Atmospheric Stability and Lifting Mechanisms
I.	To understand cloud formation, we must understand the natural processes that can cause air to cool down to the dew point, forcing water to condense. A cloud is a collection of billions of tiny, suspended liquid water droplets or ice crystals. Why are some days cloudy while other days are clear?
II.	The key to understanding cloud formation begins with adiabatic processes. Adiabatic processes describe how the temperature of an air parcel changes as it rises or falls in the atmosphere.
	A.	Basic facts:
		1.	An air parcel is an imaginary bubble of air, about the size of this room. Although it could potentially exchange energy with its surroundings, adiabatic processes occur relatively quickly, so that these energy exchanges can be ignored.
		2.	Like air pressure, the density of air is greatest at the earth's surface (thanks to the confining force of gravity). As you rise away from the surface, air density decreases (gas molecules are farther apart).
		3.	When an air parcel rises or falls, its density adjusts to that in the surrounding air.
			a.	Upward movement --> expansion of air parcel
			b.	Downward movement --> compression of air parcel
		4.	First Law of Thermodynamics: Q = T + W
			a.	When you add heat to an air parcel, the energy can be used to do two things:
			b.	Increase the internal energy of the air parcel. Molecules will increase random motion, that it, increase temperature of the parcel
			c.	Do work on the surrounding environment. The air parcel will expand in volume.
		5.	By definition, an adiabatic process involves no heat exchange. Q = 0.
			a.	If an air parcel rises, it expands (to adjust its density). This means W is +. If Q = 0, then T must be -. Air parcel cools.
			b.	If an air parcel subsides, it is compressed (to adjust its density). This means W is -. If Q = 0, then T must be +. Air parcel warms.
		6.	Rising air always expands and cools adiabatically. Subsiding air always is compressed and warmed adiabatically.
	B.	If air parcel is unsaturated, the rate of cooling / warming is constant:
		1.	Dry Adiabatic Rate (DAR) = 1.0°C / 100m.
		2.	If an unsaturated air parcel rises 500 m, it cools by 5°C.
		3.	If an unsaturated air parcel subsides 1000 m, it warms by 10°C
	C.	If air parcel is saturated, the rate of adiabatic cooling / warming changes. If saturated air is lifted, it will still expand and cool adiabatically. However, as it cools, its saturation vapor pressure decreases, forcing water vapor to be condensed into liquid water droplets (cloud forms). Recall that condensation releases latent heat from the water molecule to its surroundings. Therefore, the air parcel is cooled adiabatically, but this is partially off-set by the release of latent heat. (Subsiding air argument is the reverse: warming, evaporation, absorbs latent heat, partially off-sets warming)
		1.	Moist Adiabatic Rate (MAR) varies from 0.4 - 1.0°C / 100m, depending on water vapor content of air (more vapor, more latent heat exchange!). As an average, we will assume 0.6°C / 100m.
		2.	If a saturated air parcel rises 500 m, it cools by 3°C.
		3.	If a saturated air parcel subsides 1000 m, it warms by 6°C.
III.	Now that we know how air parcels change temperature as they rise or subside, we are closer to understanding why some days are cloudy and some are clear. The next step is to learn how to determine atmospheric stability.
	A.	stability -- measures the tendency for vertical motion in the atmosphere
		1.	A stable atmosphere resists vertical motion. If a parcel starts to rise, it sinks back down. Lack of vertical motion keeps clouds from forming. Stable = clear skies.
		2.	An unstable atmosphere promotes vertical motion. If a parcel starts to rise, it continues to rise. Rising air cools to the dew point, promoting cloud formation. Unstable = cloudy skies, possible storm.
	B.	To determine stability, we compare the temperature (density) of an air parcel to that of the surrounding atmosphere.
		1.	The temperature of the air parcel changes adiabatically. The DAR and MAR are physical constants.
		2.	The temperature of the surrounding atmosphere varies over space and with time. We must take vertical soundings of the atmosphere to measure the environmental lapse rate (ELR). The ELR is the real world pattern of temperature change with height, so it can vary a lot. Recall that the mean ELR in the troposphere is around 6.5°C / km.
