Weather
Scholastic Reading
Predicting Weather
Causes of Weather
Understand what causes
weather and how it
happens.
Page 2
Hurricanes &
Typhoons
Take a closer look at how
hurricanes and typhoons
happen.
Page 3
Weather Words
Check out some weather
words that you might
come across and what they
mean!
Page 5
Extreme Weather
Extreme weather is often destructive weather.
"Hottest Day of the Year" and "Storm of the Decade" are common
headlines in newspapers around the world. Several places compete for
the title of "rainiest spot on Earth," while other locations try to avoid the
menace of sandstorms. Extreme weather often is destructive weather.
Storms such as hurricanes and tornadoes can cause deaths and
millions of dollars worth of property damage.
Prolonged heat waves and cold spells
result in increased illness and death,
particularly among the very young and the
elderly. Interactions among three elements
— heat, air, and water — are primarily
responsible for weather. For example,
hurricanes often form as an air mass of low
pressure that moves from east to west over tropical seas. Hailstorms
form in clouds that contain a lot of water. Tornadoes form because
great differences in air pressure cause air to start spinning in a tight,
funnel-shaped formation. Weather scientists, called meteorologists, use
radar and other instruments to locate, track, and determine the extent
of storms and other types of extreme weather. They issue forecasts as
well as watches and warnings of approaching storms, so that people
can prepare themselves for bad weather.
WEATHER
What is weather? Why do we
get the weather we do?
What causes weather to
happen in our area and not
others? What types of
weather can we expect and
what makes weather so
dangerous?
weather Weather
includes all activities
in the atmosphere,
such as wind, rain,
snow, and storms.
We have seen lots of hurricanes recently. This is how a
hurricane forms!
Weather refers to the state of the atmosphere and
includes temperature, precipitation, humidity,
cloudiness, visibility, pressure, and winds.
Weather, as opposed to climate, includes the
short-term variations of the atmosphere, ranging
from minutes to months. Climate is typically
considered the weather that characterizes a
particular region over time.
The weather must be measured and records kept
to gain an understanding of the forces at work and
to yield the information on the averages and
extremes. By studying weather records,
atmospheric scientists may be able to predict the
weather ahead on scales of weeks to months with
greater accuracy and modify more successfully
the weather to increase precipitation or ameliorate
severe storms.
Causes of Weather
The five factors that
determine the weather of any land area are: the
amount of solar energy received because of
latitude; the area's elevation or proximity to
mountains; nearness to large bodies of water and
relative temperatures of land and water; the
number of such storm systems as cyclones,
hurricanes, and thunderstorms resulting from airmass differences; and the distribution of air
pressure over the land and nearest oceans, which
produces varying wind and air mass patterns.
How these five factors interact over the North
American continent is an excellent example of
how the weather of the United States is produced.
Because the landmass of North America
encompasses a greater range of latitude than
longitude (10° to 80° north latitude), a great
amount of differential heating occurs. This in turn
creates air-mass differences. The presence of a
large water area, the Gulf of Mexico, below the
southern states affects the character of air
masses and placement of pressure centers and
storm systems over the eastern half of the
continent. Warm, moist air from the south often
meets cold, dry air from the north over the central
United States. These contrasting air masses
include: continental Arctic and polar, from cold
land sources; maritime polar, from cold ocean
regions; and the warmer and moist Gulf or Atlantic
oceanic sources. Air from the Pacific Ocean
affects the weather in the western mountains of
the United States, which in turn affects the
interaction of cold and warm air in the eastern
United States.
Continue pg 5
2
The rate of condensation heating
that results from the intense rainfall
associated with tropical cyclones is
about 100 billion kW.
Structure of the Storm The
mature tropical cyclone is
characterized by a circular pattern
of stormclouds and torrential rains,
whipped by winds that may reach
velocities of 160 to 300 km/h (100
to 180 miles per hour) within a
radius of 10 to 100 km (6 to 60 mi)
from the storm center. The winds
diminish rapidly with increasing
distance. At a radius of 500 km
(300 mi), wind speed is usually
less than 30 km/h (18 mph). (The
winds rotate in a counterclockwise
direction in the Northern
Hemisphere and in a clockwise
direction in the Southern
Hemisphere.) The heaviest
precipitation occurs in this region
Continued…
Weather Around Us
Hurricanes and typhoons are large
and sometimes intensely violent
storm systems. In meteorological
terms, they are tropical cyclones
that have maximum sustained
winds of at least 120 km/h (75
mph). Atlantic and eastern Pacific
storms are called hurricanes, from
the West Indian huracan ("big
wind"), whereas western Pacific
storms are called typhoons, from
the Chinese taifun, "great wind."
