Chapter 14 Notes

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10 Science
Chapter 14 Notes
Meteorology - the study of the atmosphere and weather forecasting
Visit the Weather Network website - this can be linked to from the Balfour website or
http://www.theweathernetwork.com/
Bernoulli's Principle - the speed of a fluid is inversely proportional to it's pressure (see
Activity 14.1 for some examples
Weather Systems
A weather system is a set of temperature, pressure, wind and humidity that moves as a
unit (in North America largely from West to East because of the mid-latitude westerlies).
Weather systems are based on air masses.
Air masses are large bodies of air with relatively uniform temperature and moisture.
They assume there temperature and moisture properties from the region that they
originate. There are four main air masses in North America.
Maritime Polar - cool, moist
Continental Polar - cool, dry
Maritime Tropical - warm, moist
Continental Tropical - warm, dry
Generally speaking poor weather is associated with low pressure systems. Air typically
flows away from high pressure areas and toward low pressure areas. If two different air
masses move into a low pressure system they do not mix well and so the warmer (moist)
air mass rises (rapidly) above the cooler air mass (causing thunder storm clouds or even
tornados). As air rises at an area of low pressure, it tends to rotate counter-clockwise.
This is sometimes referred to as a cyclone.
In areas of high pressure, everything is reversed. Descending air typically rotates
clockwise and brings clear skies. This is called an anti-cyclone.
On weather maps, front are drawn to show the leading edge of air masses. They can be
viewed as the boundaries between different air masses. Warm fronts are drawn as red
lines with semi-circles and cold fronts are blue lines with triangles. Stationary fronts do
not move much and are indicated with blue triangles on one side and red semi-circles on
the other side.
Try p. 549 # 1 - 4 & 6
Try the case study 14.3 p. 550 Questions b,d,e,f,i & j
Try p. 552 # 1-3 & 4a
Regional Weather
Thermals - a small scale, local convection current
Sea Breezes - water has a very high specific heat capacity. When coastlines heat up
during the day, the land gets hotter than the water and so the air above the land heats up.
This creates low pressure over the shore and air from above the water (which is cooler)
will move toward the land.
Land Breezes - At night, the air above land cools more and the air above the water cools
less. The air above water is relatively warm and so rises, creating low pressure etc....the
result is a breeze that flows out to sea at night.
Lake-Effect Snow - When an air mass settles over a large body of water it gets warm and
moist. It gets warm because water is warmer than the surrounding land (again because of
heat capacity differences). This warm moist air rises and when the air mass moves over
the land it cools and snow results.
Chinook Winds - In the mid-latitudes, air generally moves west to east. When moist air
from the pacific coast moves eastward it encounters mountains and rises. As it rises, it
cools eventually to the dew point (the temperature at which air is saturated) and
condenses. Precipitation occurs and so the rising air is now dry and condensation is a
warming process. So the air is now warm and dry. It continues to move eastward and the
result is a warm dry wind in say Calgary in winter.
Humidity
Humidity - is a measure of the concentration of water vapour in the air (measured in g per
kg of dry air...g/kg)
Relative Humidity - is the measure of the maximum concentration of water vapour in the
air at a given temperature.
rel.hum 
concentration
x100 %
max .concentration
The maximum amount of water vapour that air can hold depends on the temperature. The
warmer the air, the more water vapour it can hold. As more and more water evaporates,
the concentration of water vapour in the air increases and approaches the maximum
concentration that it can hold. When this happens we say the air is "saturated". Once the
air is saturated no more evaporation can take place and in fact the water vapour starts to
condense; clouds/fog/precipitation result.
Much of the interesting weather that we've discussed involves warm moist air rising. As
air rises, the temperature decreases (at a rate of roughly 6 degrees per 1000m). If relative
humidity decreases with decreased temperature, there will come a point where rising air
becomes saturated. This is called the dew point.
Dew Point - is the temperature that you have to decrease air to in order for it to become
saturated.
This explains why dew forms (particularily on clear nights). Warm moist air comes in
contact with cool objects. The cool objects lower the temperature of the warm air (at
least that portion of the air in contact with the cool object) to the dew point . The water
vapour in the now saturated air condenses into water droplets (dew).
p. 561 # 1 - 7
Heat and Changing State
Temperature is a measure of the average kinetic energy of the particles of a substance.
When evaporation occurs, those particles near the surface and with enough energy,
escape the attractive forces that hold them together in the liquid state. Firstly this
explains why a liquid evaporates more readily when there is more surface area.
Secondly, this suggests that if a liquid evaporates its temperature will decrease. Afterall,
if those particles with more than average energy leave than the average energy will
decrease (and thus the temperature will decrease).
In summary and more generally, evaporation and melting are cooling processes.
Freezing and condensation are warming processes. This explains:
 why humans have evolved to sweat
 why dogs pant
 why Vancouver has more moderate temperatures than Regina
 why it's a little warmer when it's snowing
 chinook winds
 why liquids have boiling points/freezing points
 why freon evaporates (and than condenses) in re-fridgerators etc. etc.
p. 576 # 1-5,7 & 10
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