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Week2_chp2_chp3.pptxChapter 2: Energy: Warming and Cooling earth and the atmosphere chapter 3: Seasonal and daily temperatures
Week 2: 1/22-1/26
Chapter 2
Energy, temperature, and heat
What is “energy”?
We’re going to learn the relationship between energy, temperature, and heat
Basics of energy:
Energy: the ability (or capacity) to do work (push, pull, lift) on some form of matter
Potential energy (PE): the potential to do work
PE = mass (m) x gravity (g) x height (h)
Kinetic energy (KE): the energy of motion
KE = ½ mass (m) x velocity squared (v2)
Faster movement = greater KE
Radiant energy
First law of thermodynamics: The Law of Conservation of Energy states that energy cannot be created, nor can it be destroyed
How does this relate to meteorology?
Radiant energy is energy emitted by planetary bodies (Sun, Earth, other planets)
Energy changes from one form to another --> contributes to weather by transferring energy
Radiant energy from the Sun transfers to our atmosphere and is stored in clouds in the form of latent heat
temperature
Air temperature is a measure of the average speed (motion) of the molecules
In cold air, molecules move more slowly and crowd closer together (cold air = denser air)
In warm air, molecules move faster and farther apart
Stefan Boltzmann law
Everything emits radiation! Think of infrared cameras being able to see in the dark
Some objects (called blackbodies) are able to perfectly absorb all radiation that strikes it, and emit all possible radiation given its temperature
We estimate the Sun and Earth as blackbodies
As the temperature of an object increases, more total radiation is emitted each second
As temperature increases, emitted energy increases by a power of 4
Stefan Boltzmann constant = 5.67 * 10-8 W/m2*K4
Different temperature scales
Kelvin (K): Absolute zero (0 K), often used in scientific calculations
Celsius: Based on phase changes of water (100º between freezing and boiling: 0ºC and 100ºC)
Fahrenheit: Also based on phase changes of water (180º between freezing and boiling: 32ºF and 212ºF)
Heat capacity & specific heat
Heat capacity: energy required to raise a substance to a given temperature
Water has a high heat capacity: Think of the ocean or a pool on a hot summer day, it’s usually cooler than the surrounding air
Specific heat: heat capacity per unit mass; or heat energy required to raise 1 gram (g) of a substance by 1°C
Easier definition: ratio of the amount of heat energy absorbed to its corresponding temp rise
High specific heat equals slow warming (and cooling)
Low specific heat equals fast warming (and cooling)
| Table 2.1 | Specific Heat of Various Substances | |
| Substance | Specific Heat Cal / (g x oC) | J/(kg x oC) |
| Water (pure) | 1 | 4186 |
| Wet mud | 0.6 | 2512 |
| Ice (0 degrees C) | 0.5 | 2093 |
| Sandy clay | 0.33 | 1381 |
| Dry air (sea level) | 0.24 | 1005 |
| Quartz sand | 0.19 | 795 |
| Granite | 0.19 | 794 |
Latent heat – the hidden warmth
Latent heat: the energy involved in the change of state
Taken from environment during melting and evaporation (causes cooling, think of effect sweating has)
Released to environment during condensation and freezing (causes heating)
Reappears as sensible heat—as heat we can feel or measure
Phase changes, specific and latent heat
Energy input
Temperature
Solid
Phase change
Liquid
Phase change
Gas
Specific heat
Specific heat
Specific heat
Latent heat
Latent heat
Can go up or down this line
No temp change during phase changes, only latent heat
Specific heat required to raise temperature during the solid, liquid, and gas phases
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Conduction
Conduction: the transfer of heat from one molecule in a substance to another
Air is a poor conductor; metal is a good conductor
Heat always flows from warmer to colder
System always wants to be in equilibrium!
The greater the temperature difference, the more rapid the heat transfer
convection
Convection: transfer of heat by the mass movement of a fluid (such as water or air)
Convection circulation
Rising hot air expands and spreads outward, cools, and then begins to sink
Near the surface, cool air moves back into the heated region to replace rising hot air
Rising air bubbles are called thermals
Any air that rises will expand and cool; any air that sinks is compressed and warms.
Rising air has the most kinetic energy
A thermal is a rising bubble of air that carries energy upward by convection.
Path of energy: radiation from the Sun to surface conduction of surface/atmosphere convection as air rises
Electromagnetic radiation
Energy from the Sun travels through space and the atmosphere in the form of a wave (electromagnetic waves)—radiation.
