geography 12 9.1-9.2

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Chapter 9 Weather (KWP) Page of 3

Chapter9: Weather

9.1 Weather and Air masses and Fronts

To measure and determine weather, we need to collect a lot ofdata

Wind data, speed and direction is collected using an anemometer. To gather data on air pressurewe use a barometer Pressure is spoken of as the barometric pressure Data on precipitationis

collected using a raingauge

And of course, temperaturedata is gathered using athermometer

These can bewet bulb or dry thermometer An air massis an extremely large body of air whose

properties of temperature and moisture content(humidity), at any given altitude, are fairly similar in anyhorizontal direction

The air mass can cover hundreds of and thousands ofsquare miles

An air mass is characterised by its geographic origins These include: (IMPORTANT) cPcontinental polar cold, dry stable cTcontinental tropical hot dry, stable air aloft - unstable

surface air mP maritime polar cool, moist, and unstable mTmaritime tropical, warm, moist, usually unstable These air masses contribute much to local weather

patterns Any maritime air masswill contain a great deal of

moisture in its make-up Polar air masseswill be dry and cold This is important information to the agricultural

industry In a map of North America one can see the air

masses that affect our lives

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Chapter 9 Weather (KWP) Page 2of 3

When two air masses meet, they will createweatheraccording to the characteristic ofthe fronts

It also depends on the size of the front Cold air is denser than warm air A cold front meeting a warm front will have an effect like a bulldozer A warm front meeting a cold front will act differently The idea of fronts in meteorology came from a Norwegian meteorologist in World War I

named Vihelm Bjerknes During the war, he observed how the opposing forces would close in "fronts" When one force would push in it caused displacement in this opposing force. Air masses are not always moving Sometimes an air mass will remain over one area for several months This will leave the area relatively

unchanged weather -wise The preceding slide showed an

occluded front.

This type of front occurs when the coldfront and the warm front meet usuallyhead on.

Sometimes a cold front will meet witha mass of warm air.

In that case the warm air is pushed up. As it rises and it is in contact with the cold air, the water molecules condense causing

precipitation. Sometimes it is the warm air mass

that is moving This creates awarm front Since warm air is less dense than

cold air, when a warm front meets acold air mass it "coaxes" the airmasses away

Pushing against the cool air yet stillgenerating cloud formation andprecipitation

If the two air masses do not meet head on but brush up against each other, they cangenerate strong winds

These winds can sometimes generate theformation of tornadoes or other violent windpatterns

Fronts are areas of extreme cyclonic activity as well Since the fronts are either warm or cold, when they

meet they begin creating low pressure systems(cyclones)or high pressure systems(anticyclones)

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Meteorologists use a unique type of map that shows these areas of differing pressure onthe earth

*Meteorologistsstudy weather! These pressure areas are marked by the fronts and shown on the map using isobars Isobars are similar to contour lines on a topographical map. However, instead of showing differing elevations, the isobar shows differing pressure

areas

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Cloud

The cloud formation that occurs can also say much about the front Cloudsare basically water molecules in sufficient quantity so as to be visible to the

naked eye The type of cloud that forms can tell us about the weather Clouds are visible accumulations of water droplets or solid ice crystals that float in the

Earth's troposphere (the lowest part of the Earth's atmosphere), moving with the wind From space, clouds are visible as a white veil surrounding the planet Clouds form when water vapor (water that has evaporated from the surface if the Earth)

condenses (turns into liquid water or solid ice) onto microscopic dust particles (or othertiny particles) floating in the air

This condensation (cloud formation) happens when warm and cold air meet, whenwarm and cold air meet, when warm air rises up the side of a mountain and cools as itrises, and when warm air flows over a colder area, like a cool body of water

This occurs because cool air can hold less water vapor than warm air, and excess watercondenses into either liquid or ice

Clouds are defined by both the way they look and how high they are in the atmosphere For example, cirro(meaning "wisp of hair") is a prefix given to high-altitude clouds

(above 20,000 feet) Alto(meaning "high" is a prefix given to mid-altitude clouds (between 6000 and 20000

feet) There is no prefix of low-altitude clouds When clouds are by the ground we call them fog And no, fog is not just lazy clouds Nimbo(meaning "rain") as a prefix, or nimbus added as a suffix, in a cloud name

indicate that the

cloud can produceprecipitation (rain,snow, or otherforms of fallingwater)

Cumulo(meaningheap) refers topiled up clouds.

Strato(meaning"layer") refers to

flat, wide, layeredclouds

Now lets look atthe types of cloudsand their place inthe sky.

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Clouds are defined by theirheight in the atmosphere

The cloud formation isdetermined by this and manyother factors.

