Introduction
In the previous section, we have seen the sun as the source for our weather through the transfer of heat energy to
the earth. The equatorial region receives the bulk of the heat energy but not always directly. Relative to the sun, the
earth's axis is tilted approximately 23½°. The amount of radiation any one place receives each year varies throughout
the year.
In the northern hemispheric winter, the southern hemisphere received the majority of the solar radiation. The day
which the daylight hours are the shortest in the northern hemisphere is December 22. Conversely, the southern
hemisphere daylight hours are the longest. Six months later, on June 22, the earth has completed one half of its orbit
with the northern hemisphere receiving the majority of the radiation.
Twice a year, March 21 and September 23, both hemispheres receive the same amount of radiation. The days are
called the equinox meaning equal night. Both hemispheres have 12 hours of daylight and darkness.
Global Circulations
Global Circulations explain how air and storm systems travel over the
Earth's surface. The global circulation would be simple (and the weather boring) if the Earth did not rotate, the
rotation was not tilted relative to the sun, and had no water.
In a situation such as this, the sun heats the entire surface, but where the sun is more directly overhead it heats the
ground and atmosphere more. The result would be the equator becomes very hot with the hot air rising into the upper
atmosphere.
That air would then move toward the poles where it would become very cold and sink, then return to the equator
(above right). One large area of high pressure would be at each of the poles with a large belt of low pressure around
the equator.
Learning Lesson
However, since the
earth rotates, the axis is tilted, and there is more land mass in the northern hemisphere than in the southern
hemisphere, the actual global pattern is much more complicated.
Instead of one large circulation between the poles and the equator, there are three circulations...
1.
Hadley cell - Low latitude air movement toward the equator that with heating, rises vertically, with poleward
movement in the upper atmosphere. This forms a convection cell that dominates tropical and sub-tropical
climates.
2.
Ferrel cell - A mid-latitude mean atmospheric circulation cell for weather named by Ferrel in the 19th
century. In this cell the air flows poleward and eastward near the surface and equatorward and westward at
higher levels.
3.
Polar cell - Air rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the
polar highs. At the surface air diverges outward from the polar highs. Surface winds in the polar cell are
easterly (polar easterlies).
Between each of these circulation cells are bands of high and low pressure at the surface. The high pressure band is
located about 30° N/S latitude and at each pole. Low pressure bands are found at the equator and 50°-60° N/S.
Usually, fair and dry/hot weather is associated with high pressure, with rainy and stormy weather associated with low
pressure. You can see the results of these circulations on a globe. Look at the number of deserts located along the
30°N/S latitude around the world. Now, look at the region between 50°-60° N/S latitude. These areas, especially the
west coast of continents, tend to have more precipitation due to more storms moving around the earth at these
latitudes.
The Jet Stream
Jet streams are relatively narrow bands of strong wind in the upper levels
of the atmosphere. The winds blows from west to east in jet streams but
the flow often shifts to the north and south. Jet streams follow the
boundaries between hot and cold air. Since these hot and cold air
boundaries are most pronounced in winter, jet streams are the strongest
for both the northern and southern hemisphere winters.
Why do the jet stream winds blow from west to east? Recall from the
previous section what the global wind patterns would be like if the earth
was not rotating. (The warm air rising at the equator will move toward
both poles.) We saw that the earth's rotation divided this circulation into
three cells. The earth's rotation is responsible for the jet stream as well.
The motion of the air is not directly north and south but is affected by the momentum the air has as it moves away
from the equator. The reason has to do with momentum and how fast a location on or above the Earth moves relative
to the Earth's axis.
Your speed relative to the Earth's axis depends on your location. Someone standing on the equator is moving much
faster than someone standing on a 45° latitude line. In the graphic (above right) the person at the position on the
equator arrives at the yellow line sooner than the other two. Someone standing on a pole is not moving at all (except
that he or she would be slowly spinning). The speed of the rotation is great enough to cause you to weigh one pound
less at the equator than you would at the north or south pole.
The momentum the air has as it travels around the earth is conserved, which means as the air that's over the equator
starts moving toward one of the poles, it keeps its eastward motion constant. The Earth below the air, however,
moves slower as that air travels toward the poles. The result is that the air moves faster and faster in an easterly
direction (relative to the Earth's surface below) the farther it moves from the equator.
In addition, with the three-cell circulations mentioned previously, the regions around 30° N/S and 50°-60° N/S are
areas where temperature changes are the greatest. As the difference in temperature between the two locations
increase, the strength of the wind increases. Therefore, the regions around 30° N/S and 50°-60° N/S are also regions
where the wind, in the upper atmosphere, is the strongest.
