Section 1 Temperature Real Meteorology EAS-135. Section 1 Overview –How does weather occur?...

Section 1

Temperature

Real MeteorologyReal MeteorologyEAS-135EAS-135

Section 1 Overview– How does weather occur?– Energy, Temperature, and Heat– Heat Transfer– Radiation– Energy Balance– Why the Earth Has Seasons?– Local Seasonal Variations– Daily Temperature Variations– Temperature Controls


– In order to understand what “weather” is we need to understand each of the components of weather.

– Once we understand each of the components, we can learn how they interact to create what we define as weather.


– The current state of the atmosphere (what weather is defined as) is described by:

Temperature Precipitation

Pressure Visibility

Humidity Winds


– The first of the parameters we are going to look at is temperature.

– We are going to look at temperature first because most people associate the temperature forecast with weather.

– Let’s deal with the math and theory (the hard stuff) first.


Energy, Temperature, and Heat (Energy)

– Energy from the sun makes the weather “go”.

– By definition energy is the ability to do work on some form of matter.

• Matter: Has mass and takes up space.• Work: Done whenever matter is pushed, pulled, or

lifted over some distance.

– By doing work on an object we give it energy.


Energy, Temperature, and Heat (Potential Energy)– Potential energy (PE) represents the potential

to do work.– Examples:

• A lake behind a dam contains energy by virtue of its position.

• A great deal of destructive work can occur if the dam were to break.

• A volume of air aloft has more PE than the same size volume of air just above the surface.

• The air aloft has the potential to sink and warm through a greater depth of the atmosphere.

– A substance also contains PE if it can do work when a chemical change takes place.

– Food, coal, and natural gas all contain chemical PE.


Energy, Temperature, and Heat (Kinetic Energy)

– Kinetic energy (KE) is the energy of motion, and applies only to moving objects.

– It depends both on the objects mass and speed.• The faster something moves, the greater its KE.• Strong breeze has more KE than a light breeze.• The greater an objects mass, the greater its KE.• A volume of water has more KE then an equal volume of air

moving at the same speed.

– Atoms and molecules that comprise all matter also have KE due to their motion, this is called heat energy.

– The most important form of energy in terms of weather and climate is the energy from the sun, radiant energy.


Energy, Temperature, and Heat (Energy)

– The total amount of energy (PE and KE) in an object/system is its internal energy.

– Energy can take on many forms, and it can change from one form into another, but the total amount of energy in the universe remains constant.

– Energy cannot be created or destroyed. It can just change from one form into another.

– This is what is meant by energy being conserved.

– This is called the First law of Thermodynamics or the law of conservation of energy.


Energy, Temperature, and Heat (Temperature)

– The air is composed of many molecules moving about in all directions at nearly the same speed. Each of these molecules has kinetic energy.

– Temperature is simply a measure of the average KE of the air molecules.

• Faster average speed → higher temperature.• Slower average speed → lower temperature.



– The balloon is filled with air at some temperature.

– If the air in the balloon is heated, the average kinetic energy (temperature) of the balloon increases.


– Since the molecules have the same mass, only the speed increases and the molecules move farther apart and the air becomes less dense.

– The opposite is true when the air in the balloon is cooled.


– If the air in the balloon is cooled, the average kinetic energy (temperature) of the balloon decreases.


– Since the molecules have the same mass, only the speed decreases and the molecules move closer together and the air becomes more dense.

– Eventually we can cool the air so much that the molecules stop moving. Absolute zero (-273.12C)

Energy, Temperature, and Heat (Heat)

– Since temperature only tells us how “hot” or “cold” something is relative to some set value, it does not tell how much internal energy that something possesses.


– Before• Each mug has the

same amount of liquid and the same temperature.

– After• Total energy

increases in A because there is more mass. Temp has not changed.

A B

After

Energy, Temperature, and Heat (Heat)– Heat is energy in the process of being

transferred from one object to another because of the temperature difference between them.

• Now imagine if we took the cup with less internal energy (B), and heated it.

• The heated liquid (B) has a much higher temperature, but (A) has more internal energy because it has more molecules.

