Weather

Chapter 8

 “One day the Pharisees and Sadducees came to test Jesus’ claims by asking him to show them a miraculous sign from heaven. 

He replied, “You know the saying, `Red sky at night means fair weather tomorrow, red sky in the morning means foul weather all day.’

You are good at reading the weather signs in the sky, but you can’t read the obvious signs of the times!”

 Matthew 16:1-3 (NLT)

**There is a video version of this lecture here:  https://youtu.be/wp2fNKzGWnQ

**The exam is based on the content in these notes, so please print them off to study from.

Terms to know … (good exam questions)

  • Climate is the long-term average of weather conditions, including extremes in a region
  • Weather is the short-term, day-to-day condition of the atmosphere
  • Climatology is the scientific study of climate
  • Meteorology is the scientific study of the atmosphere – thus it includes study of the weather

I. Air Masses

To understand weather at a local level, global circulation patterns are important, but so are more localized weather systems associated with the specific air masses in a region.

Air masses are large, uniform bodies of air, with no major (horizontal) differences in temperature, wind and humidity.  All of the air in an air mass is more-or-less the same temperature, pressure, and relative humidity.

The character of an air mass is directly related to the conditions in the area where the air mass was formed.  Remember that air is heated by insolation warming the surface beneath it, which in turn radiates longwave radiation heating the atmosphere.  The amount of water vapour in the air is related to these temperature characteristics, AND whether the surface is dry land or water.

  • An air mass that formed in moist tropical regions, will be warm and moist.
  • An air mass that formed in cold dry regions will be cold and dry.

See Figure 8.1, “Principal air masses” 4CE, p. 209 (8.2 3CE, p. 199).

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For this reason air masses are categorized (and subdivided) into 6 classes, based on where the originate (see a map of these):

  • continental arctic – cA – formed around the North Pole … very cold and very dry.  This air often forms when a high-pressure area forms over Eastern Alaska, the Yukon, Siberia or northern Canada, usually above the Arctic Circle.  Due to a near lack of winter solar radiation, abundant surface snow/ice cover and the continuous emission of radiation from the Earth’s surface, the air will progressively become colder and colder.  Temperatures can reach -30 degrees C to -50 degrees C.   Such air masses often plunge south across Canada and the USA during winter as Rossby Waves and the Zonal Index dip south, but very rarely form during the summer because the sun warms the Arctic.
  • continental polar – cP – formed in northern regions over land … cold and dry.   These usually form farther to the south of cA air masses, and often dominate the weather picture across central Canada and the northern USA during winter.  These air masses are the ones responsible for bringing clear, cool — but not brutally cold — weather during the winter to most of Canada (the really FRIGID temperatures are the result of cA air masses).  Continental polar masses do form during the summer.  In the summer, these air masses bring clear, cool weather to the Arctic, but rarely move further south.
  • maritime polar – mP – formed over northern oceans … cold and wet.  These form over the northern Atlantic and the northern Pacific oceans.  They usually bring cloudy, damp weather to Canada, including blizzards, snow or cold rain storms.   They most often influence the Pacific Northwest and the Northeast (Labrador, and the Atlantic Provinces).  Since mP air is always near saturation, any lifting of the air mass (such as orographic lifting along mountain ranges) can produce widespread rain or snow.  This air mass is notorious for producing fog, drizzle, cloudy weather and long lasting light to moderate rain (West Coast residents and Atlantic Canadians know all about these during the winter).  The temperature of mP air ranges from well below freezing to low 20’s C.  Maritime polar air masses can form any time of the year and are usually not as cold as continental polar air masses, because of the moderating effect of water.
  • continental tropical – cT – formed over southern land masses … warm and dry.  They usually form over the Desert Southwest of the USA and northern Mexico during summer. They can bring dry weather and record heat to the US Plains and the Canadian Prairies and southern Ontario during summer, but they usually do not make it to the East Coast.  As they move eastward, moisture evaporates into the air, making the air mass more like a maritime tropical air mass.
  • maritime tropical – mT – formed over southern oceans … warm and wet.   Maritime tropical air masses are most common across the eastern USA and originate over the warm waters of the southern Atlantic Ocean and the Gulf of Mexico. These air masses can form year round, but they are most prevalent across southern Ontario, Quebec, the Atlantic Provinces, and much of the USA during summer. Maritime tropical air masses are responsible for the hot, humid days of summer across central and eastern Canada.
  • maritime equatorial – mE … formed over equatorial oceans … very warm and very wet.  These are common along the Equator.  Lots of heat.  Lots of precipitation.

