Meteorology

Chapter 26: Weather and the Atmosphere

Weather

• the state of the atmosphere at a given time and place

• i.e., type of clouds, precipitation, temperature, wind speed and direction)

Meteorology

• study of the atmosphere (including the weather).

Composition of the Atmosphere

• mixture of gases called air.

• Main gases:• Nitrogen (78%)• Oxygen (21%)• Argon (0.9%)• Carbon dioxide (0.03%)• Rare gases (He, H, Ne, Ozone, Kr, etc.)

Composition of the Atmosphere

• At higher elevations (up to 80 km), the percentages of each type of gas (the composition) stays the same, but the amount of air is reduced.

• Above 80 km, the atmosphere divides into layers of gas:• 80 – 1000 km : Oxygen• 1000 – 2400 km: Helium• 2400 km – outer space: Hydrogen

Structure of the Atmosphere:

1. troposphere (0-8 km at poles, 0-18 km at equator)

2. stratosphere (18-50 km)

3. mesosphere (50 – 80 km)

4. thermosphere (80 – 500 km)

5. Exosphere (500 – 700 km)

Troposphere

• 0-8 km at poles, 0-18 km at equator

• where weather occurs

• temperature decreases with altitude

Stratosphere

• above the clouds

• where jets fly

• temperature rises with increasing elevation

• contains the ozone layer

Mesosphere

• 50-80 km

• temperatures cool off again

Thermosphere

• 80-500 km

• temperatures rise again

• (nitrogen and oxygen atoms absorb solar energy, causing temperature to increase).

Exosphere

• 500-700 km

• Outermost layer of the atmosphere

• Merges with outer space

Two Important regions of the Atmosphere:

a) Ozone Layer (10-15 km)

• Absorbs 99% of UV rays

• Chlorofluorocarbons (CFCs) break down ozone, allowing more UV through – more skin cancer, plant damage

• Hole in ozone layer over Antarctica and arctic since 1970s;

• Since ban of CFCs in late 1980s, ozone layer has started to heal

b) Ionosphere (65 – 600 km)

• Radiation transforms gases into ions

• Radiowaves reflect off ionosphere back to earth

• Northern and southern lights (auroras) form here

Heating the Atmosphere

• Heat energy is transferred in three ways:

• conduction - transfer of kinetic energy in solids when particles of an object collide.

• convection - transfer of heat by the movement of fluids (liquids and gases) caused by their different densities. Forms currents of moving particles, with warm fluids rising and cool fluids sinking

• radiation - transfer of heat through space in the form of electromagnetic waves (i.e., infrared)

Radiation and Conduction in the Atmosphere

• The earth receives about 0.000 000 005 of the sun’s rays and yet all of the energy on earth comes from the sun

• Waves of solar radiation transfer their energy when they are absorbed by matter and converted into thermal energy.

• The amount of solar radiation that reaches a certain area is called insolation (for example, the Pacific Ocean at the equator receives more insolation than the Arctic Ocean).

• Very little solar radiation heats the atmosphere directly; most passes in short wavelength waves through to the Earth’s surface where it is absorbed.

• Earth’s surface reradiates some of this energy as longer infrared waves which are then absorbed by the atmosphere.

• Conduction transfers heat from the Earth’s surface to the air directly above it, so the temperature of the air increases.

The Greenhouse Effect

• Energy absorbed from the sun is radiated back from the earth’s surface as infrared waves.

• These infrared waves heat gases in the atmosphere (water vapour, carbon dioxide, CFCs, methane and nitrous oxide), which then radiate heat back again towards the earth

• As a result, the earth is about 30oC warmer than it would be without an atmosphere

• This warming of the earth’s surface by the atmosphere is called the greenhouse effect.

• Ideally, the Earth’s heat is balanced -the earth must lose as much heat as it gains. If it is not balanced, the Earth will quickly heat up or cool down.

Temperature Variation

• Air temperatures cool with increasing elevation.

• Temp. drop is caused by a decrease in air pressure (particles lose energy as they move farther apart)

Lapse Rate

Dry-adiabatic Lapse Rate

• If air is clean and dry, it cools at a rate of 1oC for every 100 m.

Normal Lapse Rate

• In normal air (clear or cloudy, moving or still), the average rate of cooling is slower - 1oCfor every 160 m.