• Now if we poured (B) in the cup with more internal energy (A), then (A) would heat rapidly.

• The energy transferred from the hot liquid (B) to the cool liquid (A) is called heat.

– After heat is transferred, it is stored as internal energy.

– More on this heat/energy transfer process shortly.


Energy, Temperature, and Heat (Specific Heat)

– The heat capacity of a material is the ratio of the amount of heat energy absorbed by that substance to its corresponding temperature rise.

– The heat capacity of a substance per unit mass is called specific heat. The amount of heat needed to raise one gram of a substance by 1C.

• If 1 g of water is heated on a stove, it would take 1 calorie (unit of energy) to raise its temperature 1C. So water has a specific heat of 1.

• If 1 g of “dirt” or “soil” is heated on a stove, it would only take 0.2 calorie to raise its temperature 1C.


Energy, Temperature, and Heat (Specific Heat)

– Therefore, the specific heat of water is 5 times greater than that of soil. Water must absorb 5 times as much heat as the same quantity of soil to raise its temperature by the same amount.

– Not only does water heat slowly it also cools slowly. A fact well known by people who live near oceans and large lakes.

– This is why water’s high specific heat plays an important role in the atmosphere and climate.


Energy, Temperature, and Heat (Latent Heat)

– The heat required to change a substance from one state/phase to another is called latent heat. Heat is either released or taken from the environment.


Cools Environment

Warms Environment

Energy, Temperature, and Heat (Latent Heat Example)

– Why is this heat referred to as “latent”? – Consider a very small drop of pure water. – At the drop’s surface between the air and water,

some slower water vapor molecules are striking the water surface and sticking while some faster moving water molecules are escaping the water droplet.

– The important feature here is that the faster molecules are leaving the droplet and only slower molecules are sticking (left with the water droplet).

– This means that the average KE (temperature) of the droplet is lowering.

– Evaporation is, therefore, a cooling process.


Energy, Temperature, and Heat (Latent Heat Example)

– The energy lost by the liquid water during evaporation can be thought of as carried away by, and “locked up” within, the new water vapor molecule.

– The energy is thus in a “stored” or “hidden” condition and is, therefore, called latent heat.

– The sensible heat (heat we can feel or sense) can return when the water vapor condenses back into liquid water.

– Therefore condensation is a warming process.


Energy, Temperature, and Heat (Latent Heat Example)– Heat energy released when water vapor condenses

to form liquid droplets is called latent heat of condensation.

– Heat energy used to change liquid into water vapor at the same temperature is latent heat of evaporation.

– Nearly 600 cal are required to evaporate a single gram of water at room temperature.

– With hundreds of grams of water evaporating from the body when getting out of a swimming pool, you can see why the body feels cool before drying off (some of the energy used to evaporate the water comes from our skin).

– Importance source of atmospheric energy.


Heat Transfer in the Atmosphere (Conduction)

– Conduction is the transfer of energy by contact.

– As molecules bump into each other, they transfer energy.

– This occurs most easily when the objects are in contact.

– Generally the larger the temperature difference the faster the transfer.


– Heat flows from warm to cold.– Solids are good conductors,

the air is a poor conductor.– Ground warms a shallow layer

of air through conduction at the lowest few centimeters.

Heat Transfer in the Atmosphere (Convection)– Convection is the transfer of heat by the mass

movement of a fluid. Flowing currents.– Can only occur in fluids and gases.– Occurs in the atmosphere when a surface heats up.

The air in contact with the surface is heated by conduction.


– The heated air is less dense than the surrounding air and so it rises (thermal).

– Cooler denser air flows toward the surface replacing the warmer less dense air (rising).

– The transfer of these properties by horizontal moving air is called advection.

Heat Transfer in the Atmosphere (Convection)


Brief Review– The temperature of a substance is a measure of the

average KE (average speed) of its atoms and molecules.– Evaporation (liquid into vapor) is a cooling process that

can cool the air, whereas condensation (vapor into liquid) is a warming process that can warm the air.

– Heat is energy in the process of being transferred from one object to another because of the temperature difference between them.