Consists of various wavelengths (λ) of energy; shorter λ = higher energy; contains photons (packets of energy)
Visible light is radiation that humans can see; average λ = 0.5 µm; range 0.4–0.7 µm
Ultraviolet light: <0.4 µm; penetrates skin (sunburn!)
Which waves have the most energy?
Wein’s law
Objects emit radiation at all wavelength, but there is a peak wavelength where they emit most of their radiation
Constant = 2897 µm K (~3000K)
Example: Sun’s surface temperature is 6000K, and the Earth’s surface temperature is 288K, what are their peak wavelengths of radiation?
3000/6000 = 0.5 µm
3000/300 = 10 µm
This is why we call the sun’s radiation shortwave (short wavelength) and Earth’s radiation longwave (long wavelength)
Selective absorbers and the atmospheric greenhouse effect
Most substances are selective absorbers: they only absorb certain wavelengths of radiation
Selectively emit at the same wavelengths (Kirchhoff’s law)
Think of a glass window: it absorbs some of the infrared and ultraviolet radiation, but not visible light
As gases in the atmosphere absorb infrared radiation, they gain kinetic energy, run into other air molecules, and increase air temperature
Atmospheric window: region from ~8 to 11 µm where H2O vapor and CO2 do not absorb IR, but clouds can absorb in this window
Greenhouse effect: ~60 percent due to H2O; ~26 percent due to CO2; ~7 percent due to methane (CH4); ~7 percent due to remaining greenhouse gases
% Absorption and wavelength
Greenhouse effect visualized
Gases that contribute significantly to atmospheric greenhouse effect: water vapor, CO2, N2O, methane
Cloud cover, water vapor, and oceanic circulation all have variable influence
Transfer of energy in the atmosphere
Earth’s energy balance
While Earth and the atmosphere maintain an annual energy balance, the balance is not maintained at each latitude.
High latitudes lose more energy to space than received from the sun; low latitudes retain more energy than they lose.
Energy balance is achieved at ~38° latitude.
To maintain Earth’s energy equilibrium, energy is transferred from the tropics toward the poles
Solar storms
Solar wind: Stream of charged particles from the sun that distorts Earth’s magnetic field into a teardrop shape called the magnetosphere
The aurora is when high energy particles within the magnetosphere are ejected into Earth’s upper atmosphere, where they excite atoms and molecules
Excited atmospheric gases emit visible radiation
Solar storms occur during the active phase of the 11-year solar cycle
Impacts satellites, electrical grids, airplane navigation
Chapter 3
Seasons in the NH
Earth revolves around the sun in an elliptical path: reaches is closest point in January and farthest point in July so why is it so cold in the NH in January?
More intense solar energy when the sun’s rays strikes the Earth perpendicularly compared to at an angle
More daylight hours = more energy available from the sun
Seasons in the NH cont.
Seasons in the nh cont.
During summer solstice (June 21) the sun is directly overhead the Tropic of Cancer (~23N)
Since Earth is tilted, on June 21 in the NH all latitudes experience more than 12 hours of sunlight (more extreme the higher the latitude)
Artic circle (~66N) experiences sunlight all day on June 21
During winter solstice, sun is directly above Tropic of Capricorn (~23S)
Path of sunlight at different latitudes
Seasons in the SH
Why is the SH summer not more intense if Earth is closer to the sun in January?
Think of oceans vs. land large bodies of water are able to absorb and circulate incoming solar radiation
Keeps temperatures lower in the SH in January than the NH in July
Daytime warming
Air warms during the morning and the sun reaches its highest point around noon why is noon not usually the highest temperature during the day?
The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from Earth’s surface
When incoming energy exceeds outgoing energy (orange shade), the air temperature rises
When outgoing energy exceeds incoming energy (gray shade), the air temperature falls
Where the summer sky remains cloud-free all afternoon, the maximum temperature may occur sometime between 3:00 and 5:00 p.m. standard time. Where there is afternoon cloudiness or haze, the temperature maximum usually occurs an hour or two earlier. In Denver, clouds that typically build over the mountains on warm days drift eastward early in the afternoon. These clouds reflect sunlight, sometimes causing the maximum temperature to occur as early as noon. Whenever clouds persist throughout the day, the overall daytime temperatures are usually lower.
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Instrument shelters
Instrument shelters: house thermometers and other instruments
Protects them from rain, snow, and the sun’s direct rays.