Once an air mass moves out of its source region, it is modified as it encounters surface conditions different than those found in the source region.  For example, as a polar air mass moves southward, it encounters warmer land masses and consequently, is heated by the ground below.  Air masses typically clash in the middle latitudes – much of southern Canada – producing some very “interesting” weather.

During the course of the year, our weather may be influenced by several different classes of air masses (Figure 8.1 4CE, p.209; 8.2 3CE)

On the West Coast:

  • In winter polar air dominates the Northern Hemisphere; the Arctic Front is about 38°N; the West Coast experiences cold cP (clear and cool) air masses and mP air masses (cool and wet – lots of rain and/or snow!).
  • In summer the Arctic Front retreats to about 65°N; the West Coast experiences more cT (warm and dry) and mT air masses (warm and wet).

 In the Prairies:

  • In winter polar air dominates the Northern Hemisphere; the Arctic Front is about 38°N; Alberta experiences cold cA and cP air masses.  They typically have cold, dry weather.  We also get some mP air masses (cool and wet – snow).
  • In summer the Arctic Front retreats to about 65°N; Alberta experiences more cT (warm and dry) and a very few mT air masses (warm and wet).
  • As air masses move their characteristics change because they are influenced by the surfaces they pass over.  Thus mP and mT air masses, by the time they reach the Prairies, have usually lost most of their moisture over the west coast and the various mountain ranges to the west of us.  By the time they reach us they still have some moisture, but not much.  They are relatively dry and stable.

 In Ontario/Quebec:

  • In winter polar air dominates the Northern Hemisphere; the Arctic Front is about 38°N; Central Canada experiences cold cP (clear and cool) air masses and mP air masses (cool and wet – lots of snow).
  • In summer the Arctic Front retreats to about 65°N; Central Canada experiences more cT (warm and dry) and mT air masses (warm and wet) — some of these can have hurricane strength.
  • Proximity to the Great Lakes means that air masses can pick up lots of added moisture meaning more snow in winter and more humidity (and thunderstorms) in summer than would be expected if the lakes were not there.

In Atlantic Canada:

  • In winter polar air dominates the Northern Hemisphere; the Arctic Front is about 38°N; Atlantic Canada experiences cold cP (clear and cool) air masses and mP air masses (cool and wet – lots of snow!).
  • In summer the Arctic Front retreats to about 65°N; Atlantic Canada experiences more cT (warm and dry) and mT air masses (warm and wet) — some of these can have hurricane strength.

 II. Atmospheric Lifting Mechanisms

  • See your text for diagrams, Figure 8.3 (4CE, p. 211) / Figure 8.5 (3CE, p. 202), “Atmospheric lifting mechanisms.”

Air masses can be “lifted” in one of four principal ways.  As the air lifts, it cools to form clouds and precipitation.  Lifting is important because it causes the uplift that leads to adiabatic cooling, condensation, and precipitation.

  1. Convergent lifting occurs in low pressure areas (like the ITCZ), air converges and rises (See Figure 8.3a (4CE) / 8.5a (3CE)). Convergent lifting refers to an atmospheric condition when there is a horizontal  inflow of air into a region. When air converges along the earth’s surface, it is forced to rise since it cannot go downward. Large scale convergence can lift a layer of air hundreds of kilometers across.  During a typical day at the ITCZ, the day begins clear.  As insolation heats the surface, evaporation occurs and warm moist air rises.  As it rises it cools, resulting late afternoon rain and thunderstorms.
  1. Convectional lifting occurs when the ground surface heats.  This causes the air above it to heat, and the air rises by convection (See Figure 8.3b and 8.4 (4CE, p. 211-212) / Figure 8.5b and 8.6 (3CE, p. 202), “Local heating and convection”).  This is common in the Prairies and US Mid West in summer:
  • Morning sunshine heats dark surfaces (soil, lakes), causing atmospheric heating, evaporation, and rising air.
  • By afternoon the moist air is still rising, and cumulus clouds often develop.
  • If the heating is intense enough, cumulonimbus clouds and thunderstorms may develop.  Before the rising air reaches the dew point temperature, it cools at the DAR – no condensation/clouds.   Once the dew point temperature is reached, it cools at the MAR  and clouds form.
  1. Orographic lifting occurs when a moving air mass encounters a physical barrier, such as a mountain range.  The air is forcibly lifted/pushed upslope by wind.  (See Figure 8.3c (4CE) / 8.5c (3CE).  Know Figure 8.6 (4CE, p. 212) / 8.8 (3CE, p. 204), “Orographic precipitation”)
  • Initially it cools at the DAR – no condensation/clouds.
  • If it cools to the dew point it then cools at the MAR and clouds form.
  • The altitude at which the clouds form is the altitude at which the air cools to the dew point temperature.