– In conduction, which is the transfer of heat by molecule-to-molecule contact, heat always flows from warmer to colder regions.

– Air is a poor conductor of heat.– Convection is an important mechanism of heat transfer,

as it represents the vertical movement of warmer air upward and cooler air downward.


Heat Transfer in the Atmosphere (Radiation)

– Radiation is the transfer of heat by electromagnetic (EM) waves (have both electric and magnetic properties).

– The energy travels in the form of waves that release energy when they are absorbed by an object.

– EM waves travel at the speed of light (3 x 108 ms-1) at wavelengths that vary from meters (TV and AM radio) to centimeters (Microwaves) to nanometers (Visible light).

– The longer the wavelength the less energy each wave contains.


Heat Transfer in the Atmosphere (Radiation)


– Short wavelengths (Ultraviolet and X-rays) contain a lot of energy and can cause sun burns and penetrate the skin.

– Long wavelengths (Infrared and Microwave) do not carry as much energy and typically only heat the object.

Radiation (Radiation and Temperature)– Every object that is above absolute zero emits

energy.– The energy originates from rapidly vibrating

electrons, billions that exist in every object.– The wavelength that each object emits

depends primarily on the object’s temperature.

• Higher temperatures, faster electron vibrations, shorter wavelengths.

• Lower temperatures, slower electron vibrations, longer wavelengths.

– Consider a rope shaken rapidly (vibrations). Many waves move along the rope and the wavelength is short.

– A rope shaken slowly will only produce a few waves and the wavelength is large.


Radiation (Radiation and Temperature)

– Objects at high temperatures emit energy at a greater rate than objects at lower temperatures.

– This can be expressed mathematically by the Stefan-Boltzmann law:

– Radiation is emitted (E) at a rate that is proportional to the fourth power of their absolute temperature (K).

– Small increase in temperature results in a large increase in the amount of radiation emitted. Doubling the absolute temperature of an object increases output by a factor of 16. 24 = 2•2•2•2 = 16


4TE σ=

Radiation (Radiation of the Sun and Earth)

– Related to the Stefan-Boltzmann law is Wien's Law which says that the wavelength of the emitted radiation is inversely proportional to the temperature.

– This formula will give us the wavelength at which the maximum radiation emission occurs.


T

Kmμλ 3000max=


– Using Wien's, important facts about radiation from the Sun and Earth can be pointed out.

– One is the difference in the peak emission of the Sun and Earth.

– The Sun (6000K) emits its peak energy at:

– The Earth (300K) emits its peak energy at:


mK

Km

T

Km μμμλ 5.06000

30003000max ===

mK

Km

T

Km μμμλ 10300

30003000max ===

shortwave

longwave


– Even though the sun radiates at a maximum rate at a particular wavelength (0.5 μm), it also emits some radiation at all other wavelengths.


– An area on the sun emits 160,000 more times the energy of a similar area on earth.

– Sun emits 88% of its energy at short wavelengths (< 1.5 μm). While the earth emits nearly all of its energy at long wavelengths (5-25 μm).

Balancing Act (Absorption, Emission, and Equilibrium)– The difference in the wavelength of

maximum radiation emission is extremely important to understanding what controls the Earth's temperature.

– The fact that everything emits energy leads us to a question. If the earth is emitting energy, then why doesn't it get colder?

– The answer is that all objects not only radiate energy, they absorb it as well.

– There is an overall balance between absorption (Day) and emission (Night). When an object emits and absorbs energy at equal rates, its temperature remains constant.


Balancing Act (Absorption, Emission, and Equilibrium)– Any body/object that absorbs all the energy

that strikes it and immediately re-emits it is called a blackbody.

– Blackbodies do not have to be black, they simply must absorb and emit all possible radiation.

– Since the Sun and the Earth absorb and radiate with nearly 100% efficiency, they behave as blackbodies.

– If the earth is viewed from space, it can be seen that half of it is in sunlight, while the other half is in darkness. If we assume there is no other method of transferring heat, then the earth is in a state of radiative equilibrium.