The primary energy source for a
tropical cyclone is the latent heat
released when water vapor
condenses. Only extremely moist
air can supply the energy
necessary to spawn and maintain
tropical storms, and only very
warm air contains enough
moisture. Tropical cyclones,
therefore, form only over oceans
with water temperatures of at least
27° C (80° F). After they have
formed, such storms tend to
intensify when passing over
warmer water and weaken over
colder water.
What are some extreme types of weather? Where might these types be found?
Hurricanes & Typhoons
Weather Forecasting
The task of predicting the weather that will be observed at a
future time is called weather forecasting. As one of the
primary objectives of the science of meteorology, weather
forecasting has depended critically on the scientific and technological
advances in meteorology that have taken place since the latter half of the 19th century.
While there are many ways we have forecasted weather in the past, today we use a
barometer to forecast the weather.
Making a weather forecast involves three steps: observation and analysis, extrapolation
to find the future state of the atmosphere, and prediction of particular variables. One
qualitative extrapolation technique is to assume that weather features will continue to
move as they have been moving. In some cases the third step (prediction) simply
consists of noting the results of extrapolation, but actual prediction usually involves
efforts beyond this.
3
of intense convection. Thunderstorms may
produce rainfall rates of 250 mm (10 in) a
day. The release of latent heat associated
with this rain maintains low pressure and
strong winds. The total cloud system of a
large tropical cyclone may have a diameter of
up to about 3,200 km (2,000 mi).
At the center of the storm, within a "wall" of
powerful winds, there is an "eye" — a cloudfree circular region of relatively light winds
that has a diameter of 10 to 100 km (6 to 60
mi). Surface pressure reaches its minimum in
the eye. Typical values are 950 millibars, but
values of less than 900 have been recorded.
The sinking motion in the eye, which causes
the clearing, also produces adiabatic warming
and drying. Temperatures at 5 km (3 mi)
above sea level are typically 10° C (18° F)
warmer than the tropical storm's environment.
The very-high-velocity winds surrounding the
eye are maintained in strength by the large
differences in horizontal pressure between
the eye and the outer region of the storm.
Although the winds themselves are
responsible for much of the storm damage,
the waves and tides generated by the wind
often cause most of the damage to coastal
areas. Because much of human activity near
the coast is concentrated within a few meters
above mean sea level, storm surges can
result in considerable loss of life and property.
Speed of Rotation The rapidly whirling
tangential circulation of winds in a tropical
cyclone can be explained by the conservation
of angular momentum. Just as ice skaters
spin faster as they bring their arms down
closer to the axis of rotation, so the air rotates
faster as it is pulled in toward the center of the
storm by the low pressure. Without friction,
the wind would increase as the inverse of the
distance from the center. Thus, a wind
rotating at 5 km/h (3 mph) at a radius of 500
km (300 mi.) would have a velocity of 250
km/h (160 mph) if it reached a radius of only
10 km (6 mi.). Friction reduces the predicted
speed, but the basic principle explains the
high rotational velocities near the center.
The air that spirals toward the center and
rises in the intense convection in the wall of
the eye turns outward in the upper
“I don’t know
whether I should
check the weather or
let it be a surprise!”
What’s the difference?
troposphere (about 15 km/10 mi. above sea
level). As the air moves away from the center,
its counterclockwise rotation slows, in accord
with conservation of angular momentum. At a
distance of about 300 km (190 miles) from the
center, the air acquires an anticyclonic
(clockwise) rotation.
Occurrence and Movement Tropical
cyclones move with the large-scale wind
currents in which they are embedded. The
typical speed is 25 km/h (16 mph), but some
storms may race along at twice this speed.
