1.1 The Atmosphere: Energy Transfer and Properties
Atmospheric Pressure
The air around us is made of particles that are always moving. The air closest to the surface has
the most particles and the most collisions between those particles. As the distance from the
surface increases, the number of particles and number of collisions decreases.
The particles collide with the objects on the surface such as trees, land, animals and even our
skin (though we are not aware of it). The pressure (pushing force) that the particles exert on
the Earth’s surface is called the atmospheric pressure.
Air pressure is measured in units called pascals (Pa). Since pascals are very, very small, a more
convenient unit is used – kilopascals (kPa).
1 kPa = 1000 Pa
Atmospheric Pressure and Altitude
At sea level, the atmospheric pressure is about 101.3 kPa. This is the same as 1 kg of air
pushing each square centimeter of your body. We are not aware of this force because it is
counteracted by the pressure of fluids inside your body. We only become aware of the
atmospheric pressure when it changes like when we drive up a mountainside, ride in an
elevator or travel in an airplane. The popping of our ears is evidence of changing air pressure.
As we go up in altitude, there are fewer particles with more space between them resulting in
fewer collisions. This means a lower atmospheric pressure. In order to equalize the pressure of
the fluids in your middle ear they “pop” to balance these inside and outside pressures. We
become aware of the changing atmospheric pressure at this time.
Temperature and Atmospheric Pressure
Remember… molecules in the air are always moving. Heating up the particles will cause
them to move faster and spread out (less dense). Conversely, cooling down the molecules will
slow them down and push them closer together (more dense).
When heated air particles spread out, they have fewer collisions and exert a lower atmospheric
pressure.
Cooler air has less space between molecules. This results in more collisions and a higher
atmospheric pressure.
Weather involves the movement of air masses that have different temperatures, densities and
atmospheric pressures. When these different blocks of air meet and collide, it results in
changing weather conditions. Meteorology attempts to predict these changes before they
happen to forecast the weather expected.
Humidity and Atmospheric Pressure
Humidity is a measurement of how much water vapour is in the air. The more water vapour in
the air, the lighter the air is.
Dry air is made mostly of nitrogen (N2) and oxygen (O2) gases. These molecules are heavier
than water vapor. When more water vapour moves into the air, it pushes out an equal amount
of dry air molecules (N2, O2). This makes the air lighter. Less dense air exerts less atmospheric
pressure than denser air.
More humid air has more water vapour = less dense = smaller atmospheric pressure
Less humid air has less water vapour = more dense = larger atmospheric pressure
As the air temperature increases, air particles are moving faster and are more spread out. This
means that there are more spaces for water vapour to fit in the air. Warmer air will have more
water vapour and will be more humid than cooler air. Cooler air is more closely packed and had
less space for water vapour.
When warm, humid air begins to cool down, the molecules in the air move slower and pack
together. At a certain temperature, there is no longer enough room in the air spaces for water
vapour to remain in the air. The water vapour will condense and change into the liquid form of
water and stick to surfaces as dew. The temperature that this happens is called the dew point.
Examples: Oudoors: Autumn afternoon and evening
Fall days are quite warm. The air holds a fair amount of water vapour during the
afternoon as the warm air has lots of space for water vapour. In the evening, air
temperatures cool off and particles slow down and squeeze together. If the
temperature overnight is cool enough, there will not be enough space for water vapour
to remain in the air and some will be squeezed out and change into liquid water
droplets. These droplets stick to the surface of the grass, leaves, vehicles etc. as dew. If
the temperature of the night continues to drop below freezing, the dew will freeze to
become frost.
Indoors: When you take a hot shower in a cool room
When you turn on the shower, the temperature of the hot water begins to warm the air
in the bathroom by causing the air particles to move faster and spread out. This makes
room for the water vapor to fit in the spaces. When the water vapour has completely
filled the spaces in the air, the room has reached the dew point. At this time, there are
no more spaces to hold any more water vapor and water vapour begins to condense
into small water droplets and stick to surfaces in the bathroom like the ceiling, walls and
mirror. The mirror becomes fogged up. This will happen sooner in a cold room than in
the summer when the air is hot to begin with.