Balancing Act (Absorption, Emission, and Equilibrium)

– After slightly more challenging mathematics (beyond this course), it can be shown that the radiative equilibrium temperature of the earth should be approximately 255K (-18C).

– This temperature is well below the earth’s observed average surface temperature of 288K. This is obviously to cold.

– The question becomes why is it so much warmer?


Balancing Act (Selective Absorbers and Greenhouse Effect)

– The explanation for the difference between the computed emission temperature and the actual temperature lay in the Earth's atmosphere.

– The Earth's atmosphere does not act like a blackbody. Gases in the atmosphere are selective absorbers and emitters.

– Gases in the atmosphere may allow certain wavelengths to pass unhindered, but at the same time absorb others.

• Ozone and ultraviolet light is an example of this effect.• Ozone allows visible wavelengths to pass unhindered

(transparent), but absorbs any ultraviolet light.



– Snow is a good absorber/emitter of infrared wavelengths (white snow actually behaves as a blackbody at infrared).

– Kirchoff's law states that good absorbers are also good emitters at a particular wavelength.



– What does this have to do with the fact that the earth is warmer than it's emission temperature?

– Almost all of the gases in the atmosphere (Ozone is not) are transparent at short wavelengths and opaque (not transparent) at long wavelengths.

– This allows the short waves to pass unhindered to the surface where the energy is absorbed and heats the surface.

– The surface emits in proportion to its temperature (~300K) and as we know, these wavelengths are in the infrared range.



– The atmosphere (especially CO2 and H2O), however, is opaque at these wavelengths and in turn absorbs the infrared energy from the surface.

– As these gases (CO2 and H2O) absorb infrared radiation emitted from the earth’s surface, they gain KE. The gas molecules then share this energy by colliding with its neighboring air molecules, such as nitrogen and oxygen.

– These collisions increase the KE of the air and subsequently keep the lower atmosphere warm.

– CO2 and H2O also re-emit infrared energy. About 50% of this energy is emitted back into space and 50% is redirected back toward the ground where it is re-absorbed.



– For this reason, the absorption of infrared radiation from the earth by CO2 and H2O is popularly called the greenhouse effect.

– In summary, the atmospheric greenhouse effect occurs because water vapor, CO2, CH4, N2O, and other gases are selective absorbers, they allow visible light through, but block infrared.

– This is mistakenly called the greenhouse effect. Greenhouses are warmer because they prevent air from circulating and mixing with the cooler outside air, rather than by the entrapment of infrared energy.


Balancing Act (Enhancement of the Greenhouse Effect)

– Even though accurate temperature records only go back about 200 years (SLU has over 175 years of records), there has been a measurable increase in the average global temperature of about 0.6C (1.0F).

– The primary cause for this has been the increased emission of CO2, CH4 (methane), and N2O (nitrous oxide) by human activity.

– These concentrations of gases have increased primarily from the burning fossil fuels and to deforestation.


Balancing Act (Enhancement of the Greenhouse Effect)

– Coupled General Circulation Models predict that the increases will NOT appear as an increase in the temperature (no heat waves), but rather in a shift in the wind and rainfall patterns.

– The primary concern is that the equatorial regions will see an increase in temperature and the poles will see a decrease. As a result, the weather will become more severe.

– Rising ocean temperatures will cause an increase in evaporation which will add more water vapor (GG) into the atmosphere.


Balancing Act (Enhancement of the Greenhouse Effect)– The added water vapor, the primary

greenhouse gas, will enhance the greenhouse effect and double the temperature rise in what is known as a positive feedback.

– Positive Feedback (example)• Ocean temperatures rise, leads to more water vapor

in the atmosphere, leads to enhancing the greenhouse effect, leads to increasing the ocean temperatures…etc. etc.

– Negative Feedback (example)• Clouds ultimately cool the earth as they reflect and

radiate more energy away then they retain. So an increase in global cloudiness may offset some of the global warming brought on by an enhanced greenhouse effect.

– So clouds both warm and cool the earth!!!


Brief Review– All objects with a temperature above absolute zero

emit radiation.– Higher an objects temperature, the greater the amount

of radiation emitted per unit surface area and the shorter the wavelength of maximum emission.

