January 23, 2008
Reading Surface Station Models
n
Use the
o Only need to know the fields shown here. Not the others shown in appendix B
o Go through each field’s link
§ For weather symbols, only need to know (Draw better symbols)
· Rain, snow, freezing rain, thunderstorm, fog
n Go through a reverse example, (Air Temp = 28° F, Dew Point = 23° F, Cloudy with moderate snow, Air pressure = 994.7 mb, northwest winds at 25 knots)
n
Take a
quick look at the
Reading Upper level
Station Models (Use 500 mb as examples)
n
Use
o
Only
need to know fields shown here. Not all
from appendix B.
o
Go
through each link
§
For geopotential height, point out the last digit is often
dropped.
n
Take a quick look at the upper air station
model (in review section)
Brief weather discussion for
Overview of Atmosphere, Composition, Vertical
Structure
1. What is an atmosphere? Importance of Earth’s rather thin atmosphere.
n Envelop of gases and aerosols that surround a planet. Very thin layer on Earth.
n Draw thinness diagram
n Why are atmospheres on the outside of planets?
n Provide large-scale overview of atmospheric motions. No need to try to write any of this material down during lecture
n Life-essential functions of the atmosphere
o Allows breathing (oxygen)
o Shields surface inhabitants from potentially harmful uv radiation
o Re-distributes heat and moisture
o Keeps the average surface temperature (air temperature in the lower atmosphere) much warmer than if no atmosphere through the atmospheric greenhouse effect
n Some non life-essential functions of the atmosphere
o Propagation of sound
o Sight – blue skies, white clouds, sunsets, rainbows
o Sense of smell influenced by atmosphere
2. What is a gas?
n Substance in which individual molecules are not chemically bonded together. This differs from liquids and solids.
n Gases, like liquids, are fluids, i.e., they can flow. Gases will fill in spaces around solid objects.
3. Composition of the Atmosphere (show table on the lecture page)
n Major Components
o Nitrogen (N2) and Oxygen (O2) make up ~99% of “dry air” (not including water vapor)
n Important trace (or minor) constituents
o Water vapor (H2O)
§ Concentration varies greatly from as much as 4% (near warm tropical oceans and rainforests) to near 0% (cold polar regions); average concentration is about 0.8%
§ “makes” clouds and rainfall
§ Most important greenhouse gas on Earth
§ Large quantities of energy involved in phase changes
· Evaporation (liquid à gas) energy removed from environment
· Condensation (gas à liquid) energy released to environment
o Carbon dioxide (CO2)
§ 2nd most important greenhouse gas on Earth
§ Concentration has been increasing due to human activity (fossil fuel burning and deforestation). Concern that this will strengthen the greenhouse effect on Earth, possibly causing global warming
o Methane (CH4), Nitrous Oxide (N2O), and a few others
§ Also greenhouse gases that are known to be increasing due to human activities.
o Ozone (O3)
§ Most found in the stratosphere (Ozone layer), where it absorbs harmful ultraviolet radiation (shielding the Earth’s surface)
§ Very little found naturally near ground where it is a toxic pollutant. But sometimes dangerously high levels can develop near large cities (if so, ozone alerts may be issued) in a process known as photochemical smog.
4. Vertical Structure of the atmosphere (Density)
n Density is most often described as the mass of material divided by volume (or weight/volume), but for gases, it is easier to think in terms of number density, which is the number of gas molecules per volume
o Near sea level ~ 2.7x1019 molecules/cm3 (~4.4x1020 molecules/inch3)
n Gases, like air, are easily compressed, i.e., squeeze it together and its number density increases
n Draw a schematic representation of a column of air. Considering that air is compressible, make the conclusion: Air near the Earth’s surface is compressed by the weight of the air above becoming more dense. Moving upward, air density generally decreases because there is less and less weight pushing down from above.
o Show density figure from the lecture summary page.
