Chapter 14 – The Atmosphere The modern atmosphere Two most abundant gases: 78% N 2 21% O 2 Less abundant gases (< 1%) Argon Water vapor CO 2 (only about .035%) Non-gaseous components water droplets dust, pollen, soot and other particulates Fig. 17.6, p.437
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H2O in air exists in 2 forms: vapor (invisible) and liquid e.g. rain, fog, clouds.
Saturation: Maximum quantity H2O vapor that air can hold. Additional H2O condenses into liquid water.
Saturation increases with temperature (hot air can hold more water vapor than cold air.
Conditions left of curve cause water vapor to condense.
Relative humidity (RH)- ratio of actual amount H2O vapor in air to the amount at saturation.
Often, as the days warms up, the air, more water will evaporate into the air (actual amount), but since the temperature has increased, so has the amount of water needed for saturation
Therefore, the RH stays about the same.
What happens if the air now cools down?
Actual amount of water vapor in the air stays the same but amount needed for saturation decreases.
Therefore the RH increases.
When the relative humidity reaches 100%, the air is saturated, regardless of actual amount of water vapor present.
At low temperatures, 100% RH means less water vapor than at higher temperatures.
Dew point temperature – temperature at which RH = 100 %.
If saturated air cools below the dew point temperature, water vapor will condense into liquid.
At the dew point temperature, clouds, fog rain or dew appear.
The ozone layer in the stratosphere is a good thing because it protects life on Earth from harmful UV rays
The Ozone in the troposphere is a bad thing because it damages hearts and lungs.
Ozone – O3 forms in the stratosphere and absorbs UV energy, which can be harmful
Halons and Chlorofluorocarbons (CFCs), – are organic compounds that destroy ozone in the ozone layer
Ozone hole – reported in 1985 and linked to CFCs in Antarctic ice clouds
Many nations of the world agreed to reduce or stop use of these CFCs and halons
Most industrial countries no longer produce CFCs.
Since banning CFCs, the hole may be decreasing
Clouds – made of water droplets and/or ice particles (not water vapor, which is invisible)
Cloud particles grow around a tiny solid surface; these are called condensation nuclei (singular nucleus).
Examples include dust, smoke, pollen, sea salt pollutant particulates.
clouds form when the air is saturated (100% relative humidity) and H20 vapor condenses.
air must cooled to dew point for saturation to occur.
Large scale cooling occurs when an air mass is lifted to a higher level in the atmosphere, due to the Adiabatic Principle.
When a gas is compressed, it’s temperature increases.
The adiabatic process refers to a temperature change caused by a change in pressure.
The Adiabatic Process
Adiabatic (expansional) cooling – occurs when rising air expands due to decrease in air pressure.
Adiabatic (compressional) warming - sinking air compresses, due to increase in air pressure.
For clouds to form by adiabatic cooling, the air mass must rise to where the air pressure is less; then it can expand
The Adiabatic Process
Warm air is less dense (lighter) than cool air.
Water vapor is less dense (lighter) than dry air.
Air near earth’s surface warms, picks up water vapor from near the surface of the Earth, then rises.
Adiabatic cooling lowers temperature to dew point, which causes cloud formation.
Air may be “helped” to rise by various means.
Adiabatic Lapse Rates rise to where the air pressure is less; then it can expand
Dry: 10°C/1000m Wet: approx 5°C/ 1000m
<--- lifting condensation level
Cloud families – based on height
Cloud classes - based on shape
Formed from ice crystals, not liquid water drops.
Strong winds at altitude blow them into wisps (“mare’s tails”).
No rain (or snow) comes directly from them, but they are often an indication of unsettled weather in the near future.
Altocumulus – Middle-layer Clouds rise to where the air pressure is less; then it can expand“Mackerel Sky”
Horizontally layered, sheet-like clouds.
Form in relatively stable conditions when air stops rising as soon as condensation occurs.
Causes a gray overcast sky that may persist for days.
Nimbostratus clouds bring steady rain or snowfall (nimbo = rain).
Fluffy, white clouds with flat bottoms and billowy tops that may rise as much as 4 miles up in the air.
Form from relatively unstable air which continues to rise after condensation takes place. Flat base of cloud is where condensation started.
Rain or snow comes in the form of brief, intense showers.
Fair weather cumulus
Lenticular clouds form as moist air flows up and over a mountain peak. They are classified as altocumulus. Lenticular clouds indicate high winds aloft.
Fog is a cloud that forms at or very close to the ground. The picture shows an example of radiation fog, from air near the earth’s surface which cooled by radiation during the night. Fog usually disappears during the day, when the sun warms the earth and it begins to emit heat.
The marine layer is an example of an advection fog: warm air over the ocean takes in water layer, but as prevailing west winds blow it over the land, the water vapor condenses
Precipitation mountain peak. They are classified as altocumulus. Lenticular clouds indicate high winds aloft.
Prevailing winds push air up against, and then over, a mountain range
This process is responsible for our deserts in eastern San Diego County
Orographic precipitation occurs on west side as air is lifted over mountains.
Great Central Valley has less rain than coastal regions.
Death Valley and Mojave Deserts are rainshadow deserts.
When moving warm and cold air masses meet (a “front”), more dense cold air forces the less dense warm air to rise.
This happens when a storm (cyclone) passes through
Unstable air conditions caused by:
This graph shows how the barometric pressure (x axis – in millibars) decreases as the elevation increases and the number of gas molecules above gets less and less.