– The earth absorbs solar radiation only during the daylight hours; however, it emits infrared radiation continuously, both during the day and at night.

– The earth’s surface behaves as a blackbody, making it a much better absorber and emitter of radiation than the atmosphere.

– Water vapor and carbon dioxide are important atmospheric greenhouse gases that selectively absorb and emit infrared radiation, thereby keeping the earth’s average surface temperature warmer than it otherwise would be.


Brief Review (continued)– Cloudy, calm nights are often warmer than clear,

calm nights because clouds strongly emit infrared radiation back to the earth’s surface.

– It is not the greenhouse effect itself that is of concern, but the enhancement of it due to increasing levels of greenhouse gases.

– As greenhouse gases continue to increase in concentration, the average surface air temperature is projected to rise substantially by the end of this century.


Balancing Act (Warming the Air From Below)– On a clear day the incoming solar radiation

(insolation) passes through the atmosphere unhindered until it finally reaches the surface, warming it.

– Air molecules in contact with the surface bounce against it and gain energy by conduction.

– Since the air near the surface of the earth is dense, these energized molecules only travel a short distance before colliding with other molecules.

– During this collision, the rapidly moving molecules share some of their energy with less energized molecules, thereby raising the average temperature of the air.


Balancing Act (Warming the Air From Below)

– As we know, the air is such a poor conductor of heat that this process is only important within a few centimeters of the ground.

– As the surface air warms, it becomes less dense than the air directly above it. Warmer air rises and the cooler air sinks, setting up thermals, or convection cells.

– These convection cells transfer heat upward and distribute it through a deeper layer of the atmosphere.

– If the air rises enough, it will condense into cloud droplets, releasing latent heat that also warms the air.


Balancing Act (Warming the Air From Below)– At the same time, the earth constantly

emits infrared radiation that is absorbed by the greenhouse gases in the atmosphere.

– These gases then re-emit infrared radiation upward towards space and downward towards the earth’s surface.

– Since the concentration of water vapor (the most common greenhouse gas) decreases rapidly above earth, most of the absorption occurs in a layer near the surface.

– For these reasons, the lower atmosphere is mainly heated from the ground upward.


Balancing Act (Warming the Air From Below)


Incoming Solar Radiation (Scattered and Reflected Light)

– The energy from the Sun leaves the Sun's surface and reaches the top of the Earth's atmosphere approximately 8 minutes later.

– The solar constant is the amount of energy reaching the top of the atmosphere. Using Wien's Law, the solar constant can be calculated.

– Solar constant = 1367 W/m2

– It has been previously discussed how gases absorb energy in the atmosphere.

– But air molecules and dust particles also reflect and scatter some of the energy in all directions.



– The distribution of light/energy in this matter is called scattering. Scattered light is also called diffuse light.

– This accounts for the blue sky during the day. Since the air molecules and dust particles are much smaller than the wavelengths of visible light, they are more effective scatterers of the shorter (blue) wavelengths than the longer wavelengths (red).

– Therefore, when we look away from the sun’s direct rays, blue light strikes our eyes from all directions, turning the daytime sky blue.


Incoming Solar Radiation (Scattered and Reflected Light)– Blue Sky

• Air molecules selectively scatter the shorter wavelengths of visible light more effectively than the longer wavelengths.

• When these shorter waves reach our eyes, they are processed in our brain as blue.

– White Sun• At noon, the sun is perceived as white because all

the waves of visible sunlight reach our eyes. Thinner portion of the atmosphere, so there is not as much time for scattering.

– Red Sky• At sunrise and sunset the white light from the sun

passes through a much thicker portion of the atmosphere. Scattering removes all the shorter wavelengths so just the longer waves are left.




– Another portion of the insolation/incoming energy is reflected away. Reflection differs from scattering because most of the light/energy is sent backwards.

– The percent of radiation returning from a given surface (reflected back) compared to the amount of radiation initially striking that surface is called albedo.

Incoming Solar Radiation (Scattered and Reflected Light)– Averaged for an entire year, the earth and its

atmosphere (including clouds) will redirect about 30% of the sun’s incoming radiation back to space, which gives the earth and its atmosphere an albedo of 30%.