The wetter slope is called the windward slope.

The drier side is the leeward slope.  On this side the descending air mass is heated by compression and water vapor evaporates.  It warms at the DAR (it is descending, so it is warming).  As the air warms during descent it becomes much drier, because no moisture is added to the air mass at this time (relative humidity is decreasing).

Depending on the pressure gradient between the high pressure center and the low pressure off the coast, very strong winds with an easterly component can develop…winds sometimes as strong as 140 kmh (100 mph).

It is a very hot and dry wind, as described above. These warm, down slope winds are called Chinook winds (southern Alberta), Santa Ana Winds (California), or Föhn winds (central Europe).

Leeward regions are often called “rain shadow” regions.  Precipitation falls on the windward side (where the air is rising, cooling and condensing).  But on the leeward side air is descending, warming, and not condensing.  So little precipitation falls.

Consider Figure 8.7 (4CE, p. 214) / Figure 8.9 (3CE, p. 205), “Orographic patterns in …” Notice how wet the windward slopes are (the western slopes of the Olympic peninsula and Coast Mountains).  And notice how relatively dry the leeward slopes are (especially south-central Washington, in the rain shadow of the Coast Mountains).  A similar pattern happens all along the west coast, from Alaska, through B.C., to Washington and California.

  1. Frontal lifting occurs with the passage of warm and cold fronts. See Figure 8.3d (4CE) / 8.5d (3CE).

The line along which two air masses come in contact is called a front.  Where warm air is replacing cold air it is called a warm front; where cold air is replacing warm air it is called a cold front.

a.  Cold Front

See Figure 8.8a (4CE, p. 215) / Figure 8.10 (3CE, p. 206), “A typical cold front.”

Cold fronts occur where a cold air mass is replacing a warm air mass.

Cold fronts usually have a steep slope, essentially “snow-plowing” out the warm air.  Remember cold air is dense and does not rise easily.  As this cold air plows along the surface, it may cause rapid upward movement of the warm air it is replacing, causing thunderstorms, heavy rainfall and tornadoes.

With cold fronts, temperature changes are felt first, often followed by strong winds and heavy precipitation.  Clouds progress from clear, to a few cirrus clouds, to cumulo- or cumulonimbus clouds.  A fast moving cold front can cause violent lifting creating a zone known as a squall line.  This squall line can be characterized by sudden winds, strong updrafts, cumulonimbus clouds, and intense rain, thunder, lightning, or hail.

Click here for a great graphic model of a cold front.  The same site has a good

Note that the weather map symbol is a (normally blue) line with triangles pointing in the direction the front is moving (see the example on Page 218 (4CE) / Figure 8.12 (3CE, p. 208), “Midlatitude Cyclones/ Idealized stages…”).

b.   Warm Front

See Figure 8.9 (4CE, p. 216) / Figure 8.11 (3CE, p. 202), “A typical warm front.”

Warm fronts occur where a warm air mass is replacing a cold air mass.  In a warm front the slope is relatively gradual; the warm air is pushed up over top of the existing colder air. The cold air is gradually pushed out of the way.  During the process, warm, moist air rises, forming clouds.

The normal progression goes from clear skies, to cirrus clouds (which appear well before the front arrives), then gradually altostratus and stratus clouds get thicker and lower until low nimbostratus (rain) clouds cover the entire sky.

Signs of coming changes (a transition from clear skies to cirrus clouds) are seen long before temperature changes are felt on the ground. 

Click here for a great graphic model of a warm front.  The same site has a good

Note that the weather map symbol is a line with (normally red) semi-circles pointing in the direction the front is moving (see the example on page 218 (4CE) / Figure 8.12 (3CE), “Idealized stages…”).

c.   Occluded Front

In most weather systems, the trailing cold fronts move more quickly than the leading warm fronts.  Because cold fronts move more quickly, they often “catch up to” the warm front, an occlusion or occluded front occurs.

Depending on the relative temperatures of the cooler air masses, an occluded front will either resemble a warm or cold front.

Note that the weather map symbol is a line with triangles alternating with semi-circles, pointing in the direction the front is moving (see the example in “Geosystems in Action 8:  Mid-latitude Cyclones” Figure 8.1, page 218 (4CE) / Figure 8.12 (3CE), “Idealized stages…”).