The 50°-60° N/S region is where the polar
jet located with the subtropical jet located around
30°N. Jet streams vary in height of four to eight
miles and can reach speeds of more than
275 mph (239 kts / 442 kp/h).
The actual appearance of jet streams result from
the complex interaction between many variables such as the location of high and low pressure
systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in
altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else.
Jet streams also "follow the sun" in that as the sun's elevation
increases each day in the spring, the jet streams shifts north
moving into Canada by Summer. As autumn approaches and the
sun's elevation decreases, the jet stream moves south into the
United States helping to bring cooler air to the country.
Also, the jet stream is often indicated by a line on maps and by
television meteorologist. The line generally points to the location
of the strongest wind. Jet streams are typically wider and not as distinct but a region where the wind increases toward
a core of strongest wind.
One way of visualizing this is consider a river. The river's current is generally the strongest in the center with
decreasing strength as one approaches the river's bank. It can be said that jet streams are "rivers of air".
Climate
The earth's tilt, rotation and land/sea distribution affect the global weather patterns we observe. While the weather
varies from day-to-day at any particular location, over the years, the same type of weather will reoccur. The
reoccurring "average weather" found in any particular place is called climate.
German climatologist and amateur botanist Wladimir Köppen (1846-1940) divided the world's climates into several
major categories based upon general temperature profile related to latitude. These categories are as follows:
A - Tropical Climates
B - Dry Climates
Tropical moist climates
The most obvious climatic feature of
extend north and south from the equator to about 15°
this climate is that potential evaporation and transpiration
to 25° latitude. In these climates all months
exceed precipitation. These climates extend from 20°-35°
have average temperatures greater than 64°F
North and South of the equator and in large continental
(18°C) and annual precipitation greater than 59".
regions of the mid-latitudes often surrounded by mountains.
D - Moist Continental Mid-latitude Climates
C - Moist Subtropical Mid-Latitude Climates
Moist continental mid-latitude climates
have warm to cool summers and cold winters. The location of
This climate generally has
these climates is poleward of the C climates. The average
warm and humid summers with mild winters. Its
temperature of the warmest month is greater than
extent is from 30°50° of latitude mainly on the
50°F (10°C), while the coldest month is less than -22°F (-
eastern and western borders of most continents.
30°C). Winters are severe with snowstorms, strong winds,
During the winter, the main weather feature is the
and bitter cold from Continental Polar or Arctic air masses.
mid-latitude cyclone. Convective thunderstorms
dominate summer months.
E - Polar Climates
H - Highlands
Polar climates have year-
Unique climates based on their
round cold temperatures with the warmest month
elevation. Highland climates occur in mountainous terrain
less than 50°F (10°C). Polar climates are found on
where rapid elevation changes cause rapid climatic changes
the northern coastal areas of North America, Europe,
over short distances.
Asia, and on the land masses of Greenland and
Antarctica.
The map (below) shows where these major categories occur in the mainland United States.
Take it The classical length of record to determine the climate for any particular place is 30 years, as defined by the
World Meteorological Organization (WMO). The quantities most often observed are temperature, precipitation, and
wind.
The "normals" are computed once every 10 years which helps to smooth out year-to-year variations. For example,
the current 30-year normals were calculated from the actual weather data that occurred during the 30 years of 19712000. In 2011, a new set of 30-year normals will be calculated using the data from the period of 1981-2010. So, when
you hear what the normal high and low temperature for your location, for example, they come from these 30-year
averages.
Precipitation Types
Regardless of the originating cause, the weather around the world falls into three basic categories of phenomena;
precipitation, obscurations and 'other' phenomena. Precipitation is any of the forms of water particles, whether liquid
or solid, that fall from the atmosphere and reach the ground. The different types of precipitation are as follows....
Rain
The most
commonly
observed
precipitation,
comes either
in the form of
drops larger
than that of
drizzle (0.02
inch / 0.5 mm),
or smaller
drops which, in
contrast to
drizzle, are
widely
separated.
Drizzle
Fairly uniform
precipitation
composed
exclusively of
fine drops very
close together.
Drizzle
appears to
float while
following air
currents,
although
unlike fog
droplets, it falls
to the ground.
Quite often fog
and drizzle
occur together.
Ice Pellets
(Sleet)
Precipitation of
transparent or
translucent
pellets of ice,
which are
round or
irregular hard
grains of ice
consisting of
frozen
raindrops, or
largely melted
then refrozen
snowflakes.