Temperature Inversions• On clear, dry nights, the ground and air near the ground cool

rapidly

• This bottom layer is cooler than the air above it

• Because the cooler layer is more dense, the cool air mass is trapped beneath the warmer air above it

• Smoke and other pollutants can then get trapped in this layer, resulting in poor air quality

• Usually, sunlight warms the ground, destroying the inversion by mid-morning

Angle of Insolation• When the sun’s rays are vertical

(angle of insolation = 90o), the surface gets all the energy possible.

• As the angle of insolation decreases:• the energy is spread out over a

larger area• the energy travels through

more atmosphere, so more is absorbed or reflected

• Places near the equator get nearly vertical rays year-round

• In mid-latitudes, the rays are near vertical in summer months, but less vertical in winter

• At the poles, the rays never strike the surface at near-vertical angles and are colder year-round.

Changing Temperatures• Although sunlight is strongest at

noon, the warmest hours are usually afterwards

• For several hours after noon, the lower air receives more heat from the sun than the ground loses

• Therefore, temperature continues to rise.

• When skies are clear, there is strong heating by day and rapid cooling at night, leading to large temperature variations.

• On cloudy days, the temperature range is smaller;

• The clouds keep out sunshine by day, but also keep the air from radiating its heat out into space at night.

Albedo• Albedo describes the amount of radiation

reflected by a surface.

• Dark areas of continents will absorb more radiation than areas covered in snow and ice, which are good reflectors.

• Snow-covered areas and deserts therefore have high albedos while forests and soils have low albedos.

• Human activities, from deforestation to air pollution, can change the albedo of the Earth

Heating of Land and Water• Water warms much more slowly than land and

cools off more slowly:

• Sun’s rays penetrate water several metres while only heat the surface of land

• Water spreads heat more efficiently (fluid)

• Water needs more energy to raise its temperature than soil/rock, and therefore loses energy more slowly

• Evaporation uses up some of the sun’s energy

• Therefore, in summer, continents are warmer than the surrounding ocean waters; the reverse is true in winter.

Measuring Air Temperature

• Temperature (energy of particles) is measured by thermometers

• Liquid (usually mercury or alcohol) is placed in a bulb connected to a long, narrow glass tube. When the temperature rises, the liquid expands and rises to fill the tube

• The tube is divided into degrees (either Celsius or Fahrenheit)

1oC = 1.8oF

(1 degree Celsius equals almost 2 degrees Fahrenheit)

Isotherms

• Isotherms are lines drawn on maps that connect places with the same temperature

• usually spaced 5 to 10 degrees apart

What isotherms on temperature maps show:

• Isotherms shift more over the northern hemisphere than over the southern hemisphere.

• Isotherms shift more over continents than over oceans

• The hottest and coldest places are on land.

• The coldest spot in the Northern Hemisphere is far south of the North Pole

• This behaviour indicates that the temperature range is larger over land because land heats and cools more readily than water.

Evaporation, Condensation and Precipitation(Chapter 27)

States of Water

• Water exists in the atmosphere in all the three states of water:

Solid: snow, hail, ice particles

Liquid: rain, cloud droplets

Gas: water vapour

• Water changes from one state to another through the processes:

• melting/freezing

• evaporation/condensation

• sublimation/deposition

Evaporation

• Molecules of a liquid are always moving

• Some are moving faster than others, some slower

• Molecules with enough energy near the surface of a water body can escape into the atmosphere – evaporating away.

• As temperatures increase, the water molecules have more energy and are more likely to have enough energy to escape – evaporation rate increases

• When molecules evaporate, they absorb energy from the surroundings, causing the remaining liquid cooler.

Humidity

• Warmer air can hold more water vapour than cooler air

• The amount of water vapour actually present in the air is the specific humidity

• It is measured as the number of grams of water vapour in one kilogram of water.

• When air contains the maximum amount of water possible, it is saturated.

• Relative humidity compares the actual amount of water vapour in air to the maximum amount in can hold at that temperature (its capacity).

Specific humidity: 11 g/kg

Capacity: 22 g/kg

Relative Humidity = 11g/kg x 100% = 50%22g/kg

Cooling of Air

• Air cools due to four different processes:

• Contact with a colder surface

• Radiating off heat

• Mixing with colder air

• Expanding as it rises (some energy is used up in the process of expansion, reducing the thermal energy of the air molecules)

Condensation and Dew Point

• Warm air can hold more water than cool air

• As air cools at night, the amount of water in the air may exceed its capacity, causing it to condense

Dew Point: temperature at which the air becomes saturated

• The greater the relative humidity, the higher the dew point

• Condensation doesn’t always occur when the dew point is reached; instead the air may become supersaturated.

