Lesson 3 - The Atmosphere
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 3 Quiz
By the end of this lesson you should be able to:
- Describe the composition, temperature profile, and function of the atmosphere
- Explain why the sky is blue
- Define and give examples of latent heat and sensible heat
- Evaluate the difference between an actual greenhouse and the Earth's gaseous greenhouse
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Atmospheric Composition, Temperature and Function
The atmosphere or our 'air' is a mixture of gases around the Earth which are held by the gravitational attraction of the planet. The gasses in the atmosphere bounce around to fill up the space that we call 'air'. So 'air' is not really nothingness, as we often think of it -- rather it is a critical part of our four earth systems.
Recall from your chemistry classes that a gas is one of the four states of matter (the others being solid, liquid and plasma). Gases are molecules such as O2, N2 and H2O that bounce around, filling up the space that humans call the ‘air’. The air is actually a mixture of gasses (more on that later) which are held around the earth by its gravitational field. The air is most dense (more molecules per area) at sea level, and it is much less dense at high altitudes (have you ever been up in the mountains and found yourself out of breath doing a simple activity? This is because you are breathing in fewer oxygen molecules in each breath). It is difficult to estimate the extent of the atmosphere, as it gradually merges with space. Estimates of 10,000 to 20,000 km are given (compare to the radius of Earth ~ 6400 km); however, most of atmosphere is concentrated in the lower 20-30 km (97-98% by mass - within 30 km of sea level).
The composition of our atmosphere accounts for much of the difference between Earth and other planets. Other planets (except for Mercury) have an atmosphere, but the gasses that make up their atmosphere, in combination with their distance from the sun (too close and therefore too hot, or too far away and therefore too cold) keep life as we know it from thriving. Atmosphere is essential for life on Earth as it supplies oxygen, water, CO2 and some nutrients (N) to living organisms, and protects living organisms from temperature extremes and excessive UV radiation.
Up to about 80 km, the composition of atmosphere is highly uniform; therefore, the term homosphere is applied. The lower atmosphere is composed of two groups of gasses:
I. Gases which have minimal effect on weather and climate. Concentration of these gases is constant everywhere throughout the homosphere. By volume:
- 78% - nitrogen (N2); chemically inactive, neutral
- 21% - oxygen (O2); very active chemically, reacts readily with other substances in the process of oxidation: slow (rock decay) or fast (fuel combustion)
- 0.93% - argon (Ar); inert
- < 0.04% - trace gases: Neon (Ne), Helium (He), Methane (CH4), Krypton (Kr), Hydrogen (H2)
- Ozone (O3) - extremely important as a shield for life - absorbs UV rays
II. Gases which are significant for weather and climate. Concentration of these gases (especially water vapor) can vary considerably from one place to another.
- 0-4% - water vapor (< 1% on average) - absorbs long wave radiation, emits counter radiation (greenhouse effect), transfers heat by latent heat transfer (see below)
- 0.033-0.036% - carbon dioxide (CO2) - together with water vapor is responsible for greenhouse effect
The atmosphere is also composed of water, ice and dust particles (aerosols). Aerosols are found most frequently near their sources, for example, cities, sea coasts or active volcanoes. However, particles can be carried a great distance. For example, a significant study is now being undertaken by the US Geological Survey in Texas, studying microbes carried on atmospheric dust particles blown over from Africa!
Atmospheric particles can have a significant impact on weather and climate. Some particles are hydroscopic (they absorb water), and therefore stimulate water condensation and formation of clouds. Some particles absorb or reflect solar radiation and decrease the amount of energy that reaches the surface. This can be a very good thing, as much of the insolation that intersects the earth is very short wave (high energy) and can be very harmful to life (x-rays, gamma rays and ultra violet radiation).
The vertical pattern of temperature consists of a series of layers in which temperature alternately increases and decreases with a relatively thin transitional zone in between. Based on the temperature characteristics, atmosphere can be divided into 5 layers (-spheres) and 3 transitional zones (-pauses).
- Troposphere (from Greek tropos - "turn") - lowest atmospheric layer, zone of intense vertical mixing and turbulence. This is where most weather and climate phenomena take place.