Hail
Precipitation in
the form of
small balls or
other pieces of
ice falling
separately or
frozen
together in
irregular
lumps.
Associated
with
thunderstorms,
individual hail
stone sizes
are ¼ inch (5
mm) or greater
in diameter.
Hail size 1
inch (2.5 cm)
or greater
classify a
thunderstorm
as a severe
thunderstorm.
Small Hail
(Snow
Pellets)
Precipitation of
white, opaque
grains of ice.
The grains are
round or
sometimes
conical.
Diameters are
less than ¼
inch (5 mm).
Snow
Precipitation of
snow crystals,
mostly
branched in
the form of sixpointed stars.
Snow
Grains
Precipitation of
very small,
white, and
opaque grains
of ice.
Basically, this
is frozen drizzl
e.
Ice
Crystals
Generally only
occurring in
very cold
regions, they
are falling ice
crystals in the
form of
needles,
columns, or
plates.
Also called
'diamond dust'
this is a type of
fog produced
by a cirrus
cloud on the
ground but
unlike fog (a
stratus cloud
on the ground)
the individual
particles of 'ice
crystals' form
directly as ice.
The shape of
the individual
crystals of ice
causes the
'light pillar'
optical effect
above the light
source.
Mist
Visible minute water
particles suspended in the
atmosphere that reduces
visibility to less than 7
miles (11 km) but greater
than or equal to 5/8thmile
(1 km).
There is often not much
difference in the
appearance of 'haze' and
'mist'.
When the difference
between the air
temperature and dewpoint
is 3°F (1.7°C) or less then
the obscuration is usually
called 'mist'.
Obscuration Types
An obscuration is any phenomena in the atmosphere, other than precipitation, that reduces the horizontal visibility.
The most common obscuration "seen" is usually fog. Obscurations include...
Fog
Visible minute water
particles (droplets) which
are based at the Earth's
surface and reduces
horizontal visibility to less
than 5/8th mile (1 km) and,
unlike drizzle, it does not
fall to the ground.
Smoke
A suspension in the air of
small particles produced
by combustion.
A transition to haze may
occur when smoke
particles have traveled
great distances, 25 to 100
miles (40 to 160 km) or
more, when the larger
particles have settled out
and the remaining particles
have become widely
scattered through the
atmosphere.
Volcanic Ash
Fine particles of rock
powder that originate from
a volcano and that may
remain suspended in the
atmosphere for long
periods.
Dust
Fine particles of earth or
other matter raised or
suspended in the air by the
wind that may have
occurred at or far away
from the station which may
restrict horizontal visibility.
Sand
Sand particles raised by
the wind to a height
sufficient to reduce
horizontal visibility.
Haze
A suspension in the air of
extremely small, dry
particles invisible to the
naked eye and sufficiently
numerous to give the air
an opalescent appearance.
That is the scientific way of
saying haze is "air
pollution".
There is often not much
difference in the
appearance of 'haze' and
'mist'.
When the difference
between the air
temperature and dewpoint
is greater than 3°F (1.7°C)
then the obscuration is
usually called 'haze'.
Other Weather Types
Finally, there are significant types of weather that are related to wind. These other forms of weather include...
WellDeveloped
Dust/Sand
Whirls
An ensemble of
particles of dust
or sand,
sometimes
accompanied by
small litter,
raised from the
ground in the
form of a
whirling column
of varying height
with a small
diameter and an
approximately
vertical axis.
Commonly
called a 'dust
devil'.
Squall
A strong wind
characterized by
a sudden onset
in which the
wind speed
increases at
least 18 mph
(16 knots, 30
km/h) and is
sustained at 25
mph (22 knots,
41 km/h) or
more for at least
one minute.
Often occur
from
thunderstorms
where the term
'squall line'
originates. But
the term 'squall'
only refers to
the wind speed
increase and
not any other
associated
weather.
In the image at
right, the low
arcing clouds
are not the
squall line but
mark the
approximate
location of the
squall.
Tornado
A violent,
rotating column
of air touching
the ground.
Funnel
Cloud
A violent,
rotating column
of air which
does not touch
the surface.
Waterspout
A violent,
rotating column
of air that forms
over a body of
water, and
touches the
water surface.
If it does not
touch the water
surface then it is
called a funnel
cloud.
Sand Storm
Particles of
sand carried
aloft by a strong
wind. The sand
particles are
mostly confined
to the lowest ten
feet, and rarely
rise more than
fifty feet above
the ground.
