptionally strong thunderstorms. All storms are different and smaller thunderstorms will usually produce thinner tornadoes that appear to 'slither'.

Although there are many influences on the movement of weather systems, the ones most noticeable appear to be low-pressure troughs, high-pressure ridges and jet streams. We hear of low-pressure ridges becoming stationary, and Omega blocks which appear to hold off the movement of the systems and the jet streams along which lows 'ride'. Which of these are more dominant? Do they take turns?
There really is no dominant weather system in the atmosphere. Each of the many different systems have cycles. Jet Streams are a good example, they usually start flowing straight across the country then go through a slow but progressive looping structure where you start to see the big troughs and ridges and the possible formation of Omega blocks or cut-off lows. After a while, all this breaks down and the cycle starts over again.

That's just the Jet Stream. We also have factors such as seasonal cycles, water bodies, cold and warm ocean currents and pool cycles (El Nino, La Nina) that all play a vital role in the movement of the weather systems.

What are, and what causes "cold fronts"?
A cold front is the leading edge of colder air. In front of it, you usually have warmer, more humid air. Behind the front lies much cooler or colder and drier air. The cause for cold fronts is colder air masses migrating southward from the polar regions. It is part of the world's natural energy circulation or cycle, this is how the earth "balances" out the warm and cold air masses around the earth.

On average, about how long does it take for a weather system to move across the continent?
On average three to five days. The movement of weather systems across the continent is determined by a number of factors, such as the strength of the 500 Mb wind field and if there are any blocking features in place.

How do meteorologists forecast a monsoon? And, what leads up to the forming of a monsoon?
Simply speaking, a monsoon is defined as seasonal change of winds (including speed and direction) with more than 70% of annual precipitation falling in the monsoon season. (There are many places in the world with seasonal wind changes but they do not qualify as Monsoons due to the fact that most of the precipitation falling is not during Monsoon season.) The largest Monsoon is the India-China Monsoon and a second would be the one near North Africa.

Meteorologists in eastern Asia forecast Monsoon season and its precipitation as being strongly associated with eastern Asia's subtropical high. The drier westerly flow meets with moist southeasterly and southwesterly circulation around the high and produce Monsoon precipitation. The subtropic high slowly jumps from south to north, bringing Monsoon season from India, (including Burma, Thailand) to southern China. From there it moves to eastern China (the region around Yangtze River) and finally to northern China. The subtropical high withdraws very fast from north to south, bringing the second wave of Monsoon precipitation (which is much less compared to the precipitation resulting from when the Subtropical high moves from South to North.)

The duration of forecasting Monsoons in one region is entirely dependent on how long the subtropic high remains in that area. Sometimes, the Monsoon rains can stay for more than one month in the Yangtze River area, causing very humid and uncomfortable weather in those regions.

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