The depth of troposphere depends on the
- latitude (deepest at the equator, shallowest at the poles);
- season (deepest in summer, thinnest in winter);
- changes with the passage of warm and cold air masses.
On average, the top of troposphere (including tropopause) is about 18 km above sea level at the equator, 8 km over the poles.
The Troposphere is heated by the ground surface through conduction, convection (sensible heat transfer) and evaporation (latent heat transfer); to a lesser extent - by direct absorption of shortwave and long wave radiation. Temperature decreases with altitude at an average rate of 6.4º C / 1000 m - this is known as the environmental lapse rate.
- Stratosphere (from Latin stratum - "a cover") - layered, stratified zone of atmosphere without much vertical mixing.
- 18 - 48 km above the surface
- Contains ozone layer.
- Heat in the stratopause comes from UV absorption by ozone.
- Mesosphere (meso - "middle") - layer in between stratopause and thermosphere
- 50 - 80 km
- Thermosphere - layer of increasing heat. Heat comes from reactions of UV rays with atoms and molecules. Gradually merges into.
- Exosphere - outer layer, which in turn, gradually thins out into interplanetary space.
The atmosphere is composed of two more important layers:
- Ionosphere - layer of electrically charged molecules and atoms, ions in the top half of mesosphere and lower thermosphere. Aids in long-distance communication by reflecting radio waves back to Earth.
- Ozone sphere (Ozone layer) - zone of maximum concentration of ozone (O3, three atom form of oxygen) - practically coincides with stratosphere (recall that ozone is the source of heat for the upper stratosphere; heat is generated through reaction with UV radiation). Absorbs UV, serves as a shield for living organisms. It's important to realize that ozone constitutes only a tiny fraction of atmosphere - < 0.002% by volume.
For deeper understanding, search for information on the temperature profile of the atmosphere.
Energy Pathways in the Atmosphere
Recall our discussion of the electromagnetic spectrum from last week.
- Waves with a short wavelength have high energy
- Waves with a long wavelength have low energy
The majority of the spectrum of electromagnetic waves that reach the earth do not reach the surface of the earth. The various gasses and particulate matter in the atmosphere are very important in maintaining the balance of energy on our planet. Some key gasses, such as ozone, act as ‘gatekeepers’ allowing visible electromagnetic waves to pass through but stopping harmful ultraviolet electromagnetic waves. When insolation falls on a surface of any object it can be:
- absorbed
- reflected, or
- transmitted.
The way in which electromagnetic radiation interacts with the earth helps us understand a common question: Why is the sky blue?
The English scientist Lord Rayligh predicted in 1881 that particles will scatter inversely proportional to their wavelength. Therefore, short wavelength light scatters more than long. Recall the graphic of the electromagnetic spectrum from last week that blue is the shortest visible wavelength so it scatters the most, and is most visible. For example, when you look straight up, the sky is darkest blue because you are looking through the shortest atmospheric path. When you look toward the horizon, the sky may look more blue-white. This is because you are looking through a longer path length (more atmosphere) and the short wavelengths have scattered so much that you can no longer detect them. This can also be used to explain why sunsets appear red and orange (longer wavelengths) -- the sunlight is traveling through a longer atmospheric path so the short wavelengths are 'scattered away' and all you can see are the longer wavelengths.
Albedo is the percentage of short wave radiation reflected by a surface. Light colored things have a higher albedo than dark colored things. The insolation that is not reflected is absorbed and turned into heat. That is why on a sunny day, a person wearing a black T-shirt will have a hotter back than a person wearing a white T-shirt. Albedo plays an important role in the earth's radiation budget. See figure 4.5 in your text for examples of surfaces with different albedos.
To review:
- All energy the earth receives comes from the sun in the form of electromagnetic radiation
- The amount of energy in electromagnetic radiation depends on the wavelength. Short wave length = high energy, long wavelength = low energy
- Electromagnetic radiation can either be absorbed, transmitted or reflected
- When an object absorbs electromagnetic radiation it gains heat energy
Atmosphere and Surface Energy Balances
The sun heats our planet during the daytime. The amount of heat energy that the ground absorbs depends on the albedo of the surface. For example, urban areas tend to have lower albedos and therefore form 'urban heat islands' (see your text for a more in-depth discussion of urban heat islands). The ground absorbs insolation all day, and releases heat to the atmosphere, causing air temperature increases as long as incoming solar energy exceeds outgoing energy. What time of day can you expect the air to be warmest?
