Download Understanding Weather and Climate

Understanding Weather and
Climate
The Atmosphere
(Cont.)
The Atmosphere
• Atmospheric pressure is produced by
molecules hitting a surface. Since more
molecules are found near the surface of the
Earth, the pressure is highest there. Higher up,
there are fewer and fewer molecules present,
and so pressure is less and less.
• As we change our altitude, or height, we
respond to the change in pressure. Our ears
seem to “pop” because our inner ear is adjusting
to the change in atmospheric pressure.
• Atmospheric pressure can be measured
by using a barometer.
The weight of the mercury in the tube is balanced by the force of the air on the mercury in the dish.
• The mercury will rise or fall depending on
the pressure. When a big storm is
approaching, the mercury will drop
significantly. After the storm has passed,
the mercury will rise again.
The Greenhouse Effect
• Our atmosphere is warmed in a way that it is
similar to the inside of a greenhouse. Sunlight
that reaches the surface of the Earth is
absorbed. The energy that is released back into
the atmosphere is then absorbed by water and
carbon dioxide in the atmosphere. The warmth is
“trapped” in the lower atmosphere like the
warmth “trapped” inside a greenhouse. This is
why our atmosphere is said to have the
greenhouse effect.
The Greenhouse Effect
(Cont.)
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Air that moves horizontally is called wind.
Air that rises is an updraft.
Air that sinks is a downdraft.
Unequal heating and cooling of air often
makes a pattern of rising air, sinking air,
and winds called a convection cell. A
convection cell is a part of the atmosphere
where air moves in a circular pattern
because of unequal heating and cooling.
Convection Systems
• The air around you is constantly moving. A
strong storm can make trees seem to
dance in the air. This movement is called
wind.
• Have you ever noticed that basements
seem colder than the rest of the house?
This is because cold air moves
downward. Warm air rises, so the
highest part of your house will always be
the warmest.
Convection Systems
(Cont.)
• Air expands as it is warmed. In other
words, the molecules scatter and become
active. Cooler air does the opposite. It
becomes denser and less active, therefore
losing energy. When cool air meets warm
air, the denser, cool air will flow beneath
the warm air, forcing the warm air upward.
• When the Earth absorbs the sun’s
radiation, some parts of the Earth get
hotter than others. Some places, like city
streets, absorb a lot of energy. You can
see the radiation coming up from the
street on a hot day. On the other hand,
other places do not warm up nearly as
much. Forests remain relatively cool when
compared to the city streets.
• As the cooler, denser air moves from an
area of high pressure to low pressure (the
forests to the streets), it pushes the warm,
less dense air upward. This process of
warmer air being pushed upward is called
convection.
Convection Cell
• When the warmer air rises and circles back
down on the cooler surface, a convection cell
has formed. This movement of air in a
convection cell creates wind.
One side of the room has a heater; on the
opposite wall is a window. On a cold winter day,
when the heat is on, air near the heater will warm
up. What happens to hot air? It expands,
becomes less dense, and rises. On reaching the
ceiling, it is pushed along by more hot air rising
behind it. The heated air starts to cool down the
farther it drifts from the heater, and this process is
speeded up when it meets the cold window. As
the air cools, it becomes more dense, sinks to the
floor, and eventually completes a circuit of the
room. A circular convection current is set up.
Circular currents like this are called convection
cells.
Daytime sea-breeze conditions
During the day, land gains heat
energy faster and becomes
warmer than the water offshore.
Because warm air is less
dense, it rises and triggers an
onshore flow of cooler marine
air to replace the rising warm
air—the flow is usually
strongest in the afternoon.
At night, inland areas cool (radiate heat energy) faster
than offshore waters. As a result, the cooler air over the
land subsides and flows offshore over the warmer water,
where the air is lifted. This night pattern reverses the
process that develop during the day.
Sea and Land Breezes
• A wind that blows from the sea toward the
land is called a sea breeze.
• A land breeze blows from land toward the
water.
Convection Cells Also Occur Along
Mountains
• As the sun shines on a mountain during
the day, the slope heats up faster than the
valley below. Air over the slope warms and
rises.
Daytime Valley Breeze
• Cooler air over the valley replaces the
rising warm air, creating a valley breeze
that blows up the slope.
Nighttime Mountain Breeze
• At night the mountain slope cools rapidly.
This causes a mountain breeze to blow
down the slope.
Developing Vocabulary
• Sea breeze – The name of the breeze
indicates the direction the wind is coming
from: the wind blows from the sea.
• Land breeze – The wind blows from the
land.
• Can you describe a situation when you
have experienced breezes from the land
or sea?
Remaining Classtime: Read pages D54 through D63.
THE
END!!!