Remember that nimbusmeans rain

These clouds are carriers ofmuch water vapor

When this vapor is cools, itcondenses and falls as rain

Hail

Rain clouds have a lot ofenergy

In summer when the ground is radiatinga lot of heat, that heat forces moisture upinto the atmosphere

If the ground is hot enough, the resultingcoalescing of water vapor will not have achance to fall as rain

The droplet will be forced back up intothe cloud where it will encounter verylow temperatures

The resulting droplet will freeze If the energy from the earth is great

enough, the frozen water will attempt tofall but instead will be forced back upinto the cloud where it will gain anotherlayer

Sleet

Sleet is another form of precipitation It happens in cold, unstable weather

conditions Sleet is rain or melted snow thatfreezes into ice pellets before hitting

the ground Sleet only happens under very

specific weather conditions There must be a layer of air near the

ground whose temperature is belowfreezing, where water turns into ice

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Above this layer of freezing air must be a layer of a warmer air As snow falls through the warm air, it melts or partially melts into raindrops. As the

melted snow falls through the cold layer of air, it re-freezes It forms ice pellets, or sleet, before hitting the ground The main challenge in sleet is the effect is has on driving conditions

Freezing Rain

Freezing rain happens when raindroplets fall through the air thatstemperature is below zero andfreezes on impact with the groundor another object to form clear ice -also known as glaze

Generally, freezing rain starts its lifeas snow, ice, sleet or hail, but passes

through a layer of warmer air on theway down This causes it to melt and return to

a liquid form As it continues towards the ground,

it then briefly passes through colderair again, causing the water dropletsto become 'super-cooled' - colderthan 0*C but still in liquid form

When these super-cooled dropletsfall on ground which is close to or below freezing, they can freeze on impact - quickly

creating treacherous conditions Freezing rain can have serious effects on driving and walking

Air masses and fronts

To recap, fronts and frontal systemsgenerate weather

This weather depends on thecharacteristic of the particular front

Cold dry fronts meeting warmmoist fronts will generate rain

Awarm front, because it is slopinggently, will generate long periods ofrain

Cold frontsmeeting warm airmasses may cause great torrents ofrain as the warm air is forced over

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the cold air mass, but the rain will usually not last long Severe weather is caused by the meeting of large air masses In Canada, we are concerned with the following air masses.... Continental Arctic, Maritime Arctic , Maritime Polar

Jet Stream

We spoke ofjet streamsearlier The effect they have on weather can be important

9.2 Severe weather systems (page 177-194)Equatorial weather

Near the equator the trade winds affect the weather patterns more than the air masses

The intense heat energy from the sun causes great amounts of evaporation This sends water vapor into the atmosphere and

bringing much energy with it. In some areas of the equatorial zone, storms are almost

a daily event The type of rainfall associated with the high

evaporation levels is convectional rainfall This type of rainfall occurs when the sun heats the land

surface, the rising air carries with it masses of the watervapor

When the air rises to a point where cooling takes place,rain forms

There are three main types of rainfall These are, convectional, orographic and cyclonic or frontal From the previous slide we can see that convectional rainfall occurs when there is a

large upwelling of water vapor into the atmosphere For any precipitation to occur there must be a vertical movement of air Another type of rainfall occurs when an air mass is pushed up a mountain

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In mountainous regions this type of rainfall is quite common Look at how North Vancouver is often clouded while Vancouver is clear and sunny The air cools as it rises causing the water vapor to condense Orography is the science related to mountains Rain caused by mountains

is called orographic

precipitation When moisture laden air

encounters a mountain, it ispushed up

As it rises the air coolscausing the water vapor tocondense causing it to fallas rain

One of the results of this isthe creation of a rain

shadow Once the air mass has

lost all of its moisture, itmoves to the other sideas a dry air mass

This is the situation inplaces like theOkanagan valley

Parts of the valleyreceive very littleprecipitation

Then the air picks upmoisture from the lakesand it falls on theeastern side of the lake

The last type of rainfallis the cyclonic or frontal rainfall

We have already discussed the functionof frontal systems and how when theymeet, they create various types ofunstable weather

This type of rain can be torrential in thecase of an advancing clod front, or landperiods of constant light rain as in thecase of an advancing warm front

Hurricane

Hurricanesbegin as tropical storms

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Where the surface temperature is 27 degrees Celsius for a period of time, the intenseevaporation acts like fuel to the growing storm

The process by which a tropical cyclone forms and subsequently strengthens into ahurricane depends on at least three conditions;

T1. A pre-existing disturbance with thunderstorms 2. Warm (at least 27 C) ocean temperatures to a depth of about 150 feet 3. Light upper level winds that do not change much in direction and speed

throughout the depth of the atmosphere (low and shear) Heat and energy for the storm are gathered by the disturbance through contact with

warm ocean waters The winds near the ocean surface spiral into the disturbance's low pressure area The warm ocean waters add moisture and hear to the air which rises As the moisture condenses into drops, more heat is released, contributing additional

energy to power the storm Bands of thunderstorms form, and the storm's cloud tops rise higher into the

atmosphere

If the winds at these high levels remain relativelylight (little or no wind shear), the storm canremain intact and continue to strengthen