– 6% is reflected away by the dust and gases in the atmosphere.

– Around 20% is reflected away by the clouds, although this varies strongly with the type of cloud and the percent of coverage.

– About 4% is reflected by the surface. These numbers can vary significantly based on the type of surface and the angle the beam of sunlight makes to the surface.


Incoming Solar Radiation (Earth’s Annual Energy Balance)

– Although the daily temperature at any one place might vary significantly, the earth’s overall average equilibrium temperature changes only slightly from year to year.

– This fact indicates that, each year, the earth and its atmosphere combined must send off into space just as much energy as they receive from the sun.

– If this did not occur, then the earth’s average surface temperature would change.

– How is this energy balance maintained?




– Assuming 100 units of solar energy reach the top of the atmosphere.

– 19 units are absorbed by the clouds and the atmosphere.

– 30 units are reflected and scattered back into space by clouds (20), earth (4), and the atmosphere (6).

– 51 units are absorbed at the surface.



– 51 units are absorbed by the surface.

– 23 units are used for evaporation.

– 7 units are lost by convection and conduction.

– The remaining 21 units are radiated away.

– Note: The surface actually radiates away 117 units. It does so because, although it receives solar radiation only during the day, it constantly emits infrared energy both during the day and night.

– Atmosphere only allows 6 units to pass into space, the majority (111 units) are absorbed by the atmosphere’s greenhouse gases (CO2 and H2O) and clouds.

– Much of this energy, 96 units, is radiated back to the surface.

Why the Earth Has Seasons– The earth is roughly a sphere that revolves around

the sun in an elliptical path while it rotates on its axis.

– Since the earth’s orbit is an ellipse instead of a circle, the actual distance from the earth to the sun varies during the year.

– This means that at some points in the earth’s orbit, it is closer to the sun that it is at other times.

– In fact, the earth is closer to the sun in January than it is in July.

– Shouldn't that make January warmer than July?


Why the Earth Has Seasons


Why the Earth Has Seasons– The earth is tilted on its spin axis by an angle of 23

½° from the perpendicular drawn to the plane that the earth revolves in.

– The fact that our axis is tilted 23 ½° is more important than the 3% difference that occurs from the earth’s orbit.

– The amount of insolation that we receive from the sun controls the variations in temperature that are experienced at the earth’s surface.

– The angle at which sunlight strikes the surface of the earth is an important factor in how much solar radiation is received on earth.


Why the Earth Has Seasons– The total amount of energy contained in a beam of

sunlight is always the same.– The way that energy is distributed, however, is a

function of the angle the beam makes to the surface.


• If the surface of the earth is perpendicular to the beam, the area of sunlight/radiant energy covers a small area.

• When the beam is tilted to the surface of the earth, the area of sunlight/radiant energy grows larger. The same amount of energy is distributed over a larger area.

Why the Earth Has Seasons– The second important factor determining how warm

the surface of the earth becomes is the length of time the beam of sunlight shines each day.

– Obviously, the longer the beam of sunlight strikes the surface of the earth the more it is going to heat up.

– Summer days have more daylight hours than in the winter and the noontime sun is higher in the sky in the summer than in the winter.

– Both of these events occur because the spinning earth in inclined on its axis as it revolves around the sun.

– Therefore, what controls the seasons is the tilt of the earth's spin axis to the plane of the orbit.


Why the Earth Has Seasons (Northern Hemisphere Summer)

– Although it is warmer in the summer than in the winter, it still does not get very warm.

– The reason is that although the sun is above the horizon, it still isn't very far above the horizon.

• At 66 ½°N, the sun is only 90° – 66 ½° = 23 ½° above the horizon.


– This means the sunlight must pass through a very thick layer of air at 66 ½°N even though at 23 ½°N the thickness of air is at a minimum.

Why the Earth Has Seasons (Northern Hemisphere Summer)

– Since the sunlight has a thicker column of air to pass through at 66 ½°N, it has a greater chance to be absorbed, scattered, or reflected away. This reduces the amount of insolation to reach 66 ½°N.