Others can remain stalled in the same
location for several days. This is what
happened in 1998 with the powerful
Hurricane Mitch, which spent time hovering
off the shores of Honduras and Nicaragua.
Although it eventually weakened and moved
northeastward, the flooding caused by its
torrential rains was particularly deadly to
these two countries. The full death toll may
never be certain, but many thousands of
people perished in the resulting floods and
mudslides.
In the north Atlantic Ocean, tropical storms
tend to develop primarily during the summer
months of highest humidity and warmest
water-surface temperatures and often appear
on into October. Occasional storms develop
just before or after this period but only rarely
Continued…
4
Weather Words
air mass
A very large body of air in
which weather conditions are
more or less the same is
called an air mass.
cyclone In meteorology, a
cyclone is a large system of
air circulation located
between the equator and
either the North or South
pole. The central air
pressure of a cyclone is
lower than that of the
surrounding environment. In
contrast, an anticyclone has
central air pressure that is
higher than that of its
surroundings.
hurricane Large,
sometimes intensely violent
tropical storm systems in the
Atlantic and eastern Pacific
oceans are called
hurricanes. In the western
Pacific, they are called
typhoons.
meteorology The scientific
study of weather is called
meteorology.
precipitation Precipitation
is moisture that falls from the
sky, including rain, snow,
and hail.
in other months. Usually about five of these tropical cyclones
become strong enough to be categorized as hurricanes.
Typically these storms track from east to west at low latitudes,
moving with the eastern winds of the large subtropical
anticyclone that dominates that ocean area. As the storms
approach the North American continental landmass, however,
they often begin to take a more northerly tack as they curve
around the western rim of the anticyclone. (Storms that do not
curve in this way enter the Gulf of Mexico or cross over Central
America.) As they reach higher latitudes and come under the
influence of the westerlies, they usually turn toward the
northeast, often missing the continent. This turn to the northeast
is called recurvature.
Typhoons of the western Pacific Ocean develop almost
exclusively in a band between latitudes 6° and 35°, both north
and south of the equator. Those in the Northern Hemisphere
occur most frequently in the period from July to November.
Once developed, such a typhoon generally tracks
northwestwardly while it remains in the zone of the trade winds.
Thereafter the storms most commonly recurve in a
northeastward direction, generally picking up speed as they
enter the wind zone of prevailing westerlies. Typhoons are
observed most often in the general vicinity of the South China
Sea, but devastating storms have also frequently occurred in
the Bay of Bengal.
Continued from pg 1: Weather
The air movements resulting from these five weather-producing factors of
North America provide an exceptional variety of weather among regions,
including droughts, floods, and every known form of severe storm, including
hail, ice storms, and tornadoes. These extremes alternate with calm periods
of clouds or sunshine. Thunderstorms yield about half of the total
precipitation in most of the United States 80 percent in drier mountain
climates, 65 percent in the Great Plains, 50 percent in the Midwest, and 40
percent in the East.
The weather in most places is sensitive to a few key factors. For example,
severe drought in the sub-Saharan region of Africa is thought to occur when
onshore winds from the Atlantic Ocean change direction by at least 60° in a
relatively small area. A seasonal shift of this type is presumably related to
slight differences in ocean temperatures. Such differences in turn may have
resulted from changes in cloudiness related to a slight shift in hemispheric
pressure patterns.
tornado Also called
twisters, tornadoes are
storms characterized by
rapidly rotating columns of
air hanging from
cumulonimbus clouds.
weather Weather
includes all activities
5
Research Weather!
Now that you have idea about how weather happens and what causes
weather, let’s predict some weather. Think about the time of the year,
our location, and the most common types of weather we receive during
this time of year in our area. What natural disaster or extreme type of
weather could we expect?
In our news broadcast we need to include some weather! What kind of
weather could we include and what would happen if we had that type of
weather coming?
Look at
http://teacher.scholastic.com/researchtools/researchstarters/weather/
to research a little bit more about weather.
Think of a five
day forecast
when preparing
for extreme
weather. What
would it look
like?
Extreme Weather: weather that is not normal. It can either be super hot, super cold, super rainy, super snowy, super
windy. Most of the time it’s super scary!
Ms. Howard’s ESL Class
Eisenhower School