o This is what it means to say that air gets “thinner” as you move to higher altitudes
5. Vertical Structure of the atmosphere (Pressure)
n At the micro-scale, gas pressure is caused by the sum total of all collisions of gas molecules on a surface. The physical units of pressure are force per area. Each time a gas molecule collides with a surface, it exerts a little push or force on the object. Summing all these individual tiny forces over an area causes gas pressure
n Let’s again think of a column of air. The air in the column has mass and is held in place above the surface of the Earth by the force of gravity. Thus, air in the column has weight. Since weight is a force, air pressure is weight per area of the air in the column.
n Draw figure of a 1 square in column of air, with annotations
n Air pressure at any point within the atmosphere is caused by the weight of air pushing down from above. Therefore, air pressure MUST decrease as you move upward, since there is less weight pushing down from above
n Since air is a fluid, at any point in the atmosphere, the pressure is equal in all directions (down, sideways, up)
o Make an analogy with a more tangible fluid, liquid water. Use a diagram to show that as you dive downward into water, the water pressure increases because the weight of fluid above (the water) increases.
n I would also like you to understand something about the rate at which air pressure decreases as you move upward from the ground surface. Air pressure decreases very rapidly at first, since most of the weight of the atmosphere is compressed near the ground surface. As you move upward, the air density becomes lower due to the compression effect, and the rate at which air pressure decreases as you move upward gets smaller.
n Show pressure figure from lecture summary page. Explain rate of pressure decrease in terms of the compression effect.
o 50% of all weight of atmosphere below 500 mb (lowest 5-6 km above sea level)
o 75% of all weight of atmosphere below 250 mb (the troposphere)
o 99.9% below 1 mb (~50 km above sea level: the troposphere and stratosphere)
n In fact, if you know the air pressure at your location, you can quickly estimate the fraction of the mass (or weight) of the atmosphere above or below you.
n
Classroom
demonstration for air pressure
6. Vertical Structure of the Atmosphere (Temperature)
n The atmosphere is commonly divided into vertical layers based on the change in air temperature with altitude
n In this class we are only going to discuss the lowest two layers, the troposphere and the stratosphere. Show Temperature Layers figure from lecture summary page.
n Troposphere
o Lowest layer, extending from the ground surface up to about 11 km above the surface. 11 km is an average … the top of the troposphere ranges with time and location, but is generally found between 6 and 18 km. Usually the warmer the air, the higher the top of the troposphere (tropics, summer season outside tropics)
o Characterized by a rapid decrease in air temperature with increasing altitude.
§ Average rate of decrease is 6.5° C per km (3.6° F per 1000 feet).
§ Reason air temperature generally decreases as one moves upward is that the air in the troposphere is mostly heated from below by the underlying ground surface, which is mainly heated by absorbing radiation from Sun.
§ Not that uncommon to have temperature inversions, which are regions in the troposphere where air temperature actually increases with increasing altitude. In fact temperature inversions are common near the ground during the late night/early morning. Draw diagram of a surface temperature inversion. Inversions that occur above the Earth’s surface can be important in severe weather formation.
o Roughly 75% of the total mass of the atmosphere is contained within the troposphere. Why?
o Essentially all weather and all clouds are found in the troposphere
n
The boundary between the troposphere and the
stratosphere is called the tropopause
region. In this region air
temperature stops decreasing with altitude.
n Stratosphere
o Layer above the troposphere, on average extending from 11 km to 48 km.
o Moving upward from 11 km, air temperature typically remains nearly constant up to about 20 km. From 20 km up to the top of the stratosphere, the air temperature increases.
§ Reason for increase in temperature is the absorption of ultraviolet radiation by Ozone, which shields (or protects) surface inhabitants of the Earth from exposure to this potentially harmful radiation.
· The layer of highest ozone concentration is found between about 20 and 30 km altitude. This is referred to as the Ozone layer. Note that even in this layer of higher ozone concentration, ozone is still a trace or minor component of the atmospheric gases.