• Surfaces near the ground often cool quickly at night; air reaches its dew point as it touches these surfaces – forming dew.

• If temperatures are below 0oC, the water vapour deposits as a solid, called frost.

• Water vapour needs to condense on something – tiny particles called condensation nuclei can be present in the air

• water vapour condenses onto condensation nuclei to form tiny water droplets which then form clouds or fog.

Clouds

• Clouds form when air above the earth’s surface cools below its dew point.

• Above freezing, clouds are made entirely of water droplets

• Below freezing, clouds are usually a mixture of ice crystals and super-cooled water

• Their shape depends on the air movement that forms them.

Three main cloud types:

• Cirrus

• Stratus

• Cumulus

Cirrus clouds

• thin, feathery or tufted;

• Form high in the atmosphere

• Always made of ice crystals

Stratus clouds

• Sheets or layers of cloud;

• Form low in the atmosphere

Cumulus Clouds

• Thick, puffy masses

• Formed by vertically rising air currents

• All clouds are combinations or variations of these three cloud types.

• Sometimes, prefixes are added to differentiate: alto (high), nimbus (rain cloud)

Buoyancy of Air• Air becomes buoyant and rises when it is warmer than

the surrounding air.

• Dry rising air expands, causing it to cool at a rate of 1oC for every 100 m – this is the dry-adiabatic lapse rate.

• Moist air cools more slowly, as condensing water releases heat to the air. It cools at a rate of 0.6oC per 100 m – this is the moist-adiabatic lapse rate.

• Air in a cloud has more moisture, and therefore cools slower than the surrounding

dry air and is less dense.

• This causes clouds to grow vertically (cumulus clouds), due to instable layers of air

Layer Clouds

• Air is considered to be stable when the temperature is uniform through a thick layer.

• A rising current would be cooler than the surrounding air as soon as it started rising and expanding.

• It therefore becomes heavier and sinks back down to where it started.

• The air then spreads out in layers, forming sheets of clouds.

Precipitation

Precipitation: the falling of any form of water from the air to the Earth’s surface.

• It occurs when cloud droplets grow into drops heavy enough to fall to Earth.

• Raindrops form two ways:

• Warm-Cloud Processes

• Ice Processes

Warm-Cloud Processes:

• Tiny droplets form by condensation

• Droplets grow by combining with other droplets

Ice Processes:

• Temperatures in upper layers of clouds are below freezing

• Water vapour is deposited on ice crystals and they grow until they are heavy enough to fall

• If temperatures in the lower part of the cloud is above freezing, the crystal melts and continues to grow by warm cloud processes.

Forms of Precipitation

• Drizzle: very fine drops that are close together and fall very slowly

• Rain: drops are larger, farther apart, and fall much faster

• Snow: clumps of six-sided ice crystals

• Sleet: pellets of ice formed when raindrops fall through freezing air, causing them to turn to ice.

• Hail: forms in cumulonimbus clouds, and begins as a frozen raindrop. It grows by collecting smaller ice particles as it falls. The size depends on how long it is kept up in the clouds.

• Rainfall is measured using a rain gauge – measures the depth of water that the rain would leave if it did not soak into the ground or flow away.

• Snowfall is measured at an open location, by checking its depth in centimetres

• On average, 10 cm of snow equals 1 cm of rain

Chapter 28 – Atmospheric Pressure

Air Pressure

• The weight of the atmosphere per unit area

• Measured using a barometer; unit: millibar

• Decreases as you go up in elevation

• Normal sea level pressure: 1013.2 millibars

• Air Pressure during a hurricane: 870 millibars

• Air pressure changes are related to:

• changes in air temperature (warm air is lighter)

• changes in humidity (water vapour is lighter than nitrogen and oxygen molecules)

• Falling barometer readings usually mean warmer weather and more humid air; snow or rain are likely on their way.

• Rising barometer readings usually mean cooler, drier weather

Isobars

• lines drawn on maps which connect regions experiencing the same air pressure

• “closed” isobars (circles) represent a “high” or “low” region.

• Winds form as air moves from high pressure areas to low pressure areas

• When isobars are close together, the air pressure changes quickly which results in a strong pressure gradient.

Local Winds – Sea Breezes• caused by uneven heating of the

atmosphere

• During daytime, coastal land is warmer than nearby water so the air above the land is also warmer, creating lower air pressure.

