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10 Science
Weather - Chapter 13
Energy Transfers
Energy transfers play a large role in weather related phenomena. Four ways in which
thermal energy is transferred are:
 Radiation - electromagnetic radiation of which infrared is one type requires no
material medium (no matter) for its transfer. This is how energy gets from the
sun to Earth (with no matter between the two)
 Conduction - the particles of a solid vibrate. Consider a metal rod. When one
end of the metal rod is heated the particles vibrate more. The particles "collide"
with adjacent particles and the energy is transferred down the rod. This form of
energy transfer requires matter but there is no bulk movement of that matter.
Some substances (like metals) are better conductors of heat than others.
 Convection - Consider a sample of air. The particles of air move and when they
are heated they get further apart. This decreases the density of the sample of air.
Objects that are less dense float and so the hot (less dense) air rises. This is
called a convection current. Obviously, matter is required and there is bulk
movement of matter. The air has more energy and so if the air moves the energy
is transferred (up in this case)
 Advection - Air in some places has more pressure than in other places. Fluids
(liquids and gases) always flow away from areas of high pressure and toward
areas of low pressure. This accounts for wind. Advection is therefore very
much like convection except the movement is horizontal.
The energy that we get on Earth (that is responsible for weather phenomena) comes
from the sun in the form of radiation. Of all the energy that strikes the earth:




about 27 % is reflected right back by clouds and pollutants
about 3 % is reflected right back by the surface
about 20 % is absorbed by clouds, and
about 50 % is absorbed by the surface (land and water)
The surface of land is made up of different materials; concrete, sand, grass, dirt forest
etc. Different materials heat up at different rates when heated. They have different
"specific heat capacities"
Specific heat capacity is the amount of thermal energy (in Joules) required to increase
the temperature of 1.0 kg of a substance by 1.0 degree Celcius. As an example water
has a specific heat capacity of about 4000 and metals about 400. This means that it
takes 4000 Joules of energy to get 1 kg of water to heat up 1 degree Celsius where as it
only takes a metal about 400 Joules.
So if you exposed both 1 kg of water and 1 kg of a metal to 4000 Joules, the water's
temperature would rise 1 degree while the metal's temperature would rise 10 degrees.
In fact water has a particularly high specific heat capacity. Compare different
substances heat capacities on p. 506.
Now consider a beach setting where the sun heats up the sand and water at the same
rate. During daytime heating, the sand gets much hotter than the water. The air above
the hot sand heats up and rises (a convection current) and so air is pulled onshore from
the sea (an advection current). This is called a sea breeze.
Practice p. 507 # 1 - 8
Seasons and the Tilt of Earth
Despite the fact that the earth's orbit is not circular this has little impact on the seasons.
Seasons and changes in daylight hours are caused by the tilt of the earth's axis of
rotation (23.5 degrees). It is the tilt of the Earth that causes the sun's rays to strike the
Earth's surface at different angles throughout the year. See p. 508
North
Pole
sun's ray
In Winter
axis
axis
In Summer
axis
South
Pole
sun's ray
Winter Solstice - Dec 21st
Vernal Equinox - Mar 21st
- the shortest day of the year
- rays strike the equator at 90 degrees
- days and nights equal length everywhere
Summer Solstice - Jun 21st
- the longest day of the year
Autumnal Equinox - Sep 23rd - rays strike the equator at 90 degrees
- days and nights equal length everywhere
The Atmosphere
The blanket of air and moisture that surrounds Earth. It is most dense at the surface
and is about 100 - 500 km thick. At the surface, air is made up of about 78% nitrogen,
20 % Oxygen and about 2 % other gases including carbon dioxide. Scientists have
identified 4 layers of the atmosphere according to their characteristics.
 troposphere - 8 - 16 km thick
- where most weather takes place
- temperature decreases 6 degrees with each 1000m
 Stratosphere - up to 50 km (so about 38 km thick)
- highest concentrations of ozone
- dry layer of air that increases temp with increased alt
 Mesosphere - up to 80 km (so about 30 km thick)
- temperature and density of the atmosphere is very low
** the first three layers while different in temp and water content are similiar in
composition.
