Statistics for test #1 Test
#2
105-109 2 1
100-104 2 2
95-99 3 2
90-94 4 4
85-89 9 20
As 7 16 As
80-84 8 6
75-79 27 35
Bs 11 17 Bs
70-74 21 7
65-69 22 43
Cs 14 21 Cs
60-64 21 20
55-59 17 38
Ds 26 46 Ds
50-54 16 21
40-49 24 39
30-39
8 48 Fs 13
20-29 8 78 Fs
·
Winds have both speed and direction.
·
The direction of the wind is the direction from
which it is blowing.
·
That is, a north wind blows from the north.
·
Wind direction has a numerical value in degrees on a
compass from 0º to 360º.
·
North is 0º or 360º. East is 90º. South is 180º. West is 270º.
·
Pressure imbalances cause winds.
·
Temperature differences cause pressure imbalances.
Pressure
·
Pressure is the force per unit area that air
molecules exert on all surfaces with which they are in contact.
·
The standard unit of pressure is the Pascal.
·
In meteorology, a common unit of pressure is the
millibar (mb). 1 mb = 100 Pa. Air pressure at sea level is near 1000 mb,
with a standard value of 1013.25 mb.
·
Pressure increases when the temperature increases.
·
Pressure increases when the density increases
·
The equation of state, sometimes called the ideal
gas law states:
·
Pressure = density x temperature x a constant
Pressure and
height
·
Pressure always decreases upward with height.
·
Pressure decreases upward most rapidly near the
surface.
·
The atmosphere is most compressed near the surface.
·
A surface weather map shows isobars, or lines of
constant pressure.
·
Pressure on a surface weather map is sea level
pressure.
·
If a station is above sea level, and most are, some
pressure is added to the surface pressure observation to account for an
imaginary layer of air between the station and sea level.
·
Isobars are contour lines that connect equal values
of pressure. On one side of the isobars
pressures are higher, and on the other side pressures are lower.
Measuring pressure
·
Pressure is measured with a barometer.
·
The standard instrument for measuring pressure is a
mercury barometer.
·
The mercury barometer is a long thin glass tube,
closed at the top, and open at the bottom into a reservoir of mercury.
·
The reservoir of mercury is open to the pressure of
the atmosphere.
·
Higher atmospheric pressure corresponds to a higher
mercury column in the tube.
·
Lower atmospheric pressure corresponds to a lower
mercury column in the tube.
·
Aneroid barometers are not so accurate, but they are
cheaper, easier to move, less fragile, and do not use a toxic substance.
·
Aneroid barometers use a small metal box with
flexible sides from which some of the air has been removed.
·
The box shrinks when the atmospheric pressure
increases, and expands when the atmospheric pressure increases.
·
Pivots and dials convert the pressure reading to
movement of a pointer on a scale.
How air moves
·
Air changes speed and direction when forces act on
it.
·
Newton’s Law of Motion describes how forces are
related to changes of speed and direction.
·
A change in speed and/or direction of air motion is
called acceleration
·
Newton’s Law states:
Acceleration = Sum of forces / Mass
Or
Force = Mass x Acceleration
·
Forces and accelerations have both a strength and a
direction. Like winds, they are often
indicated by arrows.
·
Pressure differences exert a force called the
pressure gradient force.
·
The pressure gradient force is always directed from
high to low pressure.
The pressure
gradient force
·
The pressure gradient force is always important in
determining the wind.
·
The pressure gradient is the difference in pressure
per unit distance.
·
The stronger the pressure gradient force, the
stronger the wind.
·
The pressure gradient force is stronger where the
isobars are closer together.
·
The pressure gradient force is directed across (at
right angles to) the isobars, from high to low pressure.
·
The vertical pressure gradient force is directed
upward.
·
The vertical pressure gradient force is nearly
balanced by the downward force of gravity.
·
This near balance is called the hydrostatic
equation.
·
Buoyancy occurs when there is a slight imbalance
between the vertical pressure gradient force and gravity.