III.     Weather Systems in Temperate (Mid-) Latitudes (Us!)

Our weather is among the least predictable in the world!  Much of our weather appears to be related to storm systems (cyclones) which form along the Arctic Front, related to the growth of Rossby Waves …

A.  Anticyclones

Anticyclones are high-pressure areas of dry, stable air and calm weather.  In summer – hot, dry weather.  In winter – cold, dry weather.  In anticyclones, air is descending and diverging.  No clouds.  No condensation.  No precipitation.

In the subtropics, anticyclone belts occur all year round at approximately 30°N and S, where air descends (subtropical highs).  Warm, dry air descends, encounters cooler air over the oceans, and a very stable condition results.  Note: major deserts in these areas.

In mid-latitudes, anticyclones periodically establish themselves for several weeks in one location, possibly due to upper wind conditions.  They cause cyclones to move around them, resulting long periods of dry weather and drought conditions.

  • When we have a hot, dry, clear period in summer, we are in an anticyclone.
  • When we have a cold, dry, clear period in winter, we are in an anticyclone.

Anticyclones are normally associated with dry, continental (cA, cP, and cT) air masses.

Anticyclones rotate clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.

Does this matter?  Yes!  For example:  in winter, when an anticyclone moves into western Canada, it brings sunny, dry – and cold – weather.  Because, in an anticyclone air rotates clockwise in the northern hemisphere, Lethbridge, in southern Alberta, experienced winds blowing from the south (bringing warmer air off the US Pacific Northwest) and temperatures well above 0°C.  But in Edmonton, further north,  the winds were blowing from the north — off northern Alberta! … BRRR … temperatures well below 0°C!  The rotation of the anticyclone resulted in a temperature difference of 15°C!

B. Cyclones

Cyclones are low-pressure areas.  In these areas air is rising, cloud is forming, and precipitation usually occurs. 

Note:  in our latitudes, any low-pressure system can be technically called a “cyclone.”  Tropical cyclones (what we often think of when we hear the word “cyclone”) – really intense, devastating hurricanes – are different (see Tropical Cyclones, below).  For an excellent satellite image, click here.

In mid-latitudes, cyclones (or as they are sometimes called “extratropical cyclones” …. meaning they originate outside the tropics) typically track off the Pacific Ocean, north-east across the continent (propelled by westerly winds).  They bring with them strong pressure and temperature gradients (causing strong winds), and moisture.

As the cyclone progresses, air mixes, gradients decrease, moisture is lost, and eventually the storm “peters out.”

Cyclones typically are preceded by a warm front, followed by a cold front as they pass by … for an on-line idealized model, click here.

  • See the example in “Geosystems in Action 8:  Mid-latitude Cyclones” Figure 8.1, page 218 (4CE) / Figure 8.12 (3CE), “Idealized stages…”, and the commentary that accompanies it (“Life Cycle of a Midlatitude Cyclone”) in the text.
  • Cyclogenesis occurs when warm air converges and begins to rise (it’s unstable air)
  • The Open Stage occurs when warm, moist air is pulled into the cold, low pressure centre
  • The Occluded Stage occurs when the cold front “catches up to” the warm front, pushing the warm air aloft
  • The Dissolving Stage occurs when the cold air mass completely cuts off any incoming warm, moist air, and the storm fades into cold, clear, high pressure.
  • See also Figure CT 8.1.1 (4CE, p. 222) /(3CE, p. 211), “Open stage …”, which shows a typical mid-latitude cyclone.

Cyclones are normally associated with moist, maritime (mP, mT) air masses.

Cyclones rotate counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

Does this matter?  Yes!  When Atlantic Canada experiences a dreaded “Nor-easter” — winds from the north east, the counter clockwise rotation of a cyclone or low pressure system bring cold, WET air off the North Atlantic into the Atlantic Provinces.  Lots of snow.  Wind.  And COLD.

C. Weather Maps and Forecasting

Weather maps try to summarize all of this information in a useful summary format (see the example in “Geosystems in Action 8:  Mid-latitude Cyclones” Figure 8.1, page 218 (4CE) / Figure 8.12 (3CE), “Idealized stages…”).

Weather maps typically show

  • cold/warm/occluded fronts
  • high and low pressure centres
  • isobars (lines of equal air pressure)
  • temperatures in specific locations
  • often a variety of other information, including cloud cover, precipitation, wind speed, wind direction, and dew point temperature.