Since the moisture of the ocean is the fuel of thehurricane, a hurricane will typically die if it isover land for an extended period of time

When hurricanes near land, the devastationcaused is extensive

Winds of several hundred kilometers per hourpick up anything in its path

The incredible pressure of the hurricane causesan imbalance

When the hurricane confronts a building that is sealed tight, the pressure difference cancause the building to explode

The majority of damage and loss of life in a hurricane is due to the flying debris Hurricanes are named alphabetically and according to their origin

Tornadoes

Tornadoesare another type of cyclonic storm The station model in the previous slide is an American example The temperature is in Fahrenheit Wind speed is calculated in knots and so is universal The wind speed flag shows the strength and the direction of the wind The difference is that tornadoes occur over land Lacking the fuel supplies by a warm body of water, the tornado is less intense than a

hurricane This allows meteorologists to track repeat hurricanes and study the causes

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A tornado is a narrow, violently rotating column of air that extends from the base of athunderstorm to the ground

Because wind is invincible, it is hard to see a tornado unless it forms a condensationfunnel made up of water droplets,dust and debris.

Tornadoes are the most violent ofout all storms

Tornadoes often happen in middleAmerica

This is an area called thetornadoes alley

Tornadoes cost millions of dollarsannually as devastation strikes lowincome areas

Meteorology

There are several ways that meteorologists use to track weather Some may have heard of the Doppler Radar This is a radar station that can detect pressure changes as they move It can also track storm systems Meteorologists use weather maps which are updated on a daily basis

This information is essential for marine traffic and airtraffic

A station modelis a symbolic representation thatshows weather conditions in an area

These are included on a weather map allowing for aninstant understanding of weather patterns

The flag pointsin the direction of the wind You think of the flagas an arrow

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The centeris the point of the arrow The barbsshow the speed of the wind The circleshows the type of cloud cover Using this a pilot can predict the type of visibility that he/she

will encounter Temperature, relative humidity and dew point are all related to

one another Temperatureis the measure of the energy in the air, relative

humidityis the measure of water vapor in the air, and the dewpointis the temperature at which the water vapor in the airwill begin to condense into liquid water

To find dew point, the temperature and wet bulb temperatureneed to be known

The temperature is read straight from a thermometer Thewet bulb temperatureis the temperature air will cool to when complete

evaporative cooling occurs

Getting the wet bulb temperature requires their use of a psychomotor or hygrometer. The dew point temperatureis the temperature at which the air can no longer hold all

of the water vapor which is mixed with it, and some of the water vapor must condenseinto liquid water

The dew pointis always lower than (or equal to) the air temperature Humidityis the water vapor in a parcel of air There is absolute humidity and relative humidity Absolute humidityis simply the actual amount of water help in a parcel of airRelatively humidity

Relative humidityis the ratio ofthe actual amount of water vaporpresent in a volume of air at a giventemperature to the maximumamount that the air could hold atthe temperature, expressed as apercentage

Warm air can hold more water vaporthan cool air, so a particular amountof water vapor will yield a lower

relative humidity in warm air than itdoes in cool air One of the worldsmost puzzling

weather challenges is the El Nioevent

It is named as such because it wasfirst noticed around Dec 25th

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El Niois Spanish for the child and is a reference to the god of the Catholics whosebirth they celebrate on that day

One of the worldsmost puzzling weather challenging is the El Nio event It is named as such because it was first noticed around December 25th El Nio is Spanish for the child and is a reference to the god of the Catholics whose

birth they celebrate on that day

The El Nio event begins in the South Pacific normally, when El Nio is not operating, a giant convection cell forms in the western

Pacific between the South American coast and Australia The trade wind push surface water westward and the ocean piles up near Indonesia (sea

level around Indonesia is actually 2 feet higher than at Peru!)

This cooler water rising off the coast of Peru brings rich nutrients from the depths This provides abundant food for the food The fishing season is very good when there is no El Nio. During an El Nio event ,for some reason, the trade winds are not strong enough to

push the surface water westward When this happens, the warmer water cannot make it to Indonesia This loss of the warm water and the accompanying low pressure system means that the

seasonal rains do not fall Indonesia experiences a significant increase in forest fires

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The results are many In an El Nio event year, the fishing in Peru is poor Droughts will occur deeper inland in North America and Africa Flooding will occur in places where it is uncommon

Test Information

Air masses Weather Station Diagram Explanation section about El Nio And more..

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