Forces in the atmosphere
·
The pressure gradient force acts from high to low
pressure.
·
There is a horizontal pressure gradient force as
well as a vertical pressure gradient force.
·
Friction always acts in the direction opposite the
wind.
·
Friction always acts to reduce the wind speed
without changing its direction.
·
The stronger the wind, the stronger is the friction
force.
·
Friction is important at the surface.
·
Friction is nearly zero by 1000 meters above the
surface.
·
The Coriolis force comes about because the earth
rotates on its axis from west to east.
·
The direction we call east changes with the time of
day. This is an acceleration, the
Coriolis acceleration. When we think of
this acceleration as a force per unit mass, we call it the Coriolis Force.
The Coriolis Force
·
The Coriolis force deflects or changes the direction
of all moving objects, including air molecules.
·
The Coriolis force has no effect on stationary
objects, that is, when the wind is calm.
·
The Coriolis force deflects moving objects to the
right in the Northern Hemisphere, and to the left in the Southern Hemisphere.
·
The Coriolis force is strongest at the poles, and is
zero at the equator.
·
The Coriolis force is stronger when the wind speed
is stronger, and weaker when the wind speed is weaker.
·
The Coriolis force changes the wind direction, but
never the wind speed.
The Geostrophic
Wind
·
The geostrophic wind blows in a straight line along
straight isobars with low pressure on the left in the Northern Hemisphere.
·
The geostrophic wind is a balanced wind.
·
The pressure gradient force (directed from high to
low pressure) exactly balances the Coriolis Force (directed to the right of the
wind in the Northern Hemisphere).
·
The geostrophic wind blows at right angles to both
the pressure gradient force and the Coriolis Force.
·
The geostrophic wind blows along the isobars.
·
Where the isobars are closer together, the
geostrophic winds are stronger.
·
For test #3 it will be important to understand the
direction of the pressure gradient force and the Coriolis force that produces
the geostrophic wind.
·
The observed wind is not the same as the geostrophic
wind.
·
The geostrophic wind is what the wind would be if
there were no friction, and the winds blew in a straight line, and the isobars
were straight lines, and the pressure gradient force exactly balances the
Coriolis Force.
·
It is possible to calculate the speed and direction
of the geostrophic wind without knowing the observed wind if the values of
pressure at the same height are available.
·
It is possible
to calculate the speed and direction of the geostrophic wind without
knowing the observed wind if the values of height at the same pressure are
available.
·
Above about 1000 meters, these calculations of the
geostrophic wind are usually at least as reliable as observations of the
observed wind.
·
There are a few exceptions to the last
statement: hurricanes, thunderstorms,
very intense low pressure centers.
The Gradient Wind
·
Isobars are usually not straight lines.
·
Isobars are generally curved.
·
The wind, particularly above a height of 1000 meters,
flows nearly parallel to (along) the isobars, even when they are curved.
·
The wind that flows along curved isobars is called
the gradient wind.
·
The gradient wind is different from the geostrophic
wind because there is an imbalance between the pressure gradient force and the
Coriolis Force.
·
The imbalance of the forces is just large enough to
produce the acceleration that changes the direction of the wind as it flows
along the isobars.
·
The acceleration that changes the direction of the
wind is called the centripetal acceleration.
·
The centripetal acceleration is in the direction of
the stronger force.
The Gradient Wind
around a Low Pressure Center
Northern
Hemisphere
·
The pressure gradient force points inward toward the
low pressure center.
·
In the Northern Hemisphere, the Coriolis force
points to the right of the gradient wind.
·
The pressure gradient force is stronger than the
Coriolis force.
·
The wind blows in a counterclockwise direction
around the low pressure center, following the isobars.
·
The centripetal (unbalanced) acceleration is inward.
·
The centripetal acceleration keeps the wind from
blowing in a straight line and away from the low pressure center.
·
The centripetal acceleration keeps the wind blowing
in a counterclockwise direction around the low pressure center.
·
The speed of the gradient wind blowing around the
low pressure center is less than the geostrophic wind (subgeostrophic).
