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?
What happens when you change the temperature of a confined gas?
Let’s take our original container full of molecules and heat it up!
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?
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.)
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