Met2: The core foundational concepts (v1.0)

 Meteorology is from Greek which in Greek means study of things high in the sky. Meteorology is the science that deals with the phenomena of the atmosphere, especially weather and weather conditions. Everyone know what the atmosphere is. In this essay we will drill down more.  We will cover some of the core foundational concepts in meteorology, which are:

1. temperature. 
2. pressure. 
3. density. 
4. the composition and layers of the air in the atmosphere. 

Temperature is a measure of the kinetic energy of the vibrating and colliding atoms (even solids). It is measured by a thermometer. There are three key temperature measures - Centigrade, Fahrenheit and kelvin. Centigrade is the most widely used where water freezes at 0 degree and boils at 100 degrees. Fahrenheit is commonly used in the US. Kelvin is the absolute temperature scale where 0 degrees represent absolute zero where there is no molecular motion. 

Fahrenheit = 9/5 * Centigrade + 32. 

Centigrade = Kelvin - 273.15. 

 

The pressure of air is primarily caused by the gravitational force due to the weight of the air above (the part called hydrostatic pressure). It is measured by a barometer. Key measures for pressure are pascal, bar, pound per square inch, and mm of mercury. Sea level pressure is 1013.25 millibars. Gravitational force is proportional to mass. Pressure decreases exponentially with height. The difference in pressure at different heights is called a pressure gradient. How high is the atmosphere? The average height of 10 millibar pressure level (99% of atmosphere weight) is 18 miles but 50% of the atmosphere weight is below 3.5 miles (500 millibar pressure level). 

 

Because of air compressibility, density decreases with height. Temperature, pressure and density are related by the ideal gas law. 

The simplified ideal gas law is: pressure = density * Temperature * constant. The more precise law is PV=nRT where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. 

For example, Warm air is less dense than cold air for the same pressure. 

 

Fronts represent a meeting place for different air masses that have different densities. They push against each other in part because they resist mixing. We talk about cold fronts and warm fronts, but the real difference is density. Air masses will resist mixing even with a density difference as small as 1% (think of oil and vinegar). 

 

Nature wants to move mass from high to low pressure. Nature responds to pressure differences by trying to eliminate them. Pressure differences can exist in all directions. A key concept is that if a fluid is not accelerating in a direction, then the forces must be balanced in that direction (example gravitational force versus pressure gradient). This is called hydrostatic balance. The answer to the question "Why do we still have an atmosphere?" is hydrostatic balance.  The earth is in hydrostatic equilibrium where the gravitational force balances the force due to atmosphere pressure gradient with height and prevents significant vertical movements. The answer to the question "why do we have a thunderstorm?" is that hydrostatic balance can be broken. The atmosphere is under great strain due to the two opposing forces (just like a rubber band stretched from two ends with opposing forces). This can break resulting in an instability like thunderstorm. 

 

The standard atmosphere is defined by averaging (night/day, winter/summer, land/sea, poles/equator) and consists of four layers distinguished by how temperature varies with height. The troposphere is from ground level to 200 millibar level (about 7.5 miles level called the tropopause). The temperature decreases quickly with height (to -50 degree centigrade at tropopause). This is where weather phenomenon occurs including clouds, rain and storms. The stratosphere ranges from 200 millibar level to 1 millibar level (about 30 miles level called the stratopause). It is a region of great stability that impedes vertical motion. The ozone layer is here. Temperature increases with height primarily due to the ozone layer where heat is generated due to the absorption of UV light (to about -10 degree centigrade at stratopause).  The mesosphere ranges from the 1 millibar level to the .01 millibar level (about 55 miles level called the mesopause). The temperature resumes decreasing with height (to about -80 degree centigrade at mesopause). This is where meteors burn up as they enter the atmosphere. The thermosphere extends from 55 miles up to where the atmosphere just fades away. Temperature resumes increasing with height (but it is a strangely cold place despite the temperature which classically illustrates the difference between temperature and heat content). This layer is characterized by the presence of charged particles (ions) and is responsible for phenomena like the auroras. Remember that these layers are not sharply defined; they blend into each other gradually. Additionally, the temperature variations are influenced by factors such as solar radiation, composition, and altitude. Overall, the Earth’s atmosphere is a dynamic and essential part of our planet! 