IV.  Violent  Weather

Environment Canada has excellent pages on both

A. Thunderstorms

Thunderstorms can be caused:

  1.      within an air mass (especially warm, moist air) – like on a hot, summer Prairie day
  2.      along a cold front – when warm moist air is shoved aloft by cold air moving in quickly
  3.      on mountain slopes when rapid orographic lifting occurs because of strong winds pushing moist air upslope quickly.

Rapidly rising moist air results in the condensation of large amounts of water vapour, massive energy release (latent energy), rapidly dropping air pressure, and strong updrafts.  These can produce huge clouds, called supercells.

Turbulence within the atmosphere is a result.  This can be problematic for aircraft!   You don’t want to fly into a thunderstorm.  The air is moving up quickly.  And there are pockets with rapid downdrafts.  Water molecules are coalescing to form bigger and bigger droplets that eventually will fall as heavy rain or hail. Because of the strong updrafts, raindrops will have to be very big and heavy to overcome the upward force of the air and fall.  The presence of large hail as well as strong winds classifies the storm as a severe thunderstorm. (Figure 7.18).

Hail is usually very isolated in its effects, but can be devastating to buildings, cars, and crops.

Lightning is created by the buildup of electrical energy (tens of millions to hundreds of millions of volts) within a cumulonimbus cloud or between the cloud and the ground. Within the cloud, rapidly moving air “rubs” together, creating electrical polarities.  Lightning may be compared to the static electricity that is created when you rub two pieces of fabric together (you see sparks) – only on a much more massive scale!  80% of lightning occurs within clouds themselves, only 20% is directed toward the ground.  Lightning briefly superheats the air immediately around its path.  Within 100ths of a second the air temperature rises from normal atmospheric temperature to 15,000 – 30,000°C.

  • The most deadly lightning event in Canadian history was July 29, 1916. Lightning ignited a forest fire which burned down the towns of Cochrane and Matheson, Ontario, killing 233 people.

For a dramatic video of lightning striking a truck, check here:  lightning strikes pickup truck | CTV Edmonton News

  • Environment Canada also has a good page on lightning
  • For current lightning information in Canada, click here.

Lightning is associated with hot weather causing rapid heating and air rising.  See Figure 8.13 (4CE, p. 224) / Figure 8.17 (3CE, p. 215), “Seasonal images …” Notice how lightning storms are common in the northern hemisphere in summer and in the southern hemisphere in winter.  Notice lightning is most intense from the equator to the tropics, as this is the region of most intense insolation.

Thunder is the noise caused by the sudden, violent expansion of this superheated air – the rapid heating and resulting expansion of the air sends shock waves moving rapidly through the atmosphere as a sonic bang.  As the shock waves pass the speed of sound, you hear a sonic boom.  This is the same sound you hear when a jet aircraft passes the speed of sound.  See Figure 8.12 (4CE, p. 223) / Figure 8.16 (3CE, p. 214), “Thunderstorm occurrence.”

Thunderstorms can result in significant impacts to buildings and crop losses through strong winds, intense rain, hail, and lightning strikes..

B.  Tornadoes (Funnel Clouds)

See:     4CE, pp. 225-226; 3CE, pp. 216-219:

A man in Lincoln, Nebraska, 1943 described a tornado thus: “We looked up into what appeared to be an enormous hollow cylinder, bright inside with lightning flashes, but black as blackest night all around.  The noise was like ten million bees, plus a roar that begs description.” 

A tornado is a violently rotating column of air in contact with the ground surface, usually visible as a spinning vortex of cloud and debris.

Why tornadoes happen is not known definitively!  Required elements are warm, moist, unstable air, producing cumulonimbus clouds.  If there is a very steep lapse rate (air cools very rapidly) – often as a cold front pushes in – very strong updrafts and instability occur which, for some reason, occasionally spawn tornadoes.  Severe thunderstorms have persistent updrafts that can reach speeds of 160 kmh.  Within the storm, a strong vertical wind shear causes a horizontally rotating cylinder of air.  The updraft lifts the rotating cylinder within the supercell.  The rotating cylinder of air narrows, becoming stretched, and spins faster and faster forming a tornado.  The rotation within the thunderstorm gives the supercell its classic “hook” appearance.

Tornadoes are usually narrow (about 0.5 km wide) and rarely travel further than 15-20 km (but some have been up to 1.5 km wide and travel 500 km).  The funnel moves at a rate of about 50-65 km/h along the ground.  How fast the air can rotate in a tornado is not known exactly; no recorder has survived the passage of a tornado.  But speeds of over 600 km/h are believed to occur.  Tornadoes can last for a few minutes to tens of minutes.

Over water, tornadoes suck up water, and are referred to as waterspouts.

Wikipedia has a good article on tornadoes.

