121-ch2-07 - The Physical and Chemical Structure of the...

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Unformatted text preview: The Physical and Chemical Structure of the Atmosphere Transparent but complicated! Composition of the Atmosphere Permanent Constituents (residence time ~10 millions of years) ~ 78% of nitrogen (N2) ~ 21% of oxygen (O2) ~ 1% of argon (A) ~0.034% of CO2 Other inert gases (e.g., helium (He), neon (Ne), krypton (Kr), xenon (Xe) Variable Constituents (residence time usually a few days to a few weeks) H2O and some trace gases (such as sulfur dioxide (SO2), ammonia (NH3), nitrogen dioxide (NO2), etc. Aerosol particles - liquid droplets and solid particles Semi-permanent Constituents e.g., methane (CH4), CO, H2 Water Vapor Minute and Variable yet very important Water can exist in all three phases (gas, liquid, and solid) in the atmosphere unique in our natural environment! Water vapor is highly variable, from nearly zero in deserts to ~ 4% over the tropical ocean surface. Water plays important roles: weather phenomena cleansing of the atmosphere chemical reactions radiation (greenhouse effect) Atmospheric Pressure Most of the atmospheric gases have maximum concentration at the surface. The gas concentration usually decreases exponentially with height. But there are some exceptions (e.g., O3). The atmospheric pressure (which can be thought as a measure of concentration, although it is also influenced by temperature) also decreases exponentially upward. This is the so-called barometric law. Barometers measuring atmospheric pressure Aneroid Barometers and Altimeters Vertical Pressure Distribution www.geog.ucsb.edu Hydrostatic Balance Why wouldn't the gases in the atmosphere fly away? Because the gravitational force of the earth holds them down! Then why wouldn't they get all pulled down to the surface? Because the pressure is higher below, and the pressure gradient force pushes them upward. In the general atmosphere, the gravitational force is about as strong as (that is, balanced by) the vertical pressure gradient force, so the two basically cancel each other. This is called the hydrostatic balance. But this is not true all the time. During thunderstorms, the local atmosphere can be highly non-hydrostatic. How to study the structure of the atmosphere? One of the simplest ways : classification. You have to choose a property based on which you design a classification scheme. Can the atmosphere be divided into vertical layers? Yes, but you have to choose a property. The traditional classification of atmospheric layers is based on temperature. We plot the air temperature versus height to see if distinct structure exists. Vertical Temperature Profile > 1000 C Troposphere The lowest layer. "Tropo" means "turning" or "change". Temperature in this layer decrease with height in general. (but sometimes inversion occurs.) Strong convection causes clouds, rain, snow, thunderstorms, tornadoes, etc. The common weather phenomena are usually restricted to this layer only. Thickness of this layer varies from place to place : ~17 km near the equator and ~7 km near the pole (so characteristic thickness ~ 10 km). The top of the troposphere is called the tropopause (a temperature minimum). Convection warmer air rises and colder air sinks. Convective clouds (such as cumulus and cumulonimbus) form this way. In the `weightless' condition (such as in a space shuttle), no such convection occurs. Stratosphere The temperature stays relatively constant with height initially, then increases with height, reaching a maximum (~ 280K) at height of about 50 km. Relatively calm , almost no cloud and other weather. mother-of-pearl (nacreous) clouds (mostly frozen sulfuric acid droplets) Commercial jets usually fly in the lowest stratosphere. A balloon can usually reach a maximum height near 40 km, i.e., in the stratosphere. Ozone concentration in this layer. The top is stratopause. Mesosphere The layer above the stratosphere is called the mesosphere. The temperature again decreases with height, reaching a minimum (~ -100C) at about 85 km (the mesopause). This is the layer where noctilucent (night-glowing) clouds may exist. We don't understand this layer as much as other layers and not many people are studying it, either. Some atmospheric scientists call it, jokingly, the "ignorosphere". photos by Pekka Parviainen Thermosphere Above the mesosphere is the thermosphere where the temperature increases with height and can reach more than 1000 C. The high temperature is caused by the heating of short wave solar radiation. Will you feel cold or hot in the thermosphere? Heat and temperature are not identical things! Are there other ways to classify atmospheric layers? Yes. For instance, we can use the influence of electric and magnetic fields. Below about 60 km the atmosphere has few free electrons, and is called the neutrosphere. Above that, the electron density and the electric conductivity increase significantly. We call this upper layer the ionosphere. At height above several earth radii, the effect of geomagnetic field becomes important. This layer is called the magnetosphere. Charged particles from the outer space become trapped in this layer and bounce back and forth between the north and south pole (and rotating with the earth at the same time) causing complicated electromagnetic phenomena (such as aurora). Magnetosphere Geomagnetic Storm and Aurora One of the more significant phenomena that occurs in the magnetosphere is the aurora, which is quite common during a geomagnetic storm when solar energetic particles bombard the earth's atmosphere. The following are some aurora pictures, some of them were taken by space shuttle crew that provides a rare view of aurora from above. Aurora Trigvy Linderson Credit & Copyright: Philippe Moussette Aurora from space Auroras in other planets Jupiter Aurora Oceans Sea Surface Temperature check out www.ssec.wisc.edu How cold is the ocean water? It depends where you dip your bottles O c e a n T e m p e ra tu re a s a F u n c tio n o f D e p th 0 250 500 750 1000 1250 1500 1750 2000 P o la r M id la t w in te r M id la t s u m m e r T ro p ic a l Depth (m) L o w L a titu d e s M id la t a t s u m m e r M id la t a t w in te r H ig h la t 0 10 20 30 T e m p e ra tu re ( C ) We can divide vertical layers in an ocean according to, for example, its temperature structure. Another property we can use as the basis for the division is salinity. Ocean Temperature as a Function of Depth 0 250 Thermocline Surface layer 500 750 1000 1250 1500 1750 2000 Depth (m) Pycnocline zone - layer with marked density change. It is called a thermocline if the density change is dominated by temperature. It is called a halocline if dominated by salinity. Think for yourself-- How would the vertical density profile look like? 0 10 20 30 Temperature (C) ...
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