		3.	Recall the First Law of Thermodynamics.
			a.	If we add heat to an air parcel, we increase its temperature and it expands (its density decreases).
			b.	Conversely, if we remove heat, an air parcel decreases in temperature and shrinks in volume (its density increases).
			c.	This is a fancy way of saying that warm air rises and cold air subsides. We know from a house. In the summer, the attic is blazing hot, while the basement remains cool.
		4.	Applying this principle to determining stability:
			a.	If, after an air parcel rises and cools adiabatically, it is warmer than the surrounding atmosphere, it will continue to rise. (Like a buoyant cork in a bathtub.)
			b.	If, after an air parcel rises and cools adiabatically, it is cooler than the surrounding atmosphere, it will subside back to its original level.
	C.	There are three possible stability outcomes in the atmosphere, and they are defined by the ELR, by how temperature changes with height (see overhead):
		1.	If ELR > DAR (1°C/100m), temperatures cool very rapidly with height. Rising air parcels, whether unsaturated or saturated, won't cool as fast. They will be warmer than the surrounding atmosphere, and continue to rise. This outcome is called absolute instability.
		2.	If ELR > MAR (0.6°C/100m), temperatures do not cool very quickly with height. [They may even warm with height, in a temperature inversion.] Rising air parcels, whether unsaturated or saturated, will cool faster than the surrounding air. Since they become colder, more dense, they will not continue to rise. This outcome is called absolute stability.
		3.	If the ELR is somewhere between the DAR and MAR, then the stability outcome depends on whether the air parcel is saturated or unsaturated. This is called conditional instability.
			a.	If air parcel is unsaturated, it cools at the DAR as it rises (1°C/100m), which means it gets colder than the surrounding air and sinks back down.
			b.	If air parcel is saturated, it cools at the MAR as it rises (0.6°C/100m), which means it gets warmer than the surrounding air and continues to rise. This occurs because condensation, which releases latent heat into the air parcel, making it more buoyant.
			c.	This reminds us of the enormous importance of water in the atmosphere. It makes air parcels more buoyant, promoting cloudiness and storminess.
IV.	Now that we understand stability, the final pieces of the puzzle for understanding cloud formation are:
	A.	Clouds form when rising air is chilled to the dew point. The cloud base forms at this point in the ascent of an air parcel. This is called the lifting condensation level. Below the cloud, air parcels are unsaturated and follow the DAR. Within the cloud, air parcels are saturated and follow the MAR.
	B.	There are four natural lifting mechanisms in our atmosphere:
		1.	Orographic lifting: air forced up the side of a mountain or other terrain barrier.
				This can have very important climatic consequences. Compare the windward vs. leeward side of a major mountain range.
				a.	windward side: relatively moist air rises, reaches dew point, clouds form. Once above the cloud base, air parcels cool more slowly (MAR). Often foggy, rainy on windward slopes.
				b.	leeward side: very little water vapor remains in the cold air at the mountain top. As it subsides, it warms at the DAR. This allows the leeward side to experience much warmer and drier conditions. This is called a rainshadow effect. The descending air forms a warming wind called a chinook wind.
		2.	Dynamic convergence. Winds converging from opposite directions, get squeezed together and must rise. E.g., Gulf and Atlantic breezes converging over the Florida peninsula. Afternoon clouds and thunderstorms common, especially in summer.
		3.	Thermal convection. The accumulating net radiation surplus at the surface during the course of a day will promote instability. By warming the surface air, it steepens the ELR and permits air parcels to rise upward. Afternoon heating often triggers thermal convection in a Georgia summer, especially when the atmosphere starts out with high humidity (lots of water vapor) and a conditionally unstable ELR. Thermal convection causes most of our summer thundershower activity, which tends to occur in the afternoon and evening hours, with the accumulation of surface heat.
		4.	Frontal wedging. When warm and cold air collide at the surface, we call it a front, or frontal zone. Since the warmer air is less dense, it rises over the cold air. The rising warm air is capable of holding lots of water vapor, which fuels buoyancy and triggers stormy weather. Cloudy, stormy weather is common along a front. Frontal wedging is common in our winter, when cold air from the polar zones sweeps down to lower latitudes.