Check out satellite images, including java animation of the Pine Lake, AB (2000) tornado, that killed 12

Tornadoes are measured on the Fujita Scale … F0 to F5.  Click here for a simulation.

See Table 8.1 (4CE, p. 226) / Table 8.1 (3CE, p. 218), “The Fujita Scale.” The Edmonton tornado of 1987 was an F4, killing 27 people.

From 1916 to the present an average of 230 people per year are killed by tornadoes in the U.S.   For the United States’ worst tornadoes, click here.

Canada’s Worst Tornadoes

Central North America is “Tornado Alley,” with frequency being highest in the Gulf in February/March, and July/August in Canada.  Typically they happen when cool moist mP air lifts very warm moist mT air.

Destruction happens two ways –

  • extremely high winds
  • severe pressure changes (very quickly dropping from normal, 1013 mb to as low as 600 mb).

This combination often causes buildings to be lifted off their foundations or to literally explode (higher pressure inside than outside).  In some cases straws have been found embedded in railway ties!

  • The movie Twister (1996) and Twisters (2024) provide reasonably accurate portrayals of tornadoes and the damage they can cause.  However the suggestion in Twisters that a tornado could be stopped by absorbing moisture and/or “seeding” a storm to create precipitation, while both possibilities that have been suggested by scientists, have never proved at all useful.  Tornadoes are simply too powerful and large to be significantly impacted by human intervention.  Mitigating climate change, prediction, and preparation are the useful approaches at this point.
  • Everything you want to know about tornadoes …  click here.
C. Tropical Cyclones (also known as Hurricanes or Typhoons)

In much of the trade wind belt the weather is very consistent – small convection clouds, occasional afternoon showers (heavier, more frequent in summer because of higher temperatures and uplift).  Mild storms, associated with easterly waves also occur.  The exceptions are known as Tropical Cyclones. The terms “hurricanes” (used in the Atlantic Ocean) and “typhoons” (used in the Pacific Ocean) are also used to describe tropical cyclones.  In the Mediterranean Sea, severe cylcones are often called “medicanes.”  They are synonyms.  Different words are used in different parts of the world.

Tropical storms are categorized based on their sustained wind strength

  • Tropical disturbance – low winds (just clouds)
  • Tropical depression – up to 63 kmh (moderate rain)
  • Tropical Storm – up to 118 kmh (heavy rain)
  • Tropical cyclone/hurricane/typhoon – over 119 kmh (very heavy rain, storm surge, etc)

Whereas mid-latitude cyclones are large, revolve slowly and are rather ponderous, tropical cyclones are usually small, revolve very quickly and move swiftly.  With intense heating, however, they can become VERY large and very devastating.  Tropical cyclones are classified as storms with winds over 119 km/h.

Necessary conditions include:

Warm ocean waters to fuel the tropical cyclone

  • Studies have shown that sea surface temperature must be at least 26.5°C
  • This temperature is required to a depth of at least 50 metres
  • Tropical cyclones cannot form outside of the tropics because the water temperatures are too cold

A warm, moist tropical atmosphere

  • Encourages thunderstorm development, the foundation of the latent heat release process that drives tropical cyclones

More than 500 kilometres (about 5° latitude) away from the equator

  • The apparent force of the rotating earth, called the Coriolis force, is necessary to generate the rotation of the growing disturbance
  • The Coriolis force is slight near the equator and gets stronger towards the poles

A pre-existing near-surface disturbance, low-pressure area, or region of convergence

  • Tropical cyclones cannot generate spontaneously and they require a trigger mechanism to begin drawing air inwards at the lowest levels of the atmosphere

Little to no vertical wind shear between the surface and the upper troposphere

  • The upper troposphere is where weather occurs, just below the stratosphere
  • Vertical wind shear is a change of wind speed or direction with increasing altitude
  • Large vertical wind shear disrupts a growing disturbance and can prevent a tropical cyclone from forming
  • Large vertical wind shear can weaken or destroy an already-formed tropical cyclone by tilting it over and poking holes in the warm core
    • This interferes with the processes of deep convection (overturning air) around the cyclone centre
    • It draws air up through the storm to help the air rise and keep the low pressure at the surface

While these conditions are needed to create a tropical cyclone, it does not mean that they are enough to create one. Often all of these conditions exist, yet a tropical cyclone does not form. This is part of the challenge in forecasting when a tropical cyclone will develop–a process known as tropical cyclogenesis.

