Atmospheric and Oceanic Circulation

Chapter 4--Part 1

Or: What goes around, comes around.

Air Pressure and Wind

Have you ever noticed changes in air pressure?

What is air pressure?Pressure is the force a gas exerts on some specified area of a container--it is the result of molecular collisions between the gas and the container

Air pressure changes with altitude, from place to place—and even in the same place, changes over time

Air pressure changes with altitude, from place to place—and even in the same place, changes over time

Pressure, Density, and Temperature

Pressure (P), density (D), and temperature (T) are all interrelated Pressure is the force of molecular collisions

per unit area (lbs/in2) Density is the weight of a material per unit

volume (g/m2) Temperature is a measure of molecular

motion Changes to one of these variables can

cause changes in the others

For example….

Changing Density Pt.I

• There are three ways to change the density of a gas:1.Change the size of the container

What happens to pressure?What happens to temperature?

2

What happens when you change the temperature of a confined gas?

Let’s take our original container full of molecules and heat it up!

- What’s happening to the pressure?

- Is density changing, or not?

A little simplification: For confined gases:

(if D↑ then P↑)(if D↑ then T↑)(if P↑ then T↑)

(if T↑ then P↑--but only if confined)Note:

(changing T will NOT affect D, if confined)

Changing Density—Pt.III

3. Change its temperature (if it is uncontained)

- What will happen to the density?

- How will pressure be affected?

In the atmosphere, gases are uncontained, like this…

A little more simplification: For unconfined gases (like in the

atmosphere):(if T↑ then D↓)(if D↓ then P↓)(if D↓ then T↓)

MeasuringAtmospheric Pressure In 1643 Evangelista Torricelli (a

student of Galileo) invented the first barometer…

Today, we use an aneroid barometer

Average Sea Level Air Pressure 29.92 in. (inches of mercury) 14 lbs/in2

1013.2 mb (millibars of mercury) 101.32 kPa (kilopascals, where 1

kilopascal is equivalent to 10 millibars)

We will use millibars, as this is the most commonly used unit of measurement

Isobars Lines on a map that connect points

of equal barometric pressure are called isobars

Isobars follow the same rules as other iso- lines (don’t cross, form closed shapes, etc.)

Isobaric Maps

The Pressure Gradient Force

Wind Wind—Air moving horizontally in

response to pressure differences

The process is called advection

Convection Cell Diagram Draw the convection cell diagram

and label it, just like you see it on the board

Practice drawing a simplified version to help you remember “out of the high, into the low” on exam day

Air always moves from regions of higher air pressure to regions of lower air pressure

In other words:“Out of the High, Into the Low!”

Local WindsConvection Cells in Motion Land and Sea Breezes Mountain and Valley Winds Katabatic Winds (a.k.a. Mistral) Chinook Winds (a.k.a. Santa Anas,

Diablo Winds, Foehn winds, etc.)

Wind Direction Wind direction is determined by

where the wind is coming from For example, an east wind is one that

is coming from the east A sea breeze is one that is coming

from the sea and moving toward the land

Sea Breeze

Land Breeze

Valley Breeze

Mountain Breeze

Chinook/Santa Ana Winds