 Thermosphere
- up to 500 km (so over 400 km thick)
- very low density but fairly warm because the highest energy
radiation from the sun is absorbed in this layer
- also called the ionosphere because this same high energy
radiation charges particles causing the auroras
Why is the atmosphere considered life supporting?
 contains oxygen necessary for animals to survive
 contains carbon dioxide necessary for plants to survive
 protects Earth from meteorites
 with water it regulates global temperatures daily and seasonally
Pressure
Pressure is defined as the force exerted (by a fluid) per unit area. One explanation for
pressure is that fluids are made up of particles that are moving. The moving particles
collide with each other and with the particles of anything in the fluids' surroundings.
The moving particles thus exert forces on other objects and these forces act in all
directions.
Atmospheric Pressure
Another explanation of pressure (at least as far as the atmosphere is concerned) is that
air has weight. Consider a column of air with a cross sectional surface area of 1 square
meter and a height of about 100 km (the thickness of the atmosphere). Gravity pulls
this column of air toward the Earth's centre. The weight of this 1 m2 column of air is
about 22,000 pounds (or about 101,300 Newtons the metric unit for weight).
Everything on the surface of Earth is essentially at the bottom of a sea of air with this
22,000 pounds of air above (each square meter). Although air pressure changes,
normal or standard atmospheric pressure is:
101,300 N/m2
or
101,300 Pascals (Pa)
101.3 kPa (the fundamental unit)
760 mm of Hg
1 atmosphere (usually used for underwater applications)
14 p.s.i. (not a metric unit)
See figure 4 p. 512 for the relationship between altitude and pressure
Measuring Atmospheric Pressure
Devices used to measure pressure are called barometers i)
ii)
aneroid
mercury
Pressure and Wind
Among other things pressure is depicted on weather maps. Locations of equal pressure
are connected with lines called isobars. Where these isobars are close the pressure
changes greatly over a given distance and where they are further apart, the pressure
changes gradually over a given distance. The amount that the pressure changes over a
given distance is called the pressure gradient. Since pressure effects the strength and
direction of winds, meterologists can use weather maps to predict winds. For instance,
where isobars are closest the winds are generally strong etc.
Practice p. 513 # 2 - 9
Prevailing Winds
Prevailing winds i)
ii)
involve moving air on a very large scale
are consistent based on global temperature differences
Coriolis Effect - the apparent change in the direction of air due to the earth's rotation.
It causes winds to turn right in the northern hemisphere and left in the southern
hemisphere
polar convection
currents
polar easterlies
mid-latitude westerlies
northeast trade winds
equatorial
convection
currents
mid-latitude
convection
currents
southeast trade winds
mid-latitude westerlies
polar easterlies
polar convection
currents
To understand the effects of prevailing winds, know that generally rising air is warm
and moist while falling air is cool and dry. So generally, at the equator (where air is
moist and warm) conditions are unstable with cloud and precipitation....tropical rain
forests, monsoons etc. At 30 degrees latitude, air is dry creating desert like
conditions...the Sahara, the Gobi, the Mojave, etc. At 60 degrees again air is rising
creating unstable cloud and precipitation (but not as unstable as at the equator...why?)
etc.
Try p. 519 # 1 - 3
Clouds & Fog
With all methods mentioned below, warm moist air is made to rise. As the warm air
rises it cools. At a certain temperature (called the dew point), some of the water
vapour condenses and the now liquid water attach to particles in the atmosphere and a
cloud is formed. So the only thing that remains is how the warm air rises.
Convective Clouds - are clouds that arise from convection currents. Daytime heating
causes the Earth's surface to heat up and this heats up the surrounding air. The warm
air (being less dense) rises carrying any water vapour that it contains with it.
Frontal Clouds - Air masses are classified into two broad groups; warm and cold.
Warm air masses are warm, less dense and contain more water vapour. The boundary
between two air masses is called a front. When a warm air mass moves into a cold air
mass, the warm air moves over the cold air. When a cold air mass "hits" a warm air
mass, the cold air pushes under the warm air mass. Either way, warm moist air rises.
Orographic Clouds - Here the warm moist air rises because an air mass moves up a
mountain.
Clouds are classified according to three criteria:
 their shape
 their altitude
 their water content
Shape
- cumulus meaning billowing
- stratus meaning spread out
Altitude
- low level clouds have no descriptor (0 - 2 km)
- mid level clouds are given the prefix "alto" (2 - 5 km)
- high level clouds are given the term "cirrus" (5 - 8 km)
Water Content - a cloud that has a lot of rain water in it is a "nimbus" cloud
See examples figure 7 p. 533
try
p. 534 # 2,3 & 4
p. 540 # 2 - 8, 11, 15 & 17