A high-pressure area located at the top of the troposphere, above the storm or growing disturbance, can also help a tropical cyclone to form. This acts as a “chimney” for the storm that does at least two important things:

  • It takes the rising air away from the storm centre so that it doesn’t pile up above the storm and cause the storm to collapse on itself

In the Northern Hemisphere hurricanes occur May-November (the warmest months).

Hurricanes consist of a spiral of thick, black clouds packing torrential rains, centered around a calm, clear, low pressure “eye” of descending air.  Around the eye winds often exceed 250 km/h.  Winds decrease in strength away from the eye but gale force winds (60 km/h +) often extend more than 300 km away from the center of the storm.

Once hurricanes hit land they weaken because of friction with the ground and because their fuel source (warm, ocean water) disappears.  However as the winds drop, precipitation often continues because so much moisture was evaporated and then condensed in the clouds.

Hurricanes often follow predictable tracks.  In the Atlantic Ocean, for instance, following the east coast of North America and in the Pacific, from the west coast of central America into the Pacific Ocean.

Hurricanes also often create large waves.  In Bangladesh, Nov.12, 1970, between 300,000-500,000 people were killed by a hurricane, mostly by storm surge waves (up to 12 m high).  This was one of the greatest natural disasters of the twentieth century.  (Have you ever heard of it?  Interesting how disasters in some parts of the world get more attention than others, isn’t it?  Why is that?).

The Canadian Hurricane Centre has an excellent introduction to everything hurricane-ish, especially as it relates to Canada’s east coast.

  • September 2022, Hurricane Fiona caused widespread destruction and power outages throughout Atlantic Canada.  Winds of 177 km/h were reported in Wreckhouse, NF, with a record high water height/storm surge (before waves) of 2.73 meters in Channel-Port aux Basques, NF.  At least 20 homes were washed away in Newfoundland, primarily in Port aux Basques. Fiona left more than 500,000 people without power, including 80% of all Nova Scotia customers and 95% of PEI customers.  Damage is estimated to be $3+ billion. 28 people died.
  • September 2003, Hurricane Juan was particularly vicious, resulting large-scale destruction.  Hurricane Juan tore down power lines, flooded waterfront properties, sank dozens of yachts, heaved sidewalks and damaged stately downtown homes in Halifax and Charlottetown.  At its peak, Juan left more than 300,000 homes without power in Nova Scotia and Prince Edward Island.  There were sustained winds at 158 km/h gusting over 185 km/h.  Thousands of trees were destroyed.  Winds generated a storm surge of over 1.5 m, and raised the water level in Halifax harbour to a record 2.9 metres.  Maximum wave heights outside the harbour were measured at nearly 20 metres.  Mercifully, rainfall amounts from Juan were not heavy at 25 to 40 mm.
  • Hurricane Hazel, 1954, was particularly devastating to Toronto and southern Ontario.  Hazel dumped an estimated 300 million tonnes of rain on Toronto, causing lost streets, washed out bridges and untold personal tragedy. In all, 83 people died — some bodies washing up on the shores of Lake Ontario in New York State days later.

For a list of major hurricanes affecting Canada, click here.

Recent global hurricanes/typhoons/medicanes of note include:

  • Daniel (Greece/Libya/Mediterranean 2023) killed an estimated 20,000 people.
  • Ian (Cuba, Florida 2022)
  • Fiona (Atlantic Canada 2022)
  • Laura (Louisiana 2020) caused at least $19 billion in damage and 77 deaths.
  • Dorian (Bahamas-Eastern Canada 2019)
  • Florence (eastern US 2018)
  • Irma (Caribbean, Florida, 2017)
  • Harvey (Texas, Louisiana 2017) – tied with Katrina as costliest hurricane, $125+ billion
  • Joaquin (Bahamas, Bermuda, US SE States, 2015)
  • Soudelor (Northern Mariana Islands, Taiwan, and eastern China 2015)
  • Haiyan  (Philippines, 2013)
  • Sandy  (Eastern USA, 2012)
  • Katrina (Gulf Coast, 2005) – tied with Harvey as costliest hurricane, $125+ billion
  • Juan, (Atlantic Canada 2003)
  • Mitch (Central America 1998)
  • Bhola (Bangladesh 1970) – deadliest hurricane/cyclone with 300,000-500,000 fatalities

One of the consequences of climate change appears to be an increase in the frequency and severity of hurricanes, incurring greater costs and new building guidelines.  Climate change is increasing the number of hurricanes and warmer water is fueling bigger, stronger storms that intensify faster, move more slowly, and dump a lot more rain. As the planet warms it is also extending the hurricane season, allowing the season to start earlier in the year, on average and last longer into the fall.  See this video from Canadian climate scientist, Katherine Hayhoe, on climate change and changes to hurricanes strength and speed:  Global Warming and Climate Change

The 2020 Atlantic hurricane season smashed records with an unprecedented 30 named storms, marking the fifth year in a row with above-average hurricane activity.  Higher sea-surface temperatures as a result of climate change are a contributing factor.

