Introduction To Introduction To Weather DynamicsWeather Dynamics

What you will learn…What you will learn…

In this chapter, you will…

•Describe Earth’s energy budget

•Explain how energy is transferred between and among land, air and water

•Describe weather-related properties of the atmosphere such as pressure and humidity

•Explain how areas of high and low pressure move air and energy around the globe

MeteorologyMeteorology

The study of The study of

the Earth’sthe Earth’s

atmosphereatmosphere

and weatherand weather

systemssystems..

ClimateClimate

AA

widespreadwidespread,,

long-lasting long-lasting and recurring and recurring conditions of conditions of

thethe

atmosphereatmosphere..

WeatherWeather(page 10)(page 10)

The day to day The day to day changes in thechanges in the

atmosphereatmosphere

at a particularat a particular

locationlocationon Earth.on Earth.

Components of WeatherComponents of Weather(see Table 1.1, page 11)(see Table 1.1, page 11)

Temperature Coolness or warmth of an object (average of kinetic energy – oC)

Precipitation Any form of water that falls to Earth from atmosphere (mm or cm)

Atmospheric Pressure The force of the atmosphere per square meter of surface below it (kPa)

Humidity The amount of water vapor in the air (%)

Wind Speed & Direction How quickly the air moves and its direction (km/h; NSEW bearings)

Sky Cover The portion of sky that is covered by clouds

Earth’s Energy BudgetEarth’s Energy Budget(page 13)(page 13)

• Requires contact between atoms; more energetic atoms collide with more energetic atoms and energy is transferred

Types of Energy Transfer Types of Energy Transfer - Conduction- Conduction

• In a gas or liquid, atoms are free to move and as they warm, they will move away from the heat source and be replaced by cooler atoms

Types of Energy Transfer Types of Energy Transfer - Convection- Convection

• Energy is transmitted as photons (electromagnetic radiation); does not require a medium

Types of Energy Transfer Types of Energy Transfer - Radiation- Radiation

• Is composed primarily of:– Infrared– Visible– Ultraviolet Light

• Solar constant is defined as the amount of radiant energy that hits one square meter of the Earth’s outer atmosphere every second (1362 J/s/m2)

• Unit of energy Joule (J)• See Figure 1.7, page 17• Greenhouse Effect – the warming of Earth as a result

of greenhouse gases (CO2, water), which trap some of the energy that would otherwise leave Earth

Solar RadiationSolar Radiation

Solar Radiation Arriving Solar Radiation Arriving at Earth’s Surfaceat Earth’s Surface (see Figure 1.4, (see Figure 1.4,

page 14) page 14)

• Earth would constantly increase in temperature if it did not radiate energy back to space.

• It would take approximately 25 years for the oceans to boil if no energy was emitted back to space

• Terrestrial Radiation is composed primarily of infrared photons (light)

• Albedo – the reflectivity of a surface

Terrestrial RadiationTerrestrial Radiation

• Specific Heat Capacity – a property of a substance that determines the rate at which energy is absorbed or release

• Water has a higher specific heat capacity than land and air

• Heat Sink – any substance that can absorb and keep a “pool of energy without changing state

• Water is a good heat sink thus major weather factor!

Energy Transfer & WaterEnergy Transfer & Water

Substance

Specific Heat Capacity at

25oC (J/kg/oC)

Water 4186

Air 1020

Iron 444

Copper 385

Sand 290

Gold 129

• Has a large specific heat capacity in all three states– Gas 2080 J/kg/°C– Liquid 4186 J/kg/°C– Solid 2050 J/kg/°C

• This means that water has the ability to absorb large amounts of energy with a small temperature change

• Has a large heat of vapourization (liquid to gas transition)

• Has a large heat of fusion (solid to liquid transition)

• This means that water will also absorb large amounts of energy as it changes state

Energy Transfer & WaterEnergy Transfer & Water

Energy Transfer & WaterEnergy Transfer & Water

Humidity

• Humidity – the amount of water vapor in the air• Warmer air can hold more water than cooler air

– Air at -20°C can hold 0.78g of water per kg– Air at 0°C can hold 3.84g of water per kg– Air at 20°C can hold 15.0g of water per kg

• Since it varies with temperature, we usually discuss relative humidity

HumidityHumidity

Relative Humidity

• Relative humidity is the measure of how saturated the air is

• It is calculated by dividing the amount of water in the air by the amount of water that can be held by the air and is reported as a percentage

%75%10053.1

15.1

%7.7%1000.15

15.1

xg

g

xg

g

Relative HumidityRelative Humidity

•Dew Point – the temperature at which air is saturated with water vapor so that it condenses and falls as precipitation

See Figure 1.9, page 20

Water CycleWater (Hydrological) Water (Hydrological)