The Gradient Wind
around a High Pressure Center
Northern
Hemisphere
·
The pressure gradient force points outward from the
high pressure center.
·
In the Northern Hemisphere, the Coriolis force
points to the right of the gradient wind.
·
The pressure gradient force is weaker than the
Coriolis force.
·
The wind blows in a clockwise direction around the
high pressure center, following the isobars.
·
The centripetal (unbalanced) acceleration is inward.
·
The centripetal acceleration keeps the wind from
blowing in a straight line and away from the high pressure center.
·
The centripetal acceleration keeps the wind blowing
in a clockwise direction around the high pressure center.
·
The speed of the gradient wind blowing around the
high pressure center is greater than the geostrophic wind (supergeostrophic).
·
The gradient wind explains why the wind flows along
(parallel to) curved isobars.
·
Usually the difference between the gradient wind
speed and the geostrophic wind speed is small.
·
Hurricanes and other intense low pressure centers
are exceptions.
·
Friction slows the wind speed near the ground.
·
Friction alters the balance of forces that governs
changes in wind speed and direction.
·
The overall effect of friction is to cause inflow
across the isobars towards a low pressure center.
·
Inflow towards a low pressure center helps create
convergence.
·
The overall effect of friction is to cause outflow
across the isobars away from a high pressure center.
·
Outflow away from a high pressure center helps
create divergence.
·
Surface convergence enhances upward vertical motions
and cloud formation.
Pressure and
Temperature
·
The pressure gradient force is related to differences
in temperature.
Surface Aloft
Low
Pressure higher T lower T
High
Pressure lower T higher T
·
Warming expands an air column
·
Cooling shrinks an air column
·
See Fig. 7-7 on p. 184
·
See Fig. 7-8 on p. 186
·
Pressure decreases upward more rapidly in the cold
air column. (Hydrostatic equation)
·
Pressure decreases upward less rapidly in the warm
air column. (Hydrostatic equation)
Weather Maps above
the Surface
·
Weather maps above the surface are constructed for
particular pressure levels, like 500 mb.
·
On a weather map at a constant pressure level, the
pressure gradient force is directed from higher to lower heights.
·
On a weather map at a constant pressure level, the
geostrophic and gradient winds are blowing parallel to the contours of constant
height, with low height on the left in the Northern Hemisiphere.
·
Aloft, higher heights are associated with warmer
columns of air, and lower heights are associated with colder columns of air.
·
Winds turn in a counterclockwise direction as they
flow through a trough (Northern Hemisphere) of low height (pressure).
·
Winds turn in a clockwise direction as they flow
through a ridge of high height (pressure). In the Northern Hemisphere.
Cyclones and
Anticyclones
Northern
Hemisphere
·
Cyclones are centers of low pressure.
·
The winds in a cyclone turn in a counterclockwise
direction.
·
Storm circulations have cyclonic air flow.
·
Anticyclones are centers of high pressure.
·
The winds in an anticyclone turn in a clockwise
direction.
·
Depends on two columns of air with different
temperatures.
·
The warmer air rises and the colder air sinks.
·
Helps explain many different wind and pressure
patterns.
Aloft H HPGFà L
Upward Downward
motion motion
Surface L ßHPGF H
Warmer air Colder air
Column Column
·
Would describe the atmosphere if:
o There
were no seasons
o The
earth rotated much more slowly
Has one thermal circulation in each hemisphere, called a
Hadley cell.
Has sinking motion in the coldest air column.
Has sinking motion at the pole.
Has high surface pressure at the pole.
Has rising motion in the warmest air column.
Has rising motion at the equator.
Has low surface pressure at the equator.
Has surface winds blowing from the pole to the equator.
Has winds deflected to the right in the NH.
Has winds blowing from the equator to the pole in the
upper troposphere.
o Would
describe the atmosphere if:
o There
were no seasons
o There
were no continents
Has three thermal circulation cells in each hemisphere.
Has alternating belts of wind and pressure at the
surface.
