Air Pressure and Winds

• Chapter 6
• Pressure gradients make the winds blow.
• Below is a review of air pressure. In class, we’ll start at the slide titled “Wind.”

Air Pressure Review

• Air Pressure: the weight of the air above
• Thus, air pressure decreases as height above sea level increases
• You can also think of air pressure as a force exerted against a surface by the continuous collision of gas molecules
• The gas molecules collide with you from all directions, so therefore, you can think of air pressure as being exerted in all directions

Air Pressure Review

• Remember pressure is related to temperature and volume (pV = nRT)
• pV ~ T; p ~ T / V
• Also, density is mass / volume
• So p ~ T * density
• Both the temp and the density of the gas determine the pressure a gas will exert

Air Pressure Review

• Cooler temps would lower pressure if density remained the same
• But in the atmosphere, cooler temps often result in greater density, which would increase pressure
• A change in one of the three variables causes changes in one or both of the others
• So it depends on which change dominates as to what the conditions become
• Many times, the density dominates, so a cold air mass from Canada usually has higher pressure since it is so dense

Air Pressure Review--Fig. 6-3

Air Pressure Review

• Air Pressure changes much more rapidly when one moves vertically in the atmosphere than when one moves horizontally
• Then why does the wind blow horizontally (North, South, East, West) and not vertically?
• Let’s find out!

Air Pressure Review—Fig. 6-7

Wind

• Wind is the result of differences in air pressure over a distance—pressure __________
• Air flows from areas of _______ pressure to areas of _______ pressure
• ______ is nature’s attempt to balance inequalities in air pressure

Wind

• Pressure differences are usually generated by unequal ________ of Earth’s surface
• So _______ ___________ is the ultimate energy source for most wind

Wind

• Air would flow directly from high to low pressure in a straight path if the Earth did not rotate and there was no friction
• However, since the Earth does rotate, and there is friction between the air and the ground, wind is controlled by a combination of forces

Wind

• The horizontal movement of air is controlled by a ________ between forces that include
• The Pressure-Gradient Force
• The _________ Force
• The Frictional Force

PGF

• Pressure-Gradient Force (PGF)
• Whenever we have a ____________ pressure gradient present, air is subjected to greater pressure on one side than on another
• The _______ pressure side exerts a net force (the PGF) on the air to push it from ______ to ______ pressure

PGF

• The greater the difference between the high pressure area and low pressure area over the same distance, the ________ the pressure gradient and PGF
• The greater the PGF, the _______ the air is going to flow from ______ to ______ pressure (________ wind speeds)

PGF—Fig. 6-9

Sea Breeze Example—Fig. 6-11

Vertical Pressure Gradient

• There also exists a vertical pressure gradient force, which is much stronger, since pressure changes more rapidly vertically than horizontally
• The vertical PGF wants to make the air flow from the surface up to space (from higher to lower pressure)
• However, gravity acts against it, trying to pull air toward the surface

Vertical Pressure Gradient

• In general in the atmosphere, gravity and the vertical PGF are balanced--no ____ upward or downward flow
• When there are ______-scale upward or downward motions they are usually very slow (less than 1 mile per day)
• Only ________-scale motions are rapid, like thunderstorm updrafts and downdrafts

Coriolis Force

• Coriolis Force: the force describing the ___________ of free-moving objects (including wind) due to Earth’s __________

Coriolis Force

• Mult. Choice Question

Coriolis Force—Fig. 6-13

Coriolis Force—Overhead Projector

Coriolis Force

• We saw how north-to-south flow is deflected, but what about west-to-east?
• It’s still deflected to the ______ in the NH
• Coriolis force does depend on _________
• The deflection is ________ toward the poles and is ________ at the equator

Coriolis Force—Fig. 6-13

Coriolis Force--See Fig. 6-13

• Notice also that the faster the object is moving, the greater distance it will be deflected
• This happens because faster winds cover a greater distance than slower winds in the same time period

Coriolis Force

• For atmospheric motions, it generally takes __ ____ ______ for the coriolis deflection to become noticeable

Coriolis Force

• True-False Question

Coriolis Force

• True-False Answer:

Coriolis Force

• Summary
• The Coriolis Force
• is always directed at right angles to the direction of airflow
• 90° to the ______ in NH, 90° to the ______ in SH
• affects only wind _________, not wind speed
• is affected by wind ______ (stronger wind, ________ deflection)
• is strongest at _______ and weakens ______________

Friction

• Friction acts to _____ _____ the movement of air
• Friction significantly influences airflow near Earth's surface (rough terrain), but its effect becomes ____________ above a height of a few km