– In addition, at 23 ½°N the sunlight is spread over the smallest amount of area, while at 66 ½°N the same amount of energy is spread over a much larger area.

– This is why even though northern areas experience 24 hours of sunlight on June 21st, they are not warmer than areas to the south.

– If you look at a map of the world, you will notice that the major deserts of the world are located near 30°N latitude.


Why the Earth Has Seasons (Northern Hemisphere Equinox’s)

– There are two times during a year when daylight is 12 hours long.

– They are called the autumnal (fall) and vernal (spring) equinox.

– The word equinox is derived from equal implying the length of day and night are equal.


Why the Earth Has Seasons (Northern Hemisphere Winter)

– The length of days is now at it's minimum; 12 hours at the equator and 0 hours at latitudes above 66 ½°N.

– The sun is directly above 23 ½°S.– The sun is at its lowest position in the sky. – The sun’s rays have to pass through the thickest

section of the atmosphere and spread over a large area on the surface.


Why the Earth Has Seasons (Northern Hemisphere Winter)

– With so little radiant energy, the earth’s surface rapidly cools.

– Snow covering an area will increase the amount of energy that is reflected which cools the air further.

– It is interesting to note that although the shortest day is December 21st and the longest day is June 21st, the coldest days occur in late January and the warmest days occur in late July and early August.


Why the Earth Has Seasons (Southern Hemisphere Summer)

– The southern hemisphere summer occurs during the northern hemisphere winter.

– The tilt of the earth's axis results in the seasons being switched.

– In theory, since the earth is closest to the sun in January and January is summer for the southern hemisphere, the summers should be warmer in the southern hemisphere.

– However, since the southern hemisphere is primarily ocean water, the increased amount of incoming energy does not translate into warmer temperatures. The added solar energy due to the closeness of the sun is absorbed by the oceans.


Why the Earth Has Seasons (Southern Hemisphere Summer)

– In addition, since the earth’s orbit around the sun is elliptical, the number of days from the vernal (March 20) to the autumnal (September 22) equinox is 7 days longer than the autumnal (September 22) to the vernal (March 20) equinox.

– The discrepancy between the two time periods is due to the fact that the earth travels more slowly when it is farther from the sun.

– Thus, the spring and summer in the northern hemisphere lasts 7 days longer. Furthermore, the shorter spring and summer in the southern hemisphere offsets the extra insolation received due to being closer to the sun.


Local Season Variations – Note that the during the winter, the sun rises in the

southeast and sets in the southwest, while in the summer the sun rises in northeast and sets in the northwest.


– Also, it is apparent that at noon the sun is higher in the sky in the summer than in the winter.

– In addition, anything facing south will receive more direct sunlight than anything facing other directions.

Local Seasonal Variations– Hills that face south will receive more insolation

and be warmer than the partially shaded northern exposures.

– Higher temperatures on the southern faces usually mean greater evaporation and drier soil conditions. Thus, south-facing hillsides are usually warmer and drier as compared to north-facing slopes.

– In many areas of the western US, only sparse vegetation grows on south-facing slopes, while on the same hill, dense vegetation grows on the cooler, moist north-facing slope.


Local Seasonal Variations– In northern latitudes, north-facing hillsides have a

longer growing season.– North-facing hillsides also keep their snow longer

than the southern faces, therefore ski slopes are generally found on northern faces.

– Homes are also dependent on these local variations. Garages should be placed on the western side of the home to provide a thermal buffer.

– Planting dedicious trees on the south side of a home will provide shade in the summer (cooling). In winter, they lose their leaves and allow winter sunshine to warm the house.


Daily Temperature Variations– Each day is essentially like a mini season that goes

through a daily cycle of warming and cooling.– The sun starts out at the beginning of the day low

to the horizon, climbs to the zenith point, and then sinks again toward the horizon.

– With this variation in the amount of incoming solar radiation, the temperature varies as well.

– Surprisingly, the maximum temperature does not occur with the maximum amount of sunshine at noontime.