Hurricanes are measured by the Saffir–Simpson hurricane wind scale (SSHWS), dividing storms into five categories distinguished by the intensities of their sustained winds (5 being the strongest).

Note:  Hurricanes are named.   The use of short, distinctive names in written as well as spoken communications is quicker and less subject to error than the older, more cumbersome latitude-longitude identification methods. These advantages are especially important in exchanging detailed storm information between hundreds of widely scattered stations, coastal bases, and ships at sea.  The use of easily remembered names greatly reduces confusion when two or more tropical storms occur at the same time.  For example, one hurricane can be moving slowly westward in the Gulf of Mexico, while at exactly the same time another hurricane can be moving rapidly northward along the Atlantic coast. In the past, confusion and false rumors have arisen when storm advisories broadcast from radio stations were mistaken for warnings concerning an entirely different storm located hundreds of kilometres away.  How names are assigned in varous parts of the world is interesting, but certainly not a “need-to-know” thing.

 D. Blizzards

Blizzards are severe winter storms characterized by the following:

  • snow or blowing snow with winds of 40 km/hr or more,
  • visibility reduced to less than one km in snow and/or blowing snow,
  • windchill of -25°C or colder.

All of the above conditions must last for four hours or more to be officially classified as a blizzard.

Canada has no recognized classification system for blizzards.  The United States has developed the The Northeast Snowfall Impact Scale (NESIS) — a four point scale that rates blizzards ranging from Notable (the weakest designation) to Significant, Major, Crippling, and Extreme. They created this measuring system due to the effects these storms have on the economy and transportation..

Blizzards can occur in any Canadian province or territory.  Alberta and the Atlantic provinces receive the most blizzards in Canada.

The Iran Blizzard of February 1972 is the deadliest blizzard in history (every hear of it?  It’s time you did!).  It resulted in the deaths of approximately 4,000 people.  In a week southern Iran received almost 8 m of snow, completely burying several villages and killing all the residents.

Other notable blizzards include:

E. Ice Storms

An ice storm is a specific type of winter storm characterized by freezing rain.

The worst ice storm in Canadian history, the massive Ice Storm of January 1998 affected a huge area from eastern Ontario, through Quebec and New Brunswick to Nova Scotia.  It caused massive damage to trees and electrical infrastructure all over the area, leading to widespread power outages. Millions were left in the dark for periods varying from days to weeks, leading to more than 30 fatalities, a shut down of activities in large cities like Montreal and Ottawa and an unprecedented reconstruction effort of the power grid.

The 2013 Central and Eastern Canada ice storm left over 700,000 people in Ontario, Quebec, New Brunswick, and Nova Scotia without power for up to 12 days.

The 2020-2021 Central North American Ice Storm resulted in wide spread power outages across the US.

A list of major ice storms is here:  List of ice storms

F. Atmospheric Rivers

Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics. While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. Exceptionally strong atmospheric rivers can transport up to 15 times that amount. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.

A well-known example is the “Pineapple Express,” a strong atmospheric river that is capable of bringing moisture from the tropics near Hawaii over to the U.S. West Coast.

In 2019, scientists established a scale to classify atmospheric rivers based on their maximum intensity and duration. Similar to the Saffir-Simpson wind scale for hurricanes and the Enhanced Fujita scale for tornadoes, it is meant to help communicate potential storm effects to the public and distinguish between hazardous and more-beneficial events. (Atmospheric rivers bring replenishing rains and contribute to winter snowpack, particularly in the western United States.)

Atmospheric River affecting BC and Alaska, Sept 24, 2024

To reflect on …

Astronomer Jennifer Wiseman, director of NASA’s Hubble Space Telescope programme,  thinks that you can’t find out whether there’s a God from looking at the universe, but here she describes what she sees – as a Christian – in the discoveries of astronomy.

Check out the video “Science affirms my faith” with Natalie, a plant geneticist

To review …

Check out the resources at www.masteringgeography.com

This page is the intellectual property of the author, Bruce Martin, and is copyrighted by Bruce Martin.  This page may be copied or printed only for educational purposes by students registered in courses taught by Dr. Bruce Martin.  Any other use constitutes a criminal offence.

Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved