Geography 1000 - Test 3

Energy, Temperature, Atmospheric Circulation, Air Masses, Cyclogenesis, Violent Weather, Geologic Time, and the Rock Cycle

Isotherms

Lines that connect places with equal temperature

Global temperature patterns → January

• Continental interiors cool strongly in Northern Hemisphere winter (Opposite for Southern Hemisphere)

• Oceans and currents moderate nearby temperatures

Global temperature patterns → July

• Thermal equator shifts North in July because Northern Hemisphere land heats strongly

• Land warms and cools faster than water

• Northern Hemisphere has more land area so seasonal temperature shifts are larger

• Temperature contrasts help create pressure gradients that drive circulation

Air pressure and atmosphere

• pressure depends on the amount and motion of air molecules above a surface

• High altitude = less density = less pressure

• Low altitude = more density = high pressure

Temperature and Pressure Relationship

• Temperature contrasts create pressure differences

• Cold air = more dense = higher surface pressure

• Warm air = less dense = lower surface pressure

• Warm air expands, becomes less dense and rises

• Cold air is denser and sinks

• This produces horizontal pressure differences that the atmosphere tries to reduce.

Horizontal and Vertical Air flow

• Wind is a vector; has speed and direction

• Horizontal flow: air moves from high pressure to low pressure

• Vertical flow: air rises where air converges near the surface. Rising air cools and can produce clouds/precip.

3 main controls of wind

1. Pressure-gradient force

2. Coriolis effect

3. Friction

Pressure-gradient force

• starts the wind

• A pressure gradient is a change in pressure over distance

• Pushes air from higher pressure to lower pressure

• The stronger the gradient, the faster the air accelerates

Isobars

• connect locations with equal pressure

• Closely spaced isobars mean a steep pressure gradient (rapid change)

• Steep pressure gradient means stronger winds

Thermal circulation and local pressure

• contrasting warm and cool surfaces can create high pressure and low pressure cells.

• Warm surface = air rises = lower surface pressure

• Cool surface = air sinks = higher surface pressure

• Near the surface, air flows from high pressure to low pressure

Coriolis effect

• Earth’s rotation makes moving air appear to curve relative to the surface.

• Does not start the wind, only changes the direction of air already moving

Coriolis effect → hemisphere, latitude, and speed

• Northern hemisphere → moving air deflects right

• Southern hemisphere → moving air deflects left

• Strongest towards poles; no deflection at the equator

• Faster motion = stronger deflection

Friction

• rough surfaces drag on moving air and weaken the Coriolis deflection

• Friction strongest close to the ground

• Rough terrain, vegetation, buildings, etc. increase drag

• When the wind slows → Coriolis less strong → surface wind cross isobars

High/low pressure vertical motion

Pressure systems are 3D and surface pressure is linked to rising and sinking air

Low pressure: Surface convergence and rising air lead to clouds and precip.

High pressure: Surface divergence and sinking air lead to more clear sky and stable weather

Global pressure belts

Polar Highs (N and S) → cold, dense dry

Subpolar lows → Cool, stormy, frontal

Subtropical highs → Sinking, hot, dry

Equatorial low → Warm, risinh, wet

Equatorial Low / ITCZ

• Surface winds converge towards equatorial low

• Warm, humid air rises, then expands and cools

• Cooling promotes condensation, clouds and heavy rainfall

Subtropical Highs

• Sinking air compresses and warms

• Warming air can hold more water vapour which suppresses cloud formation

• World’s major deserts in these bands

Subpolar lows

• Occur near strong temp contrasts between polar/mid latitude air

• Cloudy, wet, stormy conditions

• Important for mid-latitude cyclone development

Polar Highs

• very cold air near poles tends to sink and create surface high pressure

• Cold air is dense

• Cold air holds little water vapour so these regions have dry air

• Surface flow moves outward from polar highs to areas of lower pressure

Trade Winds

• Blow toward the equatorial low

• Deflected by Coriolis

• Important for tropical circulation and ocean currents

Westerlies

• Dominate much of the mid-latitudes

• Steer weather systems from West to East

Polar Easterlies

• Flow outward from polar highs

• Cold, dry, shallow

Jet streams / fast upper-level winds

• narrow bands of fast winds high in the troposphere

• Occur near strong temp. gradients and steer weather patterns

Rossby waves

• move warm and cold air north/south

• Move warmer air poleward

• Move colder air towards equator

• Influence pressure systems and storm development

Land/sea breezes night and day reversal

• Land heats and cools faster than water so the pressure gradient changes direction

