THE
ATMOSPHERE
IN
MOTION
Air Pressure and Wind
• Wind which is the horizontal movement of air,
helps to moderate surface temperatures,
distribute moisture, and generally cleanse the
atmosphere.
• Though there are several forces that affect the
direction in which air moves, differences in air
pressures are what set the air in motion.
Air Pressure
• Air pressure is the weight of the atmosphere as it pushes
down upon Earth exerting a force per unit of area.
• Air pressure is exerted in all directions because air
molecules move in all directions.
• Air pressure at any point on Earth’s surface is dependent
on the weight of the air above, note air pressure
decreases as elevation increase
• Air pressure is most noticeable to people when they
experience a change in the air pressure.
– A rapid ascent, the air pressure outside of your body rapidly
decreases causing your eardrums to hurt or pop because of
the greater internal pressure.
– A rapid descent causes an increase in external pressure
causing greater pressure on the outside of your eardrum.
Air Pressure
• Air pressure is measured with an instrument
called a barometer. There are two types
– Mercury
– Aneroid
• When recording air pressure there are several
different units of measure.
– Inches and millimeters are use to record the
height of the mercury column in a mercury
barometers.
– Millibars are used on U.S. weather maps. The air
pressure recorded on weather maps is adjusted to
eliminate the effect of elevation.
Air Pressure
• Air pressure changes are not only caused by
elevation but also temperature and humidity.
– At sea level air pressure decrease as temperature
increases.
• In warmer air the water molecules are further apart ,
thus exerting a lower force on the Earth’s surface.
• In colder air the water molecules are closer together,
thus exerting a greater force on the Earth’s surface.
Air Pressure
– As humidity increases (more water vapor in the
air) the air pressure decreases because water
vapor has less mass than oxygen or nitrogen
molecules.
• Water vapor molecules replace the more massive
oxygen and nitrogen molecules thus causing the air to
be lighter and exerting less force on the Earth’s surface.
Air Pressure
• Changes in air pressure allow for simple but not
necessarily an accurate way of forecasting the
weather.
– A decrease in air pressure often signals the
approach of warmer, more humid air, along with
rain or snow.
– An increase in air pressure often signals the
approach of cooler, drier air and fair weather.
Air Pressure
• Meteorologist analyze air pressures by plotting
isobars on weather maps.
– Isobar is a line that joins points of equal air pressure.
– When the air pressure steadily increases towards the
center of a set of closed isobars, the area defined by the
isobars is called a high-pressure area or high.
– When the air pressure steadily decreases towards the
center of a set of closed isobars, the area defined by the
isobars is called a low-pressure area or low..
– Closely spaced isobar lines indicate an area where the air
pressure changes quickly.
– Dividing the pressure change by the distance over which
the air pressure changes is called the pressure gradient.
Air Pressure
Air Pressure and Wind
• The wind blows due to the air moving from a
high pressure are to a low pressure area
assuming there are not other forces acting on it.
– The greater the difference in air pressure between
two points the stronger the wind the blows between
them
– Closely spaced isobars indicate a strong pressure
gradient and strong winds.
– Widely space isobars indicate a weaker pressure
gradient and weaker winds.
Air Pressure and Wind
• Pressure differences and winds are ultimately
caused by the unequal heating of Earth’s
surface.
– The Earth’s land absorbs more solar radiation than
the surrounding water.
– The air over the land heats up due to radiation and
becomes less dense.
– The hot air rises and the cooler air over the ocean
blows in to replace the rising warm air.
– Thus air flows from high
pressure to low pressure
Air Pressure and Wind
• Meteorologist use two instruments to
measure wind speed and direction.
– The wind vane is use to determine the direction
the wind is blowing from.
– The anemometer is used to measure the speed of
the wind 10 meters above the ground.
• Wind speed can also be estimated based on the wind’s
effect on water, smoke, trees, and other objects.
Factors Affecting Winds
• If the Earth was perfectly smooth and did not
rotate, then air would flow straight from high
pressure to low pressure.
• Earth is not perfectly smooth and it does
rotate so the winds never flow in a straight
path.
Factors Affecting Winds
• The Coriolis Effect: is the tendency of an
object moving freely over the Earth’s surface
to curve away from its path of travel.
• The rotation of the
Earth is what causes
the Coriolis effect.
Factors Affecting Winds
• The Coriolis Effect can described as follows:
– In the Northern Hemisphere, the Coriolis effect deflects
objects to their right; in the Southern Hemisphere, to
their left
– The Coriolis effect is greatest near the poles and least
near the equator.