• A pressure gradient forms between the ocean and the land, pushing cool ocean air inland.

• This is called a sea breeze.

• The air rises over land and then blows out to sea, forming convection currents.

• At night, the land cools faster than the water and the air pressure over land becomes higher.

• This reverses the process, and a land breeze forms, blowing cool air from the land out to the ocean.

Mountain-Valley Winds

• At night, cold heavy air sinks from mountain tops into valleys. The steeper the valley, the stronger the mountain wind that forms.

• During daytime, warm air rises from the sunny mountain slopes causing a valley breeze to form.

Coriolis Effect• Local winds flow from high pressure to

low pressure, but on a global scale, winds flow clockwise around highs in the northern hemisphere and counterclockwise around lows.

• This is due to the rotation of the earth, forcing the winds to the right relative to the surface in the northern hemisphere (and to the left in the S. Hemisphere).

• This is known as the Coriolis effect.

Global Wind Systems

• Wind systems are wide zones of prevailing winds, which result from the combination of convection currents and the Coriolis Effect.

• Earth has three major wind systems, which occur in both hemispheres:

• Trade winds

• Prevailing westerlies

• Polar easterlies

Trade Winds

• Located between the equator and 30o North or 30o South of the equator

• Air warms at the equator, rises, and travels north or south away from the equator

• At 30oN or 30oS, the air cools, sinks towards the surface, and moves west and back towards the equator along the surface of the Earth

Prevailing Westerlies

• Between 30o and 60oN, and 30oS and 60oS

• Air circulation is opposite of the trade winds

• Surface winds blow from the west to the east, and towards the poles

Polar Easterlies

• Between 60oN and the North Pole, and 60oS and the South Pole

• Air circulation pattern is similar to that of the Trade Winds

• Surface winds blow from east to west and away from the poles.

In addition, there are the Doldrums…

• Between 5oN and 5oS, a belt of low atmospheric pressure spans the equator.

• Air in this region rises but does not circulate with the Trade Winds.

• This results in unpredictable wind behaviour or even no wind at all

Jet Streams

• In the troposphere, global winds are slowed by friction with the Earth’s surface and the density of the air.

• In the stratosphere, there is less friction so winds can move much faster.

• A jet stream is a band of fast-moving air moving west to east across the stratosphere.

• These bands are typically hundreds of kilometres across, and meander greatly as they circle the planet.

• Each hemisphere has two jet streams – the polar jet stream and the weaker subtropical jet stream.

• Jet streams are important for air navigation, as they can greatly speed up or slow flight times.

Chapter 29: Air Masses and Fronts

Air Masses

• Bodies of air covering large areas which have the same temperature and humidity

• Air masses are classified according to their humidity and temperature.

• Most weather is determined by air masses.

Humidity of Air Masses:

• Maritime air masses – form over ocean; moist air

• Continental air masses – form over land; dry air

Temperature of Air Masses

• Polar air masses – form near poles; cold

• Tropical air masses – form near equator; warm

• (Arctic air masses – form over the poles)

Fronts

• Warm and cold air masses do not mix.

• Where they collide, weather changes occur.

• These locations are called FRONTS.

Warm Fronts

• When a moving warm air mass pushes against a cold air mass.

• Warm air moves up over cooler air mass.

• Clouds form along the front – first, thin cirrus clouds, followed by stratus clouds which get lower until eventually it rains

• Precipitation is even and steady; it continues to fall until the front passes.

• Eventually, weather becomes clearer and warmer as warm air mass moves in.

Cold Fronts• When a moving cold air mass pushes

against a warm air mass.

• Moving cold front “scoops up” the warm air it meets, lifting it high into the atmosphere

• Warm air cools and water vapour condenses, forming tall storm clouds

• Results in strong winds, heavy rain and snow, but is usually short-lived

• Quickly, the weather becomes drier and colder as the cold air mass moves in.

Stationary fronts

• Sometimes cold air masses and warm air masses meet, but stop moving

• If stationary front remains for a long time, a steady rain forms which may last for days.