CycleCycle

Layers of the Layers of the AtmosphereAtmosphere

• Atmospheric Pressure – the pressure exerted by air on its surroundings due to the weight of the air

• Measured in kilopascals (kPa)

• At sea level, the atmospheric pressure is 101.3 kPa (or 1 kg/cm3)

Layers of the AtmosphereLayers of the Atmosphere

Layers of the Layers of the AtmosphereAtmosphere

See Figure 1.8, page 19

Layers of the Atmosphere

• Troposphere– All water vapour is present here– All weather occurs here– From surface to ~10km– Temperature ranges from -50°C to 50°C

• Stratosphere– Ozone is present here– From 10km-50km– Temperature ranges -50°C to -30°C

Layers of the Layers of the AtmosphereAtmosphere

Layers of the Atmosphere

• Mesosphere– Meteorites burn up here– Some ions are present here– From 50km to 90km– Temperature ranges from -30°C to -90°C

• Thermosphere– Aurora present– Some ions are present here– From 90km to 180km (space)– Temperature ranges from -90°C to over 200°C

Ionosphere

• Ionosphere-Light from the sun is powerful enough to ionize atoms (remove electrons)

-This results in a layer of ions in the upper atmosphere that reflects radio waves

Layers of the Layers of the AtmosphereAtmosphere

The Causes of Weather

Science 10

Latitude

• Degrees north and south of the equator

• Range from 90°N (north pole) to 90°S (south pole) passing through 0° (equator)

• Lines are all equidistance apart

Longitude

• Degrees east and west of the prime meridian

• Ranges from 180°E (international date line) to 180°W (international date line) through 0° (prime meridian)

• Lines are furthest apart at the equator and closest together at the poles

Sun’s Rays

• The angle that the radiation from the Sun strikes the Earth is important to local climate conditions

• Approximately perpendicular at the equator

See Figure 1.11, page 26

Tilt of the Earth

• Earth’s poles are not oriented perpendicularly to the orbital axis

• This results in seasonal variation

• The angle is approximately 23.5°Note: seasons are not due to how close Earth is to Sun, i.e. Earth is 3%

closer to Sun during winter months in NH (see Figure 1.13, page 27)

Four Seasons

Four main positions:•Summer solstice (Jun 21)•Autumnal equinox (Sep 23)•Winter solstice

(Dec 21)•Vernal equinox

(Mar 20)

Circles and Zones

• Arctic Circle– 24 hours of daylight

during NH summer• Antarctic Circle

– 24 hours of daylight during SH summer

• Tropic of Cancer– Direct sunlight on

Jun 21• Tropic of Capricorn

– Direct sunlight on Dec 21

Air Masses

• Air will have different characteristics depending on where it forms

• Air Mass – a very large mass of air that has nearly uniform properties, i.e. temperature, humidity, pressure

• There are two main regions:– Polar– Tropical

• There are two main types:– Maritime– Continental

See Figure 1.14,

page 28

Air Masses of North AmericaNameName SourceSource CharacteristicsCharacteristics

Continental Continental Arctic (cA)Arctic (cA)

Arctic Basin, SiberiaArctic Basin, Siberia Very cold and dryVery cold and dry

Continental polar Continental polar (cP)(cP)

Canadian Interior, Canadian Interior, AlaskaAlaska

Cool and dryCool and dry

Maritime polar Maritime polar (mP)(mP)

North Atlantic & North Atlantic & Pacific OceansPacific Oceans

Cool and moistCool and moist

Continental Continental tropical (cT)tropical (cT)

SW USA & MexicoSW USA & Mexico Warm and dryWarm and dry

Maritime tropical Maritime tropical (mT)(mT)

Gulf of Mexico, Gulf of Mexico, Caribbean Sea, Caribbean Sea,

tropical/subtropical tropical/subtropical Atl./Pac. OceansAtl./Pac. Oceans

Warm and moistWarm and moist

See Table 1.2, page 28

Worldwide Wind and Ocean Currents

Science 10

Global Warming and Cooling of Air

• Air is warmest at the equator and coolest at poles

• So air should rise at the equator and sink at the poles

• However, due to the size of the Earth, air from the equator cools before it reaches the poles and air from the poles will warm before it reaches the equator

See Figures 1.16.1.17, page 30

Actual Wind Currents

• Air from the equator will cool by about 30° latitude and sink

• Air from the pole will warm by about 60° latitude and rise

• Further, air descends at 30° and rises at 60°

Coriolis Effect

• Due to the rotation of the Earth, any object that moves across the surface of the Earth will be pushed to the left or right

• The deflection direction is shown in the diagram

Global Wind Patterns

Main wind currents:•Polar Easterlies (north and south)

•Prevailing Westerlies (north and south)

•Northeast Tradewinds (north only)

•Southeast Tradewinds (south only)

See Figure 1.19, page 31Note: ‘prevailing’ means to the usual direction from which the wind blows!