• Day → land warms quickly →lower pressure over land → sea breeze moves inland

• Night→ land cools quickly → higher pressure over land →land breeze moves offshore

Mountain + Valley slope and wind

• Slopes heat and cool differently through the day, reversing the direction of airflow

Day: Warm slope promotes air flow upwards and up the valley

Night: Cool, dense air drains downslope and down valley

Topography channels the wind. Terrain strongly control local patterns

Monsoon winds

• seasonal reversal in winds driven by land-ocean heating contrasts

Summer land heating creates low pressure. Moist are flows inland from the ocean and brings seasonal rain.

In winter, the pressure contrast reverses and dry wind flows offshore

Wind and surface ocean currents

• wind pushes surface water

• Coriolis redirects moving water and air

• Currents transport heat, salt, and nutrients

Ocean gyres

Major current systems carry warm water poleward and cool water to the equator.

Warm currents moderate costal climate

Cold currents cool coast and create fog.

Air parcel

A conceptual “bubble” of air used to understand local processes such as rising, sinking, expansion and compression

Air mass

A large body of air with relatively similar temperature and humidity over a broad region, often hundred to thousands of kilometers across

Where do air masses form?

• over broad, uniform source regions

  • Large area with similar surface conditions

  • Relatively flat or topographically simple

  • Light winds allow air to stagnate and adjust

  • Persistent high-pressure regions are especially favourable

Air masses → cold air over a warm surface

• air is heated from below

• Lower layers become less stable

• Turbulent mixing/rising motion can increase

Air masses → warm air over a cold surface

• Air loses heat to the surface

• Lower layers of air become more stable

• Fog, low clouds, or inversion can develop

Air masses → Over water / wet vegetation

• evaporation adds water vapour

• Warm water supplies more vapour than cold water

• Air can become humid

Air masses → over dry land

• Little water is available for evaporation

• Air often remains relatively dry

• Continental/and surfaces produce more dry air masses

What are the 4 airmass labels for moisture and temperature?

• c (continental)

  • Dry

• m (maritime)

  • Moist

• T (tropical)

  • Warm

• P or A (Polar/Arctic)

  • Cold

Describe cP / cA air masses

• cP → continental polar

• cA → continental arctic

Source: high-latitude continents and ice covered regions

Temperature and moisture: cold and dry

Effect: Sharp cooling when air mass moves southward in winter

Describe mP air masses

• mP → Maritime Polar

Source: North Pacific and North Atlantic

Temperature and moisture: cool and moist

Effect: Clouds, precipitation, cool conditions, modified by mountains in Western Canada

Describe mT air masses

• mT → Maritime tropical

Source: subtropical oceans and Gulf of Mexico

Temperature and moisture: warm and humid

Effect: warm, muggy conditions

Describe cT air masses

• cT → Continental tropical

Source: Hot subtropical continental interiors

Temperature and moisture: Hot and dry

Effect: Very warm, dry, influential father south in North America

4 lifting mechanisms

1. Convergent lifting

2. Convective lifting

3. Orographic lifting

4. Frontal lifting

Convergent lifting

Air flows together near a low-pressure area, forcing air to rise

Convective lifting

Surface heating makes air less dense, causing it to rise

Orographic lifting

Air is forced to rise over a mountain barrier

Frontal lifting

Warm, less dense air is lifted over a colder + denser air front

Fronts

• Fronts are boundaries between contrasting air masses

Why are fronts active?

• air masses differ in temperature, moisture, and density

• Denser air stays closer to the surface

• Warmer. lighter air is lifted along the boundary

• Rising air cools → condensation → clouds and precipitation

Cold front

Blue triangles point in the direction the cold air is advancing

Warm front

Red semicircles point in the direction the warm air is advancing

Stationary front

Triangles/semicircles on opposite sides mean little net movement

Occluded front

The cold front has overtaken the warm front

Cold front

• cold/dense air undercuts warm air along a steep boundary

Cold air advances at the surface. Warm air is displaced upward quickly. The frontal slope is steep

Cold front lifespan

Before: warm/humid air at surface. Pressure may fall.

