Sometimes when you leave school in the afternoon, the
weather is different from what it was earlier in the morning.
Weather constantly changes.
What are air masses?
An air mass is a large body of air that has the same
temperature and moisture content as the area over which it
formed. For example, an air mass that develops over land is
drier than one that develops over water. An air mass that
develops in the tropics is warmer than one that develops
over northern regions. When weather changes from one day
to the next, it is because of the movement of air masses.
How does air pressure affect the weather?
Pressure in the atmosphere varies over Earth’s surface. You
may have heard a weather forecaster talk about high- and
low-pressure systems. Low-pressure systems are masses of
rising air. When air rises and cools, clouds form. That’s why
areas of low pressure usually have cloudy weather. But
high-pressure air masses have a sinking motion. As a result,
it’s hard for air to rise and for clouds to form. So, high
pressure usually means nice weather.
MARGIN NOTE!!!! Warm air causes low pressure and Cold air causes high pressure.
What are cyclones and anticyclones?
Winds blow from areas of high pressure to areas of low
pressure. In the northern hemisphere, when wind blows into
a low-pressure area, Earth’s rotation causes the wind to swirl
in a counterclockwise direction. These large, swirling areas
of low pressure are called cyclones. Cyclones are associated
with stormy weather.
Winds blow away from an area of high pressure. In the
northern hemisphere, Earth’s rotation causes these winds to
swirl in a clockwise direction. High-pressure areas are
associated with fair weather and are called anticyclones.
A boundary between two air masses that have different
temperature, density, or moisture is called a front. There are
four main types of fronts, including cold, warm, occluded,
What is a cold front?
A cold front occurs when cold air moves toward warm air,
as shown on the left in the figure below. The cold air goes
under the warm air and lifts it. As the warm air is lifted, it
cools and water vapor condenses, forming clouds. If there is
a large difference in temperature between the cold air and
the warm air, thunderstorms and tornadoes may form.
What is a warm front?
Warm fronts form when lighter, warmer air moves over
heavier, colder air, as shown on the right in the figure
below. In a warm front, wet weather may last for days.
What is an occluded front?
Most fronts involve two air masses. But occluded fronts
involve three air masses—cold air, cool air, and warm air. An
occluded front, as shown in the figure below, may form when a
cold air mass moves toward cool air with warm air in between.
The cold air forces the warm air up. The warm air is then
closed off from the surface. The term occlusion means “closure.”
What is a stationary front?
A stationary front occurs when a boundary between air
masses stops moving, as shown in the figure below. Stationary
fronts can stay in the same place for several days. Often
there is light wind and precipitation at the stationary front.
You usually can do your daily activities regardless of the
weather. However, some weather conditions, like blizzards,
tornadoes, and hurricanes, can force you to change your plans.
What causes thunderstorms?
During thunderstorms, heavy rain falls, lightning flashes,
and thunder rumbles. Hail might fall. What causes these
Thunderstorms occur in warm, moist air masses and
along fronts. Warm, moist air is forced up. It cools and
condensation begins, forming cumulonimbus clouds. When
rising air cools, water vapor condenses into water droplets
or ice crystals. Smaller droplets collide and form larger
ones. The larger, heavier droplets fall through the cloud
toward Earth’s surface. The falling droplets collide with
more droplets and get bigger. Raindrops cool the air around
them. The cool, dense air sinks. Sinking, rain-cooled air
and strong updrafts of warmer air cause the strong winds
that often come during thunderstorms. Hail may form as
ice crystals fall.
What damage do thunderstorms cause?
Sometimes thunderstorms stall in one area, causing heavy
rains. When streams can no longer hold all the water
running into them, flash floods occur. Because they occur
with little warning, flash floods are dangerous.
Thunderstorms often bring strong winds that can cause
damage. If a thunderstorm has winds over 89 km/h, it is
called a severe thunderstorm. Hail from thunderstorms can
dent cars, break windows, and flatten crops.
What causes lightning?
Inside a storm cloud, warm air is lifted rapidly as cooler
air sinks. This movement of air can cause different parts of
a cloud to have opposite charges. When an electrical current
runs between areas with opposite charges, lightning flashes.
Lightning can occur between two clouds, inside one cloud,
or between a cloud and the ground.