Daily Temperature Variations (Daytime Warming)

– As the sun rises in the morning, the sunlight warms the ground, and the ground warms the air in contact with it by conduction.

– Unfortunately, air is such a poor conductor of heat that only the lowest few centimeters of the air are heated.

– As the sun rises higher in the sky, the air in contact with the ground becomes even warmer.

– Eventually convection begins and the rising air bubbles (thermals) begin to redistribute the heat.



– On a calm day these thermal are small and do not effectively redistribute the heat.

– On windy days, however, the winds help stir the air and the surface heat is redistributed.


– This mixing helps the thermals to transfer heat away from the surface more efficiently.

– This mechanism for warming the air explains why the maximum temperature occurs latter in the day.

Daily Temperature Variations (Daytime

Warming) – In clear skies, the temperature maximum occurs

between 3 and 5pm. – In areas with afternoon cloudiness or hazy skies,

the temperature maximum occurs an hour or two earlier.

– In the cases where the temperature maximum occurs earlier in the day, the maximum may be lower.



– Just how fast the surface warms and how warm it becomes depends a number of factors including the type of soil, its moisture content, and vegetation cover.

– The greatest surface heating occurs in the deserts where there are clear skies, low humidity, and meager vegetation.

– Here, the sand is a poor heat conductor so heat energy does not readily transfer in the ground. This allows the surface layer to reach higher temperatures, and allows more energy to warm the air above.

– If the soil is moist, much of the energy is used for evaporation, which leaves less to heat the air.


Daily Temperature Variations (Nighttime Cooling)

– As the sun lowers, its energy is spread over a larger area, which reduces the heat available to heat the ground.

– Both the ground and air cool by radiating infrared energy. A process called radiational cooling. The ground, a much better radiator than the air, cools faster.

– Shortly after sunset, the earth’s surface is slightly cooler than the air directly above it.

– The air near the surface transfers heat to the ground by conduction, which the ground quickly radiates away.

– As the night continues, the ground and air in contact with it cool faster than the air a few meters higher.


Daily Temperature Variations (Nighttime

Cooling)


– This leaves the air near the ground cooler than the air above it and a radiational inversion forms.

– Inversions are where the temperature of the air increases with height.

Daily Temperature Variations (Radiation Inversions)

– Radiational inversions occur most often when the air is calm, the night is long, and the air is dry and cloud free.

– Calm Air• Wind tends to mix the colder air at the surface with the

warmer air above. • In the absence of wind, cooler, more dense air does not

readily mix with the warmer air above.

– Long Night • The longer the night, the longer the time of radiational

cooling.

– Clear Skies• The ground is able to radiate its energy to outer space and

cool rapidly.


Daily Temperature Variations (Radiation Inversions)

– Radiational inversions are extremely important in mountainous terrain.

– Radiational cooling at the tops of the mountains creates a large mass of cold dense air that settles into the valleys where it creates a cold pool.


– On very cold nights, to preserve their crops, orchard owners have to:

• Stir the air with fans. • Heat it with smudge pots.• Spray trees with water

(why?). Think latent heat.

Controls of Temperature– The primary factor that controls the temperature

around the globe is the amount of incoming solar radiation.

– Insolation is determined by the length of daylight hours and the intensity of incoming solar radiation.

– These factors are a function of latitude, therefore latitude is considered an important control of temperature.

– The main controls of temperature are:• Latitude• Land and water distribution• Ocean currents• Elevation


Controls of Temperature– Note that temperatures

decrease from the equator to the poles.

– In addition, the contours (isotherms) of temperature are more tightly packed in winter than they are in summer. This is due to the larger variation of insolation in winter.


Winter

Summer

Controls of Temperature– The effect of the

continents on temperature can be seen in the winter diagrams. Here the cold air dips further south in winter than in summer.

– Note also that the maximum temperatures are in the northern hemisphere in July and in the southern hemisphere in January.


Winter

Summer

Section 1 Temperature Real Meteorology EAS-135. Section 1 Overview –How does weather occur?...

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Transcript of Section 1 Temperature Real Meteorology EAS-135. Section 1 Overview –How does weather occur?...