Natural Environments: The Atmosphere

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Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (1 of 11) Further Reading: Chapter 07 of the text book Outline - pressure - pressure gradient force - land and sea breezes - coriolis force

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (2 of 11) Intro So far we have talked about thermodynamics (radiation budget and temperature) and hydrodynamics (moisture and precipitation) Now we will talk about Dynamics Mechanism behind winds, general circulation of the atmosphere, weather systems We will incorporate what we have learned about: Temperature and density Earth’s rotation Moisture, clouds, and precipitation Remember that from the global radiation balance there is a gradient in energy balance between the equators and poles There is a surplus at the equator There is a deficit at the poles One way to think of the general atmospheric circulation is that is “trying” to redistribute energy from the equator to the poles

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (3 of 11) Pressure Atmosphere has mass The atmospheric molecules are held near surface by gravity Pressure itself reflects the weight of the mass of overlying atmosphere Defined as force per unit area at the surface Units of Pascal (1 N/m2) In the atmosphere, we measure pressure in millibars (mb) It is important to remember that the atmosphere exerts pressure on all surfaces We don’t “feel” it because it is the same on all sides 1 bar = 1000 mb = 10,000 Pa

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (4 of 11) Pressure and Altitude As we move up in the atmosphere, the number of molecules above us decreases -> pressure decreases with altitude At the surface, the pressure is typically approximately 1013 mb

Natural Environments: The Atmosphere Pressure Gradient Force
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (5 of 11) Pressure Gradient Force The pressure at the surface (or any fixed level) is not the same everywhere. Because pressure is different at different locations, we observe pressure gradients. We can connect regions of equal pressure with lines, called “isobars” It is the force produced because there is high pressures in one are and low pressure in another Pressure gradient and pressure gradient force are always perpendicular to isobars Magnitude of pressure gradient Force is related to how close the isobars are As we saw, there is also a pressure gradient force in the vertical; however this is balanced by the force of gravity so typically we don’t see much vertical motion because of Pressure Gradient forces

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (6 of 11) Land and Sea Breeze-01 Pressure gradient forces can be produced by many processes. One example is the pressure gradients due to surface temperatures Recall thermal differences between ocean & land During the day, the land is warmer than ocean At night, the land is cooler than the ocean Also recall the relation between temperature and pressure As air gets warmer, it gets less dense so it rises As air rises, it produces a vacuum effect, i.e. low pressures As air gets cooler, it gets more dense so it sinks As air sinks, it “pushes” down on the surface, i.e. it produces high pressures This produces a horizontal pressure gradient force going from the ocean to the land The winds then blow down this pressure gradient, hence the “sea breeze” What about aloft? H L L H

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (7 of 11) Land and Sea Breeze-02 As the cool air sinks, it produces a vacuum aloft, hence low pressures As warm air rises, it creates high pressures aloft Hence, aloft the pressure gradient is in the opposite direction from the surface Therefore the winds blow in the opposite direction During the night, the temperature gradient reverses and therefore the direction of the pressure gradient and winds reverse -> becomes a “land breeze” Relative differences in temperature produce differences in pressure, this leads to pressure gradient forces and affects localized flow of air H L L H

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (8 of 11) Is not a real force; it only appears to affect objects because our reference frame (the earth) is rotating Only becomes important at scales larger than 100km Coriolis Force-01 Assume we are looking down at the north pole If the earth weren’t rotating and we through a ball from the equator to the pole it would go in a straight line Now lets look at what happens when the earth rotates We initially throw the ball from A to B But in the time it takes for the ball to go from A to B, the earth rotates underneath it. Lets see where the ball actually ends up During the time of travel, A goes to A’ and B goes to B’ But the ball, which started at A, has a velocity component the same as A, so it travels further to the right than B, hence it looks like it’s veered to the right It turns out that all objects in motion in the Northern Hemisphere have a Coriolis force directed to the right of motion In the Southern Hemisphere, objects have a Coriolis Force directed to the left of motion

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (9 of 11) Coriolis Force-02 With Rotation Without Rotation B B’ B A’ A A

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (10 of 11) Coriolis Force-03 Coriolis Force Movies First off, the Coriolis force is always directed perpendicular to the direction an object is moving; therefore it can never increase or decrease the speed of an object, only change the direction it is moving In addition, remember that the Coriolis force is always to the right of an object in motion in the Northern hemisphere and to the left of an object in motion in the southern hemisphere The Coriolis force is proportional to the sine of the object’s latitude. Therefore, at low latitudes, the Coriolis force is weak; as the object moves to higher latitudes, the Coriolis force increases In addition, the Coriolis force is proportional to the velocity of an object; Therefore, as an object’s velocity increases, the Coriolis force on it increases

Natural Environments: The Atmosphere
GE 101 – Spring 2007 Boston University Myneni Lecture 16: Pressure & Winds Feb-26-07 (11 of 11) Pressure & Winds Starting in the northern hemisphere, lets assume that we have a low pressure center Winds will start to blow into the low pressure center because of the pressure gradient force As the move inward, they become deflected to the right by the coriolis force; hence we find that at the surface, winds blow into and counterclockwise around low pressures For a high pressure center in the northern hemisphere, we can see that winds blow out of the high pressures because of the pressure gradient force As they do so, the winds again are deflected to the right so that in the northern hemisphere, winds blow out of and clockwise around high pressures Now look at the southern hemisphere For a low pressure center, winds will still blow into the low pressures but now they are deflected to the left; therefore winds blow into and clockwise around low pressures in the southern hemisphere Similarly, winds blow out of and counterclockwise around high pressures in the southern hemisphere