Extreme Weather

Thunderstorms

• Small area storms which always have lightning and thunder, sometimes strong wind, hail and tornadoes

• Two types:

• Air mass thunderstorms

• Frontal thunderstorms

Air mass thunderstorms

• Form over hot surfaces (usually land) and last approximately 1 hour; widely scattered

Frontal thunderstorms

• Form in lines within the warm, moist air on or ahead of cold fronts

• Can be 100s of km long and are often stronger than air-mass t-storms

• Can last several hours, producing heavy rains and flooding

Three basic ingredients:

• Moisture

• Rising unstable air (air that keeps rising when given a nudge)

• A lifting mechanism to provide the ‘nudge’

Life Cycle of a Thunderstorm

Developing (Cumulus) Stage

• Sun heats the surface of the earth, which warms the air above it

• The warm surface air is forced to rise (either by running up hills or a cold front)

• It will continue to rise as long as it is more buoyant (warmer and wetter) than the air around it

• As it rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere.

• The water vapour begins to cool, releases heat, condenses and forms a cumulus cloud.

• As the cloud grows, it begins to look like a tower

• There is little or no rain at this stage, but occasionally lightning

Mature (Cumulonimbus) Stage

• Updraft continues to feed storm, forming a cumulonimbus cloud

• Condensing water forms droplets and start to fall out of the storm

• Cooling air creates a downdraft, pulling precipitation with it

• Most likely time for hail, heavy rain, frequent lightning, strong winds and tornadoes

Dissipating Stage

• Eventually, downdraft cuts off the warm, moist air that is feeding the storm

• Rainfall decreases in intensity but lightning remains a danger.

Thunder and Lightning

• As the storm cloud grows upward, it reaches areas where the temperature is below freezing.

• Ice particles begin to grow by condensing water vapour and by collecting smaller supercooled liquid drops.

• When two ice particles collide, they usually bounce off each other, but can also transfer some electric charge.

• Lots of these collisions build up big regions of static electric charges.

• This static electricity is discharged to the ground or to another cloud in a bolt of lightning.

• Lightning is very hot (about 28,000oC) so air around the bolt expands explosively, creating thunder.

Tornadoes

• Narrow, funnel-shaped columns of spiral winds that extend downwards from the cloud base of a thunderstorm and touches the ground.

• Tornadoes travel with the parent thunderstorm at a speed of 40-65 km/h.

• Large tornados can last around 30 minutes and can have wind speeds of more than 500 km/h, which can rip buildings off their foundations. However, the majority have wind speeds less than 200 km/h.

• Large tornadoes can be 3 km wide at the base of the funnel, while most are less than 100 m across.

How Tornadoes Form

• Rotation appears to begin when winds at two different attitudes blow at two different speeds,

• This creates wind shear and forms a horizontal vortex (spinning tube of air)

• Warmed by the sun, buoyant air near the ground begins to lift a section of the horizontal vortex into a vertical position.

• The stronger the two vortices created by the updraft becomes the heart of the storm, the mesocyclone

• The other vortex dies.

• The area of rotation can be 3-10 km wide, and makes up the majority of the storm.

• This is known as a supercell thunderstorm.

• The updraft in the middle of the mesocyclone allows it to to keep growing, as warm air is sucked into the storm and away from the cool downdraft.

• The updraft creates an area of low pressure at the surface.

• This pulls the mesocyclone downwards.

• If the funnel cloud reaches ground level, a tornado has formed.

Tropical Storms• Some of the world’s most violent weather

results from the exchange of thermal energy in the tropics.

• Warm ocean water and winds produce lift moist air high into the atmosphere.

• The water vapour condenses, producing clouds and rain.

• This transfers large amounts of thermal energy from the warm ocean water into the atmosphere.

• At the same time, the rising air produces a low pressure area at the ocean’s surface.

• Warm air rushes towards the low pressure area to replace the rising air.

• The Coriolis effect forces the air to rotate (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere).

• This results in a massive, spinning storm known as a tropical storm.

• The storm has a central area called the eye, approximately 15-50 km in diameter.

Hurricanes, Typhoons and Cyclones

• While travelling over warm, tropical waters, the storm continues to gain speed and momentum as more and more air approaches the low pressure centre.

• As water vapour condenses, more energy is released and the cyclone rotates even faster.

• If the wind speeds reach at least 120 km/h, the storm becomes classified as either a hurricane (Atlantic Ocean), typhoon(Pacific) or Cyclone (Indian).

• Winds increase toward the centre of the hurricane and are most violent just outside the eye. This is also where the heaviest rain occurs.

• As a tropical storm or hurricane moves into cooler water or over land, less heat and moisture is supplied and the storm weakens and eventually dissipates.

• In the northern hemisphere, hurricane season extends from late summer to early fall, when the oceans store the greatest amount of thermal energy.