Passage: Narrow band of heavy showers/thunderstorms, gusty winds

After: Temperature drops, pressure rises, cool dry air follows

Steeper frontal lifting means narrower and more abrupt weather transition

Warm front

• warm air glides up and over retreating cooler air along a gentler slope'

Warm air advances but cannot easily displace denser cool air. Air mass rises gradually over the cool air. The frontal slope is shallow. Expands across a wider horizontal area

Warm front lifespan

Before: High clouds often appear first and gradually thicken

Passage: Layered clouds, longer duration of precipitation

After: Air becomes warmer, more humid, wind shifts

A shallow frontal slope produces a broad zone of gradual lifting

Stationary front

• form when neither air mass advances

• Front has little net movement

• Winds may run parallel to the boundary

• Can produce prolonged periods of clouds/precipitation

• Can later develop into a cyclone

Occluded fronts

• a faster moving cold front catches the warm front and lifts the warm air away from the surface

Cold front overtakes warm front near low pressure system. Warm surface air becomes “pinched” off. Temperature contrasts at the surface weaken

Late stage of cyclone lifecycle

Features of mid-latitude cyclones

• Large low-pressure system

• Warm and cold fronts wrap around the low

• Can travel long distances'

• Brings cloud, precipitation, wind shifts and temperature changes

Cyclogenesis stages

1. Stationary Front

2. Frontal wave

3. Open wave

4. Mature cyclone

5. Partial occlusion

6. Occluded low

Cyclogenesis → stationary front

• boundary separates colder and warmer air

• May be little movement at first

• Strong temperature contrast stores potential energy

Cyclogenesis → frontal wave

• Warm air starts to move poleward

• Cold air starts moving towards the equator

• Counter-clockwise circulation creates low pressure system (In Northern Hemisphere)

Cyclogenesis → open wave

• distinct warm and cold fronts form as the low intensifies

• cloud and precipitation bands develop along the fronts

Cyclogenesis → mature cyclone

• pressure is low but circulation is strong

• Cold front moves faster than warm front

• Cyclone reaches maximum organization and produces its strongest weather

Cyclogenesis → Partial occlusion

• The cold air wraps around the low pressure system.

• Warm section shrinks

• The storm loses temp. contrast

Cyclogenesis → occluded low

• warm air is lifted from surface and the storm weakens

• Cyclone dies

Mature cyclone sequence of weather

Before: Clouds thicken, widespread precipitation may begin

Warm Sector: Warmer, humid, windy conditions

Cold-front passage: Showers, storms, wind shift

After: cool/drier air, pressure rises, skies clear

Surface map (forecasting)

• Locates highs, lows, and fronts

Satellite image (forecasting)

Shows cloud structure and storm organization

Upper-level flow (forecasting)

• helps predict track of storm and intensity

Process of rising air forming clouds

1. air is lifted

2. Air expands and cools

3. Saturation is reached

4. condensation forms cloud

What factors make weather severe?

• Strong Winds

  • Straight-line winds, downbursts, tornadoes

• Ice

  • Large hail

• Electrical Hazards

  • Lightning

• Water

  • Heavy rain, flash flooding, storm surge

• Storm organization

  • Long-lived / rotating storms can increase hazard intensity

What is a thunderstorm?

• A convective storm produced by a cumulonimbus cloud

3 necessities/ingredients for a thunderstorm to form

• Moisture

  • Warm, humid air provides water vapour

• A trigger / lift

  • Heating, a front, convergence, or terrain required to start the ascent of air

• An atmosphere that supports rising air

  • Air needs to be in an environment where it will continue rising rather than sink back down

Key features of a thunderstorm cloud

• a thunderstorm is a deep convective cloud that connects surface air to upper troposphere

• Cumulonimbus

• Strong updrafts

• Downdrafts and precipitation

• “Anvil” shape near the top of troposphere

Stages of thunderstorm cloud formation

1. Cumulus stage

2. Mature stage

3. Dissipating stage

Storm lifecycle → the cumulus stage

• young / baby storm

• Warm and moist air rises in an updraft

• The cloud begins to grow rapidly upwards

• Little precipitation reach the ground

Storm lifecycle → mature stage

• storm is strongest/most hazardous

• Updrafts and downdrafts at the same time

• Heavy rain reaching the ground

• Possibility of hail and lightning

• Strong winds

Storm lifecycle → dissipating stage

• downdraft cuts off supply of warm moist air, causing precipitation to gradually decrease

• Cloud breaks apart

Downdrafts and surface winds

• downdrafts can create very strong and damaging surface winds

• Rain and evaporative cooling cause dense air to accelerate quickly downwards.

