AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

Miscellaneous Recording lectures? Trying to switch in?

Review of September 8 th : Ideal Gas Law Pressure = Density * Gas Constant * Temperature Or, P=ρRT Pressure is force per unit area Density equals mass divided by volume Temperature is a measure of the average kinetic energy of the molecules in a substance The Gas Constant is a number we can assume never changes, so for our purposes we can neglect it (for now)

Review Continued If more molecules exist in a given volume, then more pressure will be exerted against the container Kinetic Energy (KE=½*mass*velocity 2 ) A higher temperature (and so higher KE, meaning higher velocity) will also cause the pressure to increase (because faster molecules exert more force) Since density, pressure, and temperature are related, if one variable changes, at least one other variable must also change You can think of the ideal gas law as P~ ρ*T

Review Continued We discussed three temperatures scales: Kelvin, Celsius, and Fahrenheit Since temperature and density decrease with height in the lower atmosphere, pressure must also decrease with height This means that we can use pressure as a vertical coordinate, just like height in meters

Pressure Continued Atmospheric pressure can be thought of as the weight above that location (remember the surface has a pressure of roughly 14.7 lbs/inch 2 ) Since we know pressure decreases with height, the value of pressure as you go up in the atmosphere is an indication of what percentage of the atmosphere is above (or below) you If we say that the surface has a pressure of 1000 mb, and space has a value of 0 mb …

More Pressure … then as we go up in the atmosphere, a value of 500 mb represents the point where half the weight of the atmosphere is above you and half is below you Our atmosphere is roughly 50 km thick So how high above the surface do you think 500 mb is?

Here!

Troposphere The troposphere is the lowest layer of the atmosphere Its depth is variable, but it extends up to about 200 mb (or about 12 km) It is generally deeper near the equator and shallower near the poles It is where virtually all “weather” occurs At the top of the troposphere is the tropopause, which is the boundary between the troposphere and the stratosphere

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Horizontal Pressure Differences One possible reason for different pressures from one spot to another can be due to temperature differences of the air columns above those spots Remember that warmer air molecules move faster than cool ones This is one reason why warm air takes up more space than cool air (for a given number of air molecules) Starting with P~ρRT …

Thought Experiment Imagine two columns of air, initially with the same surface pressure Also imagine both columns start out at the same average temperature, and have the same amount of molecules mb Surface Now, let’s imagine we warm one air column, and cool the other while keeping the pressure constant Let’s also say that the air can only expand or contract upward or downward

Which then looks like … 500 mb Surface 12 If we say the pressure is constant, the density must react inversely to temperature, so column 2 becomes taller than column 1

But this isn’t stable in the real atmosphere Surface 12 More atmosphere exists up here! Top Of Atmosphere Now, these two columns are not in equilibrium, so something must happen 500 mb

But this isn’t stable in the real atmosphere Surface 12 More atmosphere exists up here! Top Of Atmosphere At any given location, the weight of the atmosphere above column 2 is greater than column 1 This is unstable, and the atmosphere will attempt to correct this 500 mb

The atmosphere is a fluid, so if we allow movement between the two columns … Surface 12 There is more weight (higher pressure) above any given level in column 2 than column 1 This will cause air to move from column 2 to column 1 to attempt to correct this This results in more air molecules within column 1 than column 2 Which means the pressure is higher at the surface of column 1! 500 mb

“Sea Level Pressure” Meteorologists commonly refer to “sea level pressure” rather than “station pressure” Obviously, most places are not at sea level In order to compare one station to another, stations not at sea-level must have their pressure converted to a sea level pressure This can be done fairly accurately with knowledge of that station’s elevation as well as a guess as to a theoretical temperature profile below that station

Station Models These display virtually all the relevant current weather observations for a given weather station

Meanings Wind Speeds