Global Wind SystemsWind SystemWind System LocationLocation PathPath

Trade windsTrade winds

• Between equator and Between equator and 3030oo N latitude N latitude

• Between equator and Between equator and 3030oo S latitude S latitude

• Air at equator warms, Air at equator warms, rises and travels to 30rises and travels to 30oo N N or S lat.or S lat.

• At 30At 30oo N or S, air cools, N or S, air cools, sinks & moves W toward sinks & moves W toward Equator & deflected WEquator & deflected W

Prevailing Prevailing westerlieswesterlies

• Between 30Between 30oo and 60 and 60oo N N latitudelatitude

• Between 30Between 30oo and 60 and 60oo S S latitudelatitude

• Air circulation pattern is Air circulation pattern is opposite to that of the opposite to that of the trade windstrade winds

• Surface winds blow from Surface winds blow from W to E and towards PolesW to E and towards Poles

Polar EasterliesPolar Easterlies

• Between 60Between 60oo N latitude N latitude and NPand NP

• Between 60Between 60oo S latitude S latitude and SPand SP

• Air circulation pattern is Air circulation pattern is similar to that of the trade similar to that of the trade windswinds

• Surface winds blow from E Surface winds blow from E to W and away from Polesto W and away from Poles

See Table 1.3, page 31

Jet Streams

• A narrow band of fast-moving wind

• Polar Easterlies collide with Prevailing Westerlies

• Warmer Prevailing Westerlies climb over the cooler Polar Easterlies and result in a fast moving stream of air at the edge of the troposphere

• Aided by Coriolis Effect• Up to 300 kph• Altitude 10 – 12 km

Polar Jet Streams

• Major jet streams are polar jet streams

• Separate the polar easterlies from prevailing westerlies in NH and SH

• Occur between 40o to 60o N & 40o to 60o S

• Move west to east

See Figure 1.20, page 32

Subtropical Jet Streams

• Minor jet streams are subtropical jet streams

• Occur where tradewinds meet polar westerlies

• Occur between 20o to 30o N & 20o to 30o S

• Move west to east• Storms form along jet

streams & cause large-scale weather systems (i.e. their intensity

• Weather systems usually follow path of jet streams

See Figure 1.20, page 32

Ocean Currents

• Due to the uneven heating of the oceans, convection currents are established

• Cold currents carry cold air and warm currents carry warm air so continental weather is affected by the currents

Great Ocean Conveyor Belt

• The largest continuous ocean current that covers most of the globe

• Warm water is near the surface while cooler water travels at depth

Upwelling

• When wind blows surface water away from land, the cool water from below will rush up to replace it

• This leads to colder surface water than would be expected

Frontal Systems &High and Low Pressure

Science 10

Frontal Systems

• Front is the boundary between 2 air masses

• created when one air mass overtakes another

• They will typically be classified into two categories:

– Warm front – when warm air runs into cold air

– Cold front – when cold air runs into warm air

Warm Front

Warm Front

• The warm air gradually climbs over the colder air• Typical clouds associated with a warm front are:

– Cirrus– Cirrostratus– Altostratus– Nimbostratus– Stratus

• Associated with periods of steady precipitation• May exist over 50-100km

Frontal Systems&

High and Low Pressure

Science 10

Frontal Systems

• Fronts are created when one air mass overtakes another

• They will typically be classified into two categories:– Warm front – when warm air runs into cold air– Cold front – when cold air runs into warm air

Warm Front

Warm Front• The warm air gradually climbs over the colder air

• Typical clouds associated with a warm front are:

– Cirrus

– Cirrostratus

– Altostratus

– Nimbostratus

– Stratus

• Associated with periods of steady precipitation

• May exist over 50-100km

Cold Front

Cold Front• Occurs when warm air is

violently pushed up by advancing cold air

• Cumulus clouds are associated with a cold front

• Can result in cumulonimbus clouds

• Short periods of heavy precipitation

• Covers a small geographical area (2-5km)

Stationary Front• A non-moving (or stalled) boundary between two

air masses, neither of which is strong enough to replace the other

• They tend to remain essentially in the same area for extended periods of time, i.e. several days

Occluded Front• Occurs when a warm front is

overtaken or occluded by a faster moving cold front

• When an occluded front passes overhead, changes in temperature and wind speed are observed

• Occluded fronts can generate stormy weather as they pass over

Weather Map

High Pressure

• High pressure has “more” air above it

• It is pulled down by gravity

• Due to Coriolis Effect, it spins clockwise and out in the Northern Hemisphere

• Clear skies• Light winds

Low Pressure

• Low pressure has “less” air above it

• Air from a high pressure flows towards a low pressure

• Due to Coriolis Effect, it spins counter clockwise and in in the Northern Hemisphere

• Clouds• Strong winds

High and Low Pressures