You may have noticed that I referred to heat as 'heat energy'. This is because when we discuss 'heat' in Geography, we are not generally talking about the type of heat that you can measure with a thermometer. Heat that can be measured with a thermometer is sensible heat energy. When we are talking about the global energy budget, we tend to focus on heat energy that is absorbed and then stored in the substance as latent heat energy.
Latent heat is a critical part of the global energy budget. Latent heat is released when vapor turns to water, and again when water turns to ice. This creates a 'heating effect'. For example, consider how condensation warms an area, or heat must be released from a freezer to make ice.
Latent heat is absorbed (stored) when ice melts to water, and again, when water evaporates to water vapor. This is a cooling effect as heat is absorbed. Think about how sweat cools us off when it evaporates.
The amount of latent heat energy required to get to change liquid water to water vapor is called the latent heat of evaporation. This is not necessarily the same as boiling! For example, when you sweat, the sweat does not boil as it evaporates off your skin. Rather, it absorbs latent heat energy from your skin and the sun, and changes phase.
Latent heat is a critical factor in the ocean/continent effect. Different materials heat up at different rates, depending on their heat capacity, or the amount of heat energy that a substance must absorb before it changes temperature. For example, water and concrete have different heat capacities. You may have noticed this if you have every walked barefoot on a pool deck on a sunny day. While the concrete pool deck will burn your feet, the water in the pool is still quite cool, yet both substances have absorbed about the same amount of insolation. This is because concrete has a lower heat capacity (it heats up and cools down faster) than water. Conversely, if you were to visit the same pool at night, the water temperature would likely be about the same, while the concrete pool deck would be much colder. This is because water must loose a lot of energy before it cools down.
Because oceans have a large heat capacity, they are big stores of energy (a concept called the 'Ocean/Continent effect which we will discuss in greater depth next week). Between the tropics where most of the insolation is concentrated, there is an excess of energy. Therefore a lot of latent heat energy stored in evaporation in the tropics. Turn to pages 16-17 in Goodes Atlas and look at the January and July normal temperature maps as well as the normal average temperature range maps. Note that the temperature of the land changes drastically during the year, but the temperature of the ocean remains about the same. We will explore this concept more in Lab 2.
The global energy balance is the balance between flows of energy entering the atmosphere, hydrosphere and lithosphere (also called the geosphere), and leaving it. We have already established that the incoming energy almost exclusively comes from the Sun. The Earth then dissipates this energy in the outer space. What are the mechanisms by which energy enters and leaves the planet?
Intuitively, it is clear that there should be a balance in energy flows in and out of the geosphere. Otherwise, we would have progressive cooling or warming of the planet. Which brings us to:
The Greenhouse Effect
In a sense, atmosphere functions like a blanket, keeping Earth's heat from escaping into space. It has also been compared to a greenhouse: like glass it lets short wave insolation inside, but keeps most of long wave ground radiation from going out. Your car is an example of a 'real' greenhouse. Short and long wave radiation enter and are absorbed. Some short wave radiation is reflected (especially if the interior of your car has a high albedo), but the longwave radiation that is released from your heated up interior is trapped by the glass. That is why when your car has been sitting in a parking lot all day it is much hotter than the outside temperature. A true greenhouse lacks circulation. When you get into your hot car, it is likely that you immediately roll down the windows to cool the car off (or let all that trapped longwave radiation out!).
The term greenhouse effect is often used to describe this key property of the atmosphere. However, it is important to note that our atmosphere is not a true greenhouse. Water vapor and CO2 are known as "greenhouse gases" (there are other "greenhouse gases" as well) because they act like the glass in your car, trapping longwave radiation, and encouraging the formation of thin high altitude clouds. You might want to research and study the greenhouse effect carefully. Why is the ‘greenhouse’ analogy not fully applicable to our atmosphere?
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
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