Painted white to reflect sunlight, faces north to avoid direct exposure to sunlight, and has louvered sides, so that air is free to flow through it.
Mounted about 1.5-2m (5-6 ft) above the ground
Nighttime cooling
On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion/nocturnal inversion
The ground, being a much better radiator than air, is able to cool more quickly. Consequently, shortly after sunset, Earth’s surface is slightly cooler than the air directly above it. The surface air transfers some energy to the ground by conduction, which the ground, in turn, quickly radiates away.
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What causes Cold air at the surface?
A windless night (breeze tends to mix the colder air at the surface with the warmer air above)
Radiation inversions are more likely with a clear sky and dry air.
Snow, a good emitter of infrared energy, radiates away energy rapidly at night, which helps keep the air temperature above a snow surface low
Cold, heavy surface air slowly drains downhill during the night and eventually settles in low-lying basins and valleys. Valley bottoms are colder than the surrounding hillsides (thermal belts)
A windless night is essential for a strong radiation inversion because a stiff breeze tends to mix the colder air at the surface with the warmer air above.
This mixing, along with the cooling of the warmer air as it comes in contact with the cold ground, causes a vertical temperature profile that is almost isothermal (a constant temperature) in a layer several meters thick. In the absence of wind, the cooler, more-dense surface air does not readily mix with the warmer, less-dense air above, and the inversion is more strongly developed,
Generally, the longer the night, the longer the time of radiational cooling and the better are the chances that the air near the ground will be much colder than the air above. Consequently, winter nights provide the best conditions for a strong radiation inversion, other factors being equal.
For example, a surface that is wet or covered with vegetation can add water vapor to the air, retarding nighttime cooling. Likewise, if the soil is a good heat conductor, heat ascending toward the surface during the night adds warmth to the air, which restricts cooling
Cold, heavy surface air slowly drains downhill during the night and eventually settles in low-lying basins and valleys. Valley bottoms are thus colder than the surrounding hillsides (see Fig. 3.18). In middle latitudes, these warmer hillsides, called thermal belts, are less likely to experience freezing temperatures than the valley below. This encourages farmers to plant on hillsides those trees unable to survive the valley’s low temperature.
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Protecting crops from cold air
Orchard heaters: warm the air around the trees by setting up convection currents close to the ground
Wind machines: mix the cold air at the ground with the warmer air above, thus raising the temperature of the air next to the ground
Flooding the orchard: water has a high heat capacity, it cools more slowly than dry soil, and so the surface does not become as cold as it would if it were dry.
Sprinkling system: water freezes around the branches and buds, coating them with a thin veneer of ice. The latent heat—given off as the water changes into ice—keeps the ice temperature at (0ºC)
Furthermore, wet soil has a higher thermal conductivity than dry soil. In wet soil, heat is conducted upward from subsurface soil more rapidly, which helps to keep the surface warmer.
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Daily temperature variations
Daily (diurnal) range of temperature greatest next to the ground and becomes progressively smaller further away from the surface
The largest diurnal range of temperature occurs on high deserts
Air is dry, often cloud-free, and there is little water vapor to radiate much infrared energy back to the surface.
High humidity lowers the daily range of temperature.
Haze and clouds lower the maximum temperature by preventing some of the sun’s energy from reaching the surface
At night, the moist air keeps the minimum temperature high by absorbing Earth’s infrared radiation and radiating a portion of it
Nighttime city warmth (urban heat island)
Sun’s energy is absorbed by urban structures and concrete; then, during the night, this heat energy is slowly released into the city air
This daily variation in temperature is also much larger on clear days than on cloudy ones.
By day, clear summer skies allow the sun’s energy to quickly warm the ground which, in turn, warms the air above to a temperature often exceeding. At night, the ground cools rapidly by radiating infrared energy to space, and the minimum temperature in these regions occasionally dips below, thus giving an extremely high daily temperature range of more than.
Cities near large bodies of water typically have smaller diurnal temperature ranges than cities farther from a shoreline. This phenomenon is caused in part by the additional water vapor in the air and by the fact that water warms and cools much more slowly than land. to the ground are excellent absorbers and emitters of infrared radiation
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Diurnal Temperature variations example
Clouds lower the daily range of temperature
Good reflectors of incoming solar radiation, prevent much of the sun’s energy from reaching the surface lowers daytime temps
Keep nighttime temps higher clouds emit infrared energy back to the surface
Regional temperature variations
What controls regional temperature variations?