– The effect increases as the speed of the object increases.
– The effect does not depend on an object’s direction of
movement.
– The effect is generally noticeable only for objects
traveling over great distances, such as planes and winds.
• Due to the Coriolis Effect winds in the Northern
Hemisphere rotate clockwise out of high pressure
and counterclockwise into areas of low pressure.
(The pattern is reversed in the Southern
Hemisphere).
Factors Affecting Winds
• Wind is also affect by friction between the air
mass and the Earth’s surface. The friction causes
surfaces winds to slow down.
• The Coriolis effect is weaker on slower moving
objects, thus frictions reduces the Coriolis effect
on surface winds.
• The more friction the less the deflection of
surface winds due to the Coriolis effect.
• The smoother the surface the faster the surface
winds thus the stronger the Coriolis effect.
• The higher the altitude the less friction so the
stronger the Coriolis effect on the winds, they are
deflected more.
• Coriolis effect on wind in the Northern
Hemisphere:
Factors Affecting Winds
• In the top of the troposphere the friction has literally no
affect on the wind speeds so the Coriolis effect is very
strong on these winds.
• The winds at this altitude form what is call the Jet Stream.
– The jet stream is bands of swiftly moving winds that are
typically thousands of kilometers long and hundreds of
kilometers wide and about one kilometer from top to bottom.
– The Polar-Front jet stream is cool air from the poles that mixes
with warmer air to the south.
– This polar-front jet stream has a great effect on the United
States weather, it supplies the energy to storms and directing
their paths.
– The polar-front jet stream is fastest in the winter when there is
the greatest temperature contrasts on the Earth’s surface.
– There is a tropical-easterly jet stream that is weaker than the
polar-front jet stream.
Global Wind Patterns
• Many factors affect wind speeds, including the
temperature differences between equatorial and
polar regions, the rotation of Earth, the location of
the continents, the time of year, and local
topography.
• Lets first look at Earth’s wind patterns if Earth did not
rotate.
– Warm air at equators
would rise allowing for
the cooler air that is
sinking at the poles to
move towards the
equators to replace
the rising warm air.
– This would result in
one large convection
cell in each hemisphere.
Global Wind Patterns
• Because of the Coriolis effect and the fact that
air cools and sinks long before it reaches the
poles a three-celled circulation model in each
hemisphere
better represents
what is happening.
Global Wind Patterns
• Circulation cells are caused by alternating bands
of high and low pressures at Earth’s surface.
• The Coriolis effect causes the winds in the
Northern Hemisphere to deflect to the right
– The air flowing southward from the pole is deflected
to the right producing easterlies (prevailing winds
blowing from east to west).
– Between 30o and 60o the northward air is shifted to
the right to produce westerlies (prevailing winds
blowing from west to east).
– Between 30o and the equator the southward flowing
air that is shifted to the right produces easterlies
(prevailing winds blowing east to west).
– In the Southern Hemisphere the pattern is reversed.
Global Wind Patterns
• Like with any model the three-cell model is
accurate in some ways and inaccurate in other
ways.
– The main weakness is with the mid-latitudes (30o
and 60o ) circulation is a simplified view which
does not take into account the continents and
seasons.
– The main strength of the three-cell model is that it
gives fairly accurate view of general patterns of
surfaces winds and pressures outside of the midlatitudes.
Global Wind Patterns
• Special areas of wind circulation occur at
and/or near the equator.
– Intertropical convergence zone (ITCZ) is an area
that the surface winds of the two hemispheres
come together. Here it is
hot, humid and there is
little to no wind present.
This area can be referred
to as the doldrums.
– Trade Winds are areas
where there is are constant
and steady winds. This is
where merchants wanted
to sail their ships.
Continental and Local Winds
• Because of Earth’s tilt and relative position of
the sun changes over the year, Earth’s surface
temperatures change with seasons so does
the global winds.
– The seasonal changes in temperatures cause
seasonal changes in air pressures.
– Because air pressures change seasonally thus the
winds change seasonally.
Continental and Local Winds
• Local wind is a wind that extends for a distance of 100
kilometers or less.
– Local winds are caused mainly by differences in temperature.
– Example of local winds are sea and land breezes.
– Temperature difference also cause mountain and valley
breezes.
• At night the mountain cools so the air cools and sinks causing the
breezes to flow down the mountain, thus called a mountain
breeze.
• During the day the mountain warms so the air rises causing the
breeze to flow up the mountain, thus called valley breeze.
• Valley breezes are much lower in speed than that of a mountain
breeze.