 

If air is cold at the tropopause and warm at sea level why doesn't the cold air at the tropopause sink and the warm air at sea level rise thereby inverting the troposphere? This is incorrect because of a fallacy. The correct statement should be less dense air rises and more dense air sinks. This latter statement is one of the most important concepts.

 

What is air? There is dry air and water vapor. Dry air is largely fixed in quantity and well mixed throughout the atmosphere. Water vapor is extremely variable in horizontal axis, vertical axis and in time. 

 

Dry air is 78% nitrogen (removed by bacteria and soil, added by plants and animals), it is 21% oxygen (added by plant photosynthesis, removed by plants and animals and oxidation), it is 1% argon, and it is .0382% carbon dioxide. There are other traces. I will talk more about greenhouse effect in the next essay. Carbon dioxide is a greenhouse gas and plays a major role in regulating the temperature of the earth surface. The most important greenhouse gas though is water vapor. Other important greenhouse gases are methane (2 parts per million), Nitrous oxide (300 parts per billion), and ozone (40 parts per billion). Others are Nitrogen trifluoride, Sulphur hexafluoride, Hexafluoroethane, Chlorodifluoromethane, Dichlorodifluoromethane, and Tetrafluoromethane. All these greenhouse gases, though very low in concentration, play a big role in regulating earth surface temperature. 

 

Carbon dioxide concentrations are rising due in large part to fossil fuel burning. CO2 concentration oscillates during the year (highest in April/June, lowest in sept/oct). It is lowest in fall because of increased plant activity to remove it in summer. But there is a steady increase year to year in the average. Between 1960 and 2010, it increased by a little over 20% (measured at Mauna Loa, Hawaii). That means the removal from plants is not enough to compensate for the additions from human activity.  

 

Methane is added by 6 sources. Cattle/sheep/etc, swamps/landfills/abandoned oil wells/etc, insects like termites, biomass burning, oceans, and volcanic activity. It is 25 times more potent per unit weight as a greenhouse gas than CO2. Methane also has a season cycle like CO2 and is also steadily increasing. 

 

Nitrous oxide is 300 times more potent per unit weight than CO2 as a greenhouse gas. It is produced in the soil by bacteria and destroyed by sunlight mainly in the stratosphere. Its concentration has also been rising. 

 

Ozone in the lower troposphere level is detrimental for many reasons. It is produced by lightening for one. Most of the ozone though is largely concentrated in the stratosphere (where concentration is 250 times larger). Ozone is produced from oxygen with incoming solar radiation. It is destroyed by free radical catalysts (example Chlorene radical, bromine radical, hydroxyl radical and nitric oxide radical). Stratosphere ozone blocks most of the ultraviolet light from the sun which are biologically damaging so it plays a crucial role for life. The absorption of UV light generates heat in the ozone layer. Ozone therefore has a role in the temperature structure of the atmosphere also. 

 

The ozone hole (first detected in 1979) is over the southern pole. It has a very strong annual cycle that is most pronounced in October. The hole expansion was getting worse and caused primarily by CFC's (1 or 2 parts per 10 billion). Chlorofluorocarbons (CFC) which are spray can propellants are not only greenhouse molecules but destroys ozone in a cascading Pacman style chain fashion. CFCs are also used in air conditioners and fire extinguishers. I will not detail here how CFC emissions at the surface in the northern hemisphere makes its way to the stratosphere and ozone hole in the southern hemisphere. Today use of CFC's is outlawed in 197 countries (thanks to 1987 Montreal Protocol). Scientists concur the ozone layer is slowly recovering. 

 

Water vapor is literally the fuel of thunderstorms and hurricanes. It constitutes 0 to 4% of the atmosphere. Water vapor is extremely variable in horizontal axis, vertical axis and in time. It is concentrated near the earth's surface in the lower troposphere. 90% of the water vapor is due to evaporation while 10% is due to transpiration from plants. The ability of air to hold water vapor is a very strong function of temperature. Warm air can hold much more water vapor than cold air. Higher up in the atmosphere it gets much colder. As water holding air rises in the atmosphere, it cools, leading to condensation and cloud formation.