Latitude
Land/water distribution
Ocean currents
Elevation
Isotherms = lines of constant temperature
Oriented mostly east-west
Locations at the same latitude receive nearly the same amount of solar energy
Average air temperature near sea level in January (ºF).
Regional temperature variations Cont.
Highest average temperatures do not occur near the equator, but rather in the subtropical deserts of NH closer to 30°N
Why? Sinking air associated with high-pressure areas generally produces clear skies and low humidity.
High sun, relatively barren landscape
Average air temperature near sea level in July (ºF)
As a result of the warming and cooling properties of water, even large lakes can modify the temperature around them. In summer, for example, the Great Lakes remain cooler than the land and refreshing breezes blow inland, bringing relief from the sometimes sweltering heat. As winter approaches, the water cools more slowly than the land. The first blast of cold air from Canada is modified as it crosses the lakes, and so the first freeze is delayed on the eastern shores of Lake Michigan.
At any location, the difference in average temperature between the warmest month (often July in the Northern Hemisphere) and coldest month (often January) is called the annual range of temperature. As we would expect, annual temperature ranges are largest over interior continental landmasses and much smaller over larger bodies of water (see Fig. 3.29). Moreover, inland cities have larger annual temperature ranges than do coastal cities. Near the equator (because daylight length varies little and the sun is always high in the noon sky), annual temperature ranges are small, usually less than (). Quito, Ecuador—on the equator at an elevation of 2850 m (9350 ft)—experiences an annual range of less than . In middle and high latitudes, annual ranges are large, especially in the middle of a continent. Yakutsk, in northeastern Siberia near the Arctic Circle, has an extremely large annual temperature range of ().
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air temperature Applications
Heating degree day: based on the assumption that people will begin to use their furnaces when the mean daily temperature drops below 65ºF
Determined by subtracting the mean temperature for the day from 65ºF.
If the mean temperature for a day is 64ºF, the heating degree day for this day is 1
Mean annual total heating degree days across the US
Air temperature applications cont.
Cooling degree day: based on the assumption that people will begin to use their A/C when the mean daily temperature is above 65ºF
Determined by subtracting the mean temperature for the day by 65ºF
If the mean temperature for a day is 70ºF, the heating degree day for this day is 5
Mean annual total cooling degree days across the US
Farmers use an index called growing degree days as a guide to planting and for determining the approximate dates when a crop will be ready for harvesting. A growing degree day for a particular crop is defined as a day on which the mean daily temperature is one degree above the base temperature (also known as the zero temperature)—the minimum temperature required for growth of that crop. For sweet corn, the base temperature is and, for peas, it is .
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How we feel air temperature
Wind chill: when wind starts to blow, the insulating layer of warm air is swept away, and heat is rapidly removed from the skin.
The most recent version of the wind-chill index (see Table 3.3) was formulated in 2001 by a joint action group of the National Weather Service and other agencies.
Wind speed at about 1.5 m (5 ft) above the ground (close to where an adult’s upper body would be) instead of the 10 m (33 ft) where “official” readings are usually taken
Measuring air temperature
Maximum thermometer indicates maximum temperature for the day
Small constriction within the bore just above the bulb
As the air temperature increases, the mercury expands and freely moves past the constriction up the tube, until the maximum temperature occurs
As the air temperature begins to drop, the small constriction prevents the mercury from flowing back into the bulb
Minimum thermometer indicated lowest temperature for a given time
Contains a small barbell-shaped index marker in the bore, free to slide back and forth within the liquid
As air temp drops, the liquid moves the index marker
Maximum thermometer
Minimum thermometer
Measuring air temperature cont.
Thermocouple is a type of electrical thermometer
Temperature difference between the junction of two dissimilar metals sets up a weak electrical current.
When one end of the junction is maintained at a temperature different from that of the other end, an electrical current will flow in the circuit. This current is proportional to the temperature difference between the two junctions
Radiometers (infrared sensors) measure emitted radiation
By measuring both the intensity of radiant energy and the wavelength of maximum emission of a particular gas, radiometers in orbiting satellites can obtain temperature measurements at selected levels in the atmosphere
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Week 2 Lab tips
Lab is in D2L
Some math questions first using Stefan Boltzmann and Wein’s laws
Stefan Boltzmann constant = 5.67 * 10-8 W/m2*K4
Wein’s law constant = 2897 µm K
Excel file with temperature dataset
Plot a time series, find minimum and maximum
Watch instructions video