What causes thunder?
Thunder comes from the rapid heating of air around a
bolt of lightning. Lightning can reach temperatures of about
30,000° C. That’s five times hotter than the surface of the
Sun. This heat causes air around the lightning to expand
rapidly. Then the air cools quickly and shrinks. Because of
the sudden expanding and shrinking, molecules in the air
move more rapidly. The rapid movement of molecules
creates sound waves. Thunder is the sound waves you hear.
What are tornadoes?
Some severe thunderstorms produce tornadoes. A tornado
is a violently rotating column of air that touches the
ground. Severe thunderstorms produce wind at different
heights which blow at different speeds and in different
directions. This difference in wind speed and direction is
called wind shear. Wind shear creates a rotating column
parallel to the ground. Updrafts in a thunderstorm can tilt
the rotating column upward, creating a funnel cloud. If the
funnel cloud touches the ground, it is called a tornado.
The figure below shows a diagram of a tornado. Notice
the different levels of winds and the rotating updraft. The strong updraft usually forms at the base of a type of cumulonimbus
cloud called a wall cloud.
How much damage can a tornado do?
Winds from tornadoes can rip apart buildings and tear
trees from the ground. If the winds of a tornado blow
through a house, they can lift off the roof and blow out the
walls. It can look as though the building exploded. In the
center of a tornado is a powerful updraft. The updraft can
lift animals, cars, and even houses into the air. Tornados do
not last long, but they are very destructive. In May of 1999,
thunderstorms produced more than 70 tornadoes in Kansas,
Oklahoma, and Texas. These tornadoes caused 40 deaths,
100 injuries, and more than $1.2 billion in damage.
How are tornadoes ranked?
As you have read, winds from tornadoes can cause severe
damage. Theodore Fujita, a tornado expert, created a scale
to describe and rank tornadoes. The scale, named the Fujita
Scale after him, is shown below. The Fujita Scale ranks
tornadoes based on how much damage they cause.
Tornadoes range from F0 which cause only light damage to
F5 which cause incredible damage. Luckily, only about one
percent of all tornadoes are in the category of F4 and F5.
What is a hurricane?
The most powerful storm is a hurricane. A hurricane is a
large, low-pressure system that forms over the warm Atlantic
Ocean and has winds of at least 119 km/h. It is like a
machine that turns heat energy from the ocean into wind.
Similar storms are called typhoons in the Pacific Ocean and
cyclones in the Indian Ocean.
Hurricanes are similar to low-pressure systems over
land—only stronger. In the Atlantic and Pacific Oceans,
low-pressure systems sometimes develop near the equator.
In the northern hemisphere, winds around this low pressure
rotate counterclockwise. As the storms move across the
ocean, they gain strength from the heat and moisture of
warm ocean water.
What happens when a hurricane reaches land?
Hurricanes can strike land with great force. The high
winds sometimes produce tornadoes. Heavy rains and high
waves cause large amounts of damage. Sometimes floods
follow the heavy rains and cause additional damage.
Hurricanes can destroy crops, tear down buildings, and kill
humans and animals.
What happens to the hurricane on land?
As long as the hurricane remains over water, it gets
energy from the warm moist air rising from the ocean. In
the figure below, small rising arrows show the movement of
warm air from the water below. Cool air goes down through
the eye, or center, of the hurricane. The storm needs this
energy from the ocean water. When a hurricane reaches
land, it loses its energy supply and the storm loses its power.
What is a blizzard?
Severe storms also can occur in the winter. If you live in
the northern United States, you may have experienced the
howling wind and blowing snow of a blizzard. A blizzard is
a winter storm with conditions that include very cold
temperatures, high winds, and blowing snow that makes it
difficult to see. A blizzard usually lasts at least three hours.
How can you stay safe during severe storms?
When severe weather approaches, the National Weather
Service issues a watch or a warning. A watch tells you that
even though the weather isn’t dangerous yet, it may become
dangerous soon. During a watch, stay tuned to a radio or
television station that is reporting the weather.
When a warning is given, the weather is already severe.
During a severe thunderstorm or tornado warning, go to a
basement or to a room in the middle of the house away
from windows. When a hurricane or flood watch is given,
be prepared to leave home. During a blizzard, stay indoors.