1AIR: THE GREAT, THE LARGE and the small. Lecture 3-4. P.B.Rhines 5 ii 2003 AIR: THE LARGE SCALE Let’s return to simpler science and talk about the large-scale properties of the Earth’s atmosphere (and its circulation). Here our earlier ideas about sun’s radiation are important too. The atmosphere is a shell of air, about 10 km thick, thin compared with the radius of the Earth (6380 km). Some of its dominant properties are: • the atmosphere is layered, with density about 1.25 kg/meter3 near the ground, decreasing upward. [Like the ocean, even though air is ~ 800 times less dense than water.] This layering causes winds to be dominantly horizontal: vertical motion of the air is opposed by the density layers, which trap pollutants in the lower atmosphere. • the basic temperature decreases by about 500C from the ground to the ‘half-way house’ at 10km above the ground. At the altitude of jet aircraft, say 35,000 feet or 10.6 km, the temperature is about –500C . This colder air at high altitude is not denser than the warm air beneath as you might expect. The pressure (p) decrease more than compensates the temperature decrease, in the determination of density, ρ, as you move upward. As you climb a mountain it becomes cooler at a rate of about 100C per km of elevation (this is known as the “adiabatic lapse rate”). • the basic atmospheric pressure decreases almost like an exponential curve; at any point most of the pressure can be attributed to the weight of the column of air overhead (this is known as ‘hydrostatic balance’). • At the ground the pressure is about 105 Newton/m2 or 14.7 lbs/inch2 in old-fashioned British units. Approximately 80% of the mass of the atmosphere is in the layer known as the troposphere below about 10 km, and capped by a transition to a more layered (density-stratified) region known as the stratosphere. • the atmosphere is heated by the sun and by clouds, which carry the ocean’s heat upward in the form of water vapor. Clouds are small heat engines, taking the gaseous water (invisible water vapor) from the sea or near the ground, raising it up to where it condenses into liquid water (cloud and rain). This cools the sea or ground and warms the cloud, making it buoyant and pushing it to greater heights. • contrasting temperatures lead to contrasting densities, and these lead to winds, as dense (cold) air sinks, less dense (warm) air rises. • the sun heats the tropics more than the Poles, leading to a distribution of air temperature which is always trying to convect…to flow up in the tropics, downward at higher latitude and north-south in between: this is roughly what a ‘Hadley Cell’ is. • beside the density layering, the dominant feature that shapes the circulation of the atmosphere is the Earth’s rotation (through the Coriolis force). This force turns the north-south motions into east-west winds, which are the most visible part of the atmospheric circulation.2 • air behaves nearly like an ‘ideal gas’, for which pressure p, density ρ and temperature T are related by p = ρRT. R is known as the gas constant, R = 287.04 Joules per kg per 0C. The units of p are force/area or newtons/m2 or Pascals. • the atmosphere is a ‘greenhouse’ in which solar radiation enters, warms the Earth and the ‘heat radiation’ (long-wave infrared waves) goes back to outer space, but is partially absorbed by the atmosphere. Like a blanket, the atmosphere keeps us about 320C warmer than a airless planet like the moon. Variation of temperature with height in the atmosphere. Most of the mass of the atmosphere is below 10 km altitude, where the temperature decreases with height (initially at about 100C per km). You experience this when driving up into the mountains. That drop in temperature is almost what would occur if a volume of air were simply carried upward and allowed to expand as the pressure decreased (‘adiabatic cooling’). But not quite: like the ocean, the atmosphere is ‘layered’ with important stratification of ‘true’ density that prevents easy up- or down- motion. The temperature rises at the top of the stratosphere (50 km up) where a layer of ozone intercepts the sun and heats up.3Hadley circulation. The north-south, up-down circulation of the atmosphere carries heat from tropics where it is received from the sun, to the poles where heat is radiated back to outer space. Over a very long time the Earth gives off about the same amount of heat as it receives…it is nearly a steady balance. Hence what comes in must, nearly, go out. There is some additional heat rising from the molten core of the Earth (where it is cooling down from its original creation, and manufacturing a bit more heat by nuclear processes). This ‘geothermal heat’ makes Yellowstone park and volcanoes what they are, and gently warms the bottom of the ocean. But the hotter parts of the sea floor average only put out about 1 watt/m2 of heating, compared with an average of 300 watts/m2 typical of the solar heating, and hence we can probably ignore its direct effect on weather (James Lovelock would rise and say..but don’t neglect the effect of volcanoes on Gaia’s chemistry, or the effect of mantle-rock convection on Gaia’s recycling of chemicals and minerals). In textbooks you will see that the Hadley cell does not extend farther poleward than about 300 latitude, where it meets a reverse ‘Ferrel cell’. The downward flowing air at the outskirts of the tropics is real…and causes deserts to form from the lack of clouds. But in a sense the Hadley cell nearly fills each hemisphere carrying air toward the poles. East-west winds. This picture of a rolling-over circulation of Hadley cells is obtained by averaging the winds around latitude circles…east and west. At any given4place, the structure of the winds has more to it. In particular, looking at satellite images what stands out is the intense swirling of weather patterns and the general east-west motion of the atmosphere. We scarcely see any evidence for the Hadley cell. This is typical of ‘geophysical fluid dynamics’, the basic science behind atmosphere/ocean circulation: the obvious and strong motions are connected to almost invisible, slower winds. Imagine a bit of air moving northward in a Hadley circulation. Think of a ring of air all the way round the Earth, and imagine looking down on the