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 an introductory video here:  http://youtu.be/eZYGY29apjs

 

 

Terms to know ... (good exam questions)

 

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. 

 

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

 

The character of an air mass is directly related to the conditions in the area where the air mass was formed

 

·        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.

 

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

 

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

 

 On the West Coast:

 In the Prairies:

 In Ontario/Quebec:

In Atlantic Canada:

 

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).

 

2.     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 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.  Once the dew point temperature is reached, it cools at the MAR.

 

3.      Orographic lifting occurs when a moving air mass encounters a physical barrier, such as a mountain range.  The air is forcibly lifted/pushed upslope. 

 

·        Initially it cools at the DAR. 

 

·        If it cools to the dew point it then cools at the MAR. 

 

See Figure 8.3c (4CE) / 8.5c (3CE).  Know Figure 8.6 (4CE, p. 212) / 8.8 (3CE, p. 204), “Orographic precipitation.”

 

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).  This descent causes temperature warming in the air due to the pressure increase with the descent. As the air warms during descent it becomes much drier, because no moisture is added to the air mass at this time.  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 Chinooks, Santa Anna Winds, or föhns

 

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.

 

 

4.      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 usually have a steep slope, essentially "snow-plowing" out the warm air.  Cold fronts may cause rapid upward movement of the warm air causing thunderstorms, heavy rainfall and tornadoes.

 

With cold fronts, temperature changes are felt first, often followed by strong winds and heavy precipitation.

 

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 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.”

 

In a warm front the slope is relatively gradual; thus, cirrus clouds appear well before the front arrives ... gradually clouds get thicker and thicker until low nimbostratus (rain) clouds arise.

 

Signs of coming changes (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 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" 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 on 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 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.  Cyclones rotate counter clockwise in the northern hemisphere and clockwise in the southern hemisphere. 

 

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 on page 218 (4CE) / Figure 8.12 (3CE), “Idealized stages…”, and the commentary that accompanies it (“Life Cycle of a Midlatitude Cyclone”) in the text.

 

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.

 

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!

 

 

Worth reflecting on ...

 

In his novel, Annals of a Quiet Neighborhood, Victorian-age writer, George McDonald writes, "It was a lovely day.  The sun shone so warm that you could not help thinking of what he would be able to do before long -- draw primroses and buttercups out of the Earth by force of sweet persuasive influences.  But in the shadows lay fine webs and laces of ice, so delicately lovely that one could not but be glad of the cold that made the water able to please itself by taking such graceful forms and I wondered over again for the hundredth time what could be the principal which, in the wildest, most lawless, fantastically chaotic, apparently capricious work of nature, always kept it beautiful. The beauty of the holiness must be at the heart of it somehow, I thought.  Because our God is so free from stain, so loving, so unselfish, so good, so altogether what he wants us to be, so wholly, therefore all his works declared him in beauty; his fingers can touch nothing but to mold it into nothingness; and even the play of his elements is in grace and tenderness of form."

 

How do you feel about that?  Do you agree?  Feel free to discuss this quote on the course discussion site ...

 

 

IV.    Violent  Weather

 

Environment Canada has excellent pages on both

 

A. Thunderstorms (click here for a graphic introduction)

 

Thunderstorms can be caused:

 

a.      within an air mass (especially warm, moist air) – like on a hot, summer Prairie day

 

b.      along a cold front – when warm moist air is shoved aloft by cold air moving in quickly

 

c.      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 (click here for more on 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.

 

Heavy rain normally results.  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.

 

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

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.  For a graphic model of the production of thunder, click here.  Environment Canada has a good page on The Sounds of Thunder.

 

See Figure 8.12 (4CE, p. 223) / Figure 8.16 (3CE, p. 214), “Thunderstorm occurrence.”

 

  

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."  For more on what a tornado is (with graphics), click here.

 

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. 

 

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. 

 

Wikipedia has a good article on tornadoes.

 

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

 

Tornadoes are measured on the Fujita Scale (Click for pictures) ... F0 to F5.  See Table 8.1 (4CE, p. 226) / Table 8.1 (3CE, p. 218), "The Fujita Scale." The Edmonton tornado of 1987 was an F4.

  

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 provides a reasonably accurate portrayal of tornadoes and the damage they can cause.

 

Everything you want to know about tornadoes ...  click here.

 

 

For links to specific major tornado events in the U.S., click here.

 

 

 

C.     Weather Systems in the Tropics (and on the East Coast of Canada!!!) -- Hurricanes (a.k.a Tropical Cyclones 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 and typhoons are also used to describe tropical cyclones.  They are synonyms.

 

Whereas mid-latitude cyclones are large, revolve slowly and are rather ponderous, tropical cyclones are small, revolve very quickly and move swiftly.

 

·        Tropical cyclones are classified as storms with winds over 120 km/h. 

 

·        They form over tropical seas (27°C +), from about 5°N and S to 20°N and S (but not at the Equator).  For a map of source regions, click here.

 

Exactly how they form is unknown - it appears that localized strong updrafts can produce small storms or squalls that then intensify as they travel across the ocean.  Rapid evaporation, uplift and condensation causes great quantities of latent heat to be released, fueling the storm with much energy.  The Coriolis force causes the spiral effect that, although weak, causes the air to begin to rotate.

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. 

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

 

Recent global hurricanes/typhoons of note include:

 

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:

 

 

        D.  Blizzards

 

Blizzards are severe winter storms characterized by the following:

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  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, Qubec, New Brunswick, and Nova Scotia without power for up to 12 days.

 

A list of major ice storms is here:  List of ice storms - Wikipedia, the free encyclopedia

 

 

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.

If you want to think about these issues further:  John Houghton, the former Director-General of the British Meteorological (Met) office,  describes how he answers the question often put to him: 'Should we pray about the weather?' He explores the nature of prayer, drawing on his knowledge of science, and his experiences as a Christian.  Read his story here.

 

 

To review … 

 

 

Check out the resources at Welcome to mygeoscience place.

 

 

This page is the intellectual property of the author, Bruce Martin, and is copyrighted © 2015 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