• Can result in an abrupt change in speed/direction and temperature can drop quickly

3 types of thunderstorm organization

1. single-cell

2. Multicell

3. Supercell

Single-cell thunderstorm

• Short lived (1 hr ish)

• Relatively weak

• Usually a result of afternoon heat

Multicell thunderstorm

• New cells repeatedly created

• Lasts for a long time (12 hr ish)

Supercell thunderstorm

• rotating updraft

• Most intense severe weather

• Lasts (2-4 hr ish)

Wind shear

• wind speed or direction changes with height

  • tilts the storm

  • Separates downdrafts and updrafts, meaning the storm has a constant inflow that is not cut off by downward flowing air

  • This can cause some storms to persist and intensify → multicell

Hail formation

• need a strong thunderstorm updraft

• Ice particles form above the freezing level

• Cooled droplets freeze onto each other

• Updraft keeps particles aloft in the cloud longer than normal

• Ice eventually becomes too heavy to be supported by updraft and fall to the ground

What conditions create bigger and stronger hail?

Weaker updraft = smaller particles that fall out sooner

Stronger updraft = larger particles; remain in cloud longer and continue to grow

How does lightning form?

• collision among ice particles transfer electrical charge

  • small ice crystals above, larger ice crystals below

• Electrical field strengthens until the air can no longer insulate the difference in charges

• Results in rapid electrical discharge through the atmosphere

Lightning vs thunder

Lightning = electrical discharge

Thunder = sound wave

Thunder

• rapid heating makes the air expand explosively and produces a pressure wave that we hear as thunder

• Light travels faster than sound, that is why you see lightning before you hear thunder

Where does lightning occur?

• Areas with deep convection storms

  • The tropics

• More common over land than ocean because land heats more strongly.

Tornado vs. funnel cloud

• a funnel cloud is not a tornado until the circulation reaches the ground

How does a tornado develop?

1. wind shear that creates horizontal spinning

2. Updraft tilts the spin upright

3. Stretching increases speed of rotation

4. Mesocyclone forms'

5. Tornado forms once funnel cloud reaches the ground

How is tornado intensity measured?

• Enhanced Fujita scale (EF)

• Based on damage evidence, not wind speed.

EF0 - EF1 → Weaker damage

EF2 - EF3 → Strong/severe damage

EF4 - EF5 → Extreme/devastating damage

Where do tornadoes occur in Canada?

• concentrated where the air is warm, moist, and there is strong environmental conditions

• Southern Prairies and Ontario

• Tornadoes vary by season

• Areas with less population or weather stations may result in more remote area tornadoes not being reported

Mid-latitude cyclones vs tropical cyclones

Mid latitude cyclone

• Strong horizontal temperature contrast

• Warm and cold fronts

• Common in Mid/high latitudes

Tropical Cyclone

• Warm ocean supplies heat and moisture

• No fronts needed

• warm-core storm

What is a hurricane?

A large, rotating tropical cyclone that forms over the warm ocean

Where do hurricanes develop?

• Over warm tropical oceans, but not directly at the equator (5 to 30 degrees latitude)

• Don’t happen at equator because coriolis forces clouds North and South

Tropical cyclone structure

Eye: relatively calm, sinking air

Eyewall: strongest winds and deepest convection

Rainbands: curved bands of thunderstorms

Outflow

Tropical disturbance → tropical depression → tropical storm → hurricane

1. Tropical disturbance

• Cluster of thunderstorms

2. Tropical depression

• a closed, low-pressure circulation with winds less than 63km/h

3. Tropical storm

• more organized circulation with winds from 63-118 km/h and receives a name

4. Hurricane

• Maximum sustained winds of over 119 km/h