Duststorm
A severe
weather
condition
characterized by
strong winds
and dust-filled
air over an
extensive area.
Heat Index
With basic climate regions around the world, there are some places where the weather
is considered hot. But just heat alone does not make high temperatures a threat. There is an old saying stating "It's
not the heat, it's the humidity". Well, actually it's both.
Heat waves are not easily photographed, like the destruction of tornadoes, hurricanes and floods and therefore tend
to not have the same visual impact as these other disasters. Yet, heat waves kill more people in the United States
than all of the other weather related disasters combined. The 10-year average (1997-2006) for heat related deaths in
the U.S. is 170 in a typical year.
Heat waves form when high pressure aloft (from 10,000 - 25,000 feet / 3,000 - 7,600 meters) strengthens and
remains over a region for several days up to several weeks. This is common in summer (in both Northern and
Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the middle layers
of the atmosphere, is the high pressure area.
High pressure in the middle layers of the atmosphere acts as a dome or cap allowing heat to
build up at the earth's surface.
Summertime weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure
also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome
capping the atmosphere.
This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little
or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat
at the surface that we experience as a heat wave.
Our bodies dissipate heat by varying the rate and depth of blood circulation, by losing
water through the skin and sweat glands, and, as the last extremity is reached, by panting. As the body heats up, the
heart begins to pump more blood, blood vessels dilate to accommodate the increased flow, and the tiny capillaries in
the upper layers of skin are put into operation.
The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere by
one or a combination of three ways...



radiation,
convection, and
evaporation.
At lower temperatures, radiation and convection are efficient methods of removing heat. However, once the air
temperature reaches 95°F (35°C), heat loss by radiation and convection ceases. It is at this point that heat loss by
sweating becomes all-important. But sweating, by itself, does nothing to cool the body, unless the water is removed
by evaporation (sweat changing to water vapor). The downside of this method of cooling is that high relative
humidity retards evaporation.
Relative humidity is a measure of the amount of water vapor contained in the air, divided by the maximum amount the
air can hold, expressed as a percent. A relative humidity of 50% means the air contains ½ of the water vapor it can
actually hold. The maximum amount of water vapor the air can hold is dependent upon the temperature (the
"relative" in relative humidity).
The higher the temperature, the more water (actually water vapor) the air can hold. For example, air with a
temperature of 32°F (0°C) can hold about 0.16 ounce of water. Air with a temperature of 80°F (27°C) can hold about
an ounce of water.
So, what does this all mean? Sweat is evaporated (changes from a liquid to a gas, i.e. water vapor) when heat is
added. The heat is supplied by your body. The results are summed up in the table below...
Relative
Humidity
Capacity for air
to hold water
Amount of
Evaporation
HEAT removed
from the body
low
LARGER
HIGHER
MORE
HIGH
smaller
lower
less
We, at the National Weather Service, as part of our mission for protecting life and property, have a measure of how
the hot weather "feels" to the body. The Heat Index is based on work by R.G. Steadman and published in 1979
under the title "The Assessment of Sultriness, Parts 1 and 2." In this work, Steadman constructed a table which
uses relative humidity and dry bulb temperature to produce the "apparent temperature" or the temperature the
body "feels".
We use this table to provide you with Heat Index values. These values are for shady locations only. Exposure
to full sunshine can increase heat index values by up to 15°F (8°C). Also, strong winds, particularly with very hot,
dry air, can be extremely hazardous as the wind adds heat to the body. The Heat Index Chart is below.
How to read the chart...Follow the temperature line until it intersects the relative humidity line. Then read the Heat
Index on the curved line. For example, an air temperature of 100°F (38°C) and Relative Humidity of 40%. Follow the
100°F (38°C) temperature line until it intersects the 40% relative humidity line. Then curved line that also intersects is
the Heat Index of 110°F (43°C), or Very Hot.
That is the temperature the body thinks it is and attempts to compensate for that level of heat. Remember, these
values are in the SHADE. You can add up to 15°F (8°C) to these values if you are in direct sunlight.
The chart below tells you the risk to the body from continued exposure to the excessive heat.