Resembles the single cell model
Differs from the single cell model
o The
Hadley Cell
o The
Equatorial Low / Intertropical Convergence Zone (ITCZ) / Doldrums
o The
Northeast (NH) and Southeast (SH) Trade Winds
o The
Subtropical Highs
o The
Horse Latitudes
o The
Ferrel Cell / indirect cell / center log of three
o The
Subpolar Low
o The
Westerlies / prevailing westerlies / mid-latitude westerlies
o The
Polar Cell
o The
Polar Easterlies
o The
Polar High
In
Describing the
real atmosphere
o The
three cell model doesn’t describe the upper-level winds well at all.
o The
earth does have seasons. The “thermal
equator” or the ITCZ shifts away from the equator into the summer hemisphere.
o The
earth does have continents. These set
up thermal circulations with adjacent ocean waters.
o These
thermal circulations are seasonal.
o These
thermal circulations are called monsoons.
o In
winter, the warm air column is over the ocean.
o In
summer, the warm air column is over the land.
o The
summer monsoon
o The
warmer air column is over the land
o The
colder air column is over the ocean
o The
wind flows from ocean to land at the surface
o Lower
pressure is over the land at the surface
o Higher
pressure is over the ocean at the surface
o Rising
motions are over the land
o Lots
of precipitation falls over the land
o The
winter monsoon
o The warmer air column is over the ocean
o The colder air column is over the land
o The wind flows from land to ocean at the surface
o Higher pressure is over the land at the surface
o Sinking motions are over the land
o Less precipitation falls over the land
o Lower
pressure is over the ocean at the surface
o Tibetan
Low / Siberian High over land
o Aleutian
Low / Icelandic Low over northern oceans in winter
o Hawaiian
High / Bermuda-Azores High over oceans; strongest in summer
o These
reverse on a diurnal (daily) cycle
o The
sea breeze
o Occurs
in the daylight hours
o The
warmer column is over land
o The
colder column is over water
o At
the surface, wind flows from water to land
o Aloft,
wind flows from land to water
o At
the surface, lower pressure over the land, higher pressure over the water
o Aloft,
the horizontal pressure gradient is reversed
o Rising
air motions over land, sinking over the water
o Are
zonal (blow along lines of constant latitude) and are westerlies on average.
o Have
Rossby waves embedded in the average westerly flow on individual weather maps.
o Show
ridges and troughs that define the Rossby wave
o The
Rossby waves move from west to east (except for the longest waves)
o Are
strongest in the winter hemisphere
o Are
related to the temperature difference between equator and the pole
o Have
maxima called jet streams
o The
polar jet stream is located above the subpolar low.
o The
polar front is another name for the subpolar low.
Air masses are large areas covered by air of nearly uniform properties.
Types of air
masses
Source: Source:
Higher latitudes Lower latitudes
Source: continental
continental
Land polar tropical
Source: maritime maritime
Ocean polar tropical
A front is a boundary between air masses where the
temperature changes rapidly over a short distance in the horizontal.
Pressure, humidity, wind speed, and wind direction may
also change rapidly in the vicinity of a front.
Material covered for test #3
Chapter 7: the whole chapter, except for the special
interest section on p. 197.
Chapter 8: Everything from p. 201-215 and fig. 8-12 on p.
216.
There will be no questions from the section on the oceans
that goes from page 215-220.
In the section called “Major wind systems”, pp. 220-228,
the test will include:
Monsoons, pp. 220-223
Sea and land breezes, pp. 226-227
There will be no questions from the subsections on: Foehn, Chinook, and Santa Ana Winds, pp.
223-226; Katabatic winds, p. 226; and Valley and mountain breeze, pp. 227-228.
There will be no questions from the section on air sea
interactions, pp. 228-233.
Chapter 9: The first section, from p. 237 to 246, on air
masses, will be covered on the test.
Skip the special interest sections on pp. 242-245 and on
p. 247.
In the section on fronts, starting on p. 246, the test
covers only the first paragraph, beginning at the bottom of p. 246 and ending a
few sentences into p. 247. This defines
fronts.