Category Classification
Heat Index/Apparent
Temperature
General Affect on People in High Risk Groups
I
Extremely
Hot
130°F or Higher
(54°C or Higher)
Heat/Sunstroke HIGHLY LIKELY with continued
exposure
II
Very Hot
105°F - 130°F
(41°C - 54°C)
Sunstroke, heat cramps, or heat exhaustion LIKELY,
and heat stroke POSSIBLE with prolonged exposure
and/or physical activity
III
Hot
90°F - 105°F
(32°C - 41°C)
Sunstroke, heat cramps, or heat
exhaustion POSSIBLE with prolonged exposure and/or
physical activity
IV
Very Warm
80°F - 90°F
(27°C - 32°C)
Fatigue POSSIBLE with prolonged exposure and/or
physical activity
Next
Wind Chill
Just as there are persistent hot places around the world, there are persistent cold
places. The cold air alone can be deadly but when the air is moving if feels much colder. The wind chill is the effect
of the wind on people and animals. The wind chill temperature is based on the rate of heat loss from exposed skin
caused by wind and cold and is to give you an approximation of how cold the air feels on your body.
As the wind increases, it removes heat from the body, driving down skin temperature and eventually the internal body
temperature. If the temperature is 0°F (-18°C) and the wind is blowing at 15 mph (13 kts / 24 kp/h), the wind chill
temperature is -19°F (-28°C). At this level, exposed skin can freeze in just a few minutes.
The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of
time for the object to cool. The inanimate object will not cool below the actual air temperature. For example, if the
temperature outside is -5°F (-21°C) and the wind chill temperature is -31°F (-35°C), then your car's radiator
temperature will be no lower than the air temperature of -5°F (-21°C).
The Wind Chill Chart
To determine the wind chill temperature, find the value closest to your outside air temperature. Find the value that
most closely represents your present wind speed. Your wind chill temperature is the value where lines drawn from the
air temperature and wind cross.
What is important about the wind chill besides feeling colder than the actual air temperature? The lower the wind chill
temperature, the greater you are at risk for developing frost bite and/or hypothermia.
Frostbite occurs when your body tissue freezes. The most susceptible parts of the body are fingers, toes, ear lobes,
and the tip of the nose. Hypothermia occurs when body core temperature, normally around 98.6°F (37°C) falls below
95°F (35°C). The following table shows how fast frostbite can occur at various wind chill temperatures.
Wind Chill
Cold Threat
40°F to 21°F (4°C to COLD. Unpleasant.
6°C)
20°F to 1°F (-7°C to 17°C)
VERY COLD. Very unpleasant.
0°F to -19°F (-18°C to BITTER COLD. Frostbite possible. Exposed skin can freeze within 5 minutes.
28°C)
-20°F to -69°F (-29°C to EXTREMELY COLD. Frostbite likely. Exposed skin can freeze within 1 minute. Outdoor
-56°C)
activity becomes dangerous.
≤ -70°F (≤ -57°C)
FRIGIDLY COLD. Exposed skin can freeze in 30 seconds.
The best way to avoid hypothermia and frostbite is to stay warm and dry indoors. When you must go outside, dress
appropriately. Wear several layers of loose-fitting, lightweight, warm clothing. Trapped air between the layers will
insulate you. Remove layers to avoid sweating and subsequent chill.
Outer garments should be tightly woven, water repellant, and hooded. Wear a hat, because half of your body heat
can be lost from your head. Cover your mouth to protect your lungs from extreme cold. Mittens, snug at the wrist, are
better than gloves. Try to stay dry and out of the wind.
Temperature (°F)
40
35
30
25
20
15
10
5
0
-5
10
15
20
25
30
35
40
45
50
-55
-60
0
40
35
30
25
20
15
10
5
0
-5
10
15
20
25
30
35
40
45
-50
-55
-60
5
37
31
25
19
13
7
1
-5
11
16
22
28
34
40
46
52
58
63
-69
-75
-81
10
34
27
21
15
11
3
-4
10
16
22
28
35
41
47
53
59
66
72
-78
-84
-90
15
32
25
19
13
6
0
-7
13
19
26
32
39
45
51
58
64
71
77
-83
-90
-96
20
30
24
17
11
4
-2
-9
15
22
29
35
42
48
55
61
68
74
81
-88
-94
101
25
29
23
16
9
3
-4
11
17
24
31
38
44
51
58
64
71
78
84
-91
-98
104
30
28
22
15
8
1
-6
12
19
26
33
39
46
53
60
67
73
80
87
-94
101
107
35
28
21
14
7
0
-7
14
21
27
34
41
48
55
62
69
76
83
89
-96
103
110
40
27
20
13
6
-1
-8
15
22
29
36
43
50
57
64
71
78
84
91
-98
105
112
45
26
19
12
5
-2
-9
16
23
30
37
44
51
58
65
72
79
86
93
100
107
114
50
26
19
12
4
-3
10
17
24
31
38
45
52
60
67
74
81
88
95
102
109
116