Stability Weather Balloons Skew-T Diagrams
We have learned this semester that in the troposphere, which is the layer of the atmosphere closest to the Earth, that temperature usually decreases with increasing elevation. What we will learn today is that how quickly temperature drops off with increasing elevation plays a very large role in determining the weather.
The Parcel Method
Meteorologists will often think in terms of parcels of air. A parcel of air can be thought of as an imaginary volume of air that moves through the atmosphere. By considering individual parcels, meteorologists can describe what changes will take place within them as they move through space and time.
Lapse rate
Temperature lapse rate is defined as the rate temperature changes as you move upward vertically in the atmosphere. How quickly temperature changes with elevation determines what type of bouyancy forces a parcel of air will be subjected to.
Bouyancy refers to forces that will cause a displacement in the vertical direction. Positive bouyancy forces will prompt upward motion while negative forces will prompt downward motion.
If a parcel of air is underneath denser fluids, it will become positively bouyant and will rise upward. Whereas if a parcel of air is above denser fluids, it will tend to stay at its same level.
As we have learned, density of a gas is related to temperature and pressure as described by the Ideal Gas Law. If we solve the Ideal Gas Law for density, we see that density is equal to the pressure of the gas divided by the Gas Constant times temperature. So we see that density is directly proportional to pressure and inversely proportional to temperature.
Suppose we displace a parcel upward. We know that temperature and pressure both will typically decrease. What will happen to the density though? If pressure was to remain constant and temperature would decrease, then density would increase. But if temperature were to remain constant and pressure were to decrease then density would decrease.
As we move upward, it seems that temperature will decrease with height acting to increase the density, while pressure will decrease with height as well acting to decrease the density. Therefore, it is which term falls more rapidly, temperature versus pressure, that determines how density will behave.
There is an equation we will learn about in a few weeks called the hydrostatic equation. It basically describes how pressure changes with altitude in the atmosphere. To a reasonable approximation, the rate of change with pressure with elevation is fairly constant. Then when determining how density changes with elevation, how temperature is changing is the important relation to consider.
We can define the stability of the atmosphere by considering this temperature lapse rate. To understand the basic principles, it is most convenient to consider completely dry air. If a parcel of dry air was lifted vertically into the atmosphere adiabatically, it would cool at a rate classified as the Dry Adiabatic Lapse Rate. Adiabatic means a process that is occurring to the parcel without any heat being exchanged across the boundaries. The Dry Adiabatic Lapse Rate is roughly 9.8 degrees Celsius per kilometer. That means if you could take a parcel of air and lift it adiabatically into the atmosphere a distance of 1 km, it would cool by a temperature of 9.8 C. In a process such as the one under consideration, the ratio of pressure changing with elevation to temperature change with elevation is such that the change in density would be zero. In this situation, the atmosphere would be classified as neutral.
However, if the ratio were lower, then density would be tending to get higher with elevation. That would mean that less dense fluid was underneath and would have a tendency to become positively bouyant. This case is called unstable.
If the ratio were higher, then density would be tending to get lower with elevation. The atmosphere would be stable.
In other terms, in an unstable atmosphere, if you displace a parcel of air vertically, it will continue to accelerate vertically. In a stable atmosphere, if you displace a parcel of air vertically, it will return to its originaly level. If the atmosphere were neutral, the parcel of air would simply remain at its new level and not be subjected to any restoring forces.
In the real world, we do not encounter purely dry air very often. The air has moisture and when air is lifted, as we know, the water vapor in the air will often eventually condense into cloud droplets. Then latent heat is released which affects the pressure-temperature-density relationship and will affect how density will change with height. Stability considerations thus are often made by considering the moist adiabatic lapse rate.
Since we are talking about vertical motion, we are talking about an atmospheric quantity (lapse rate) that has huge implications on determining cloud or precipitation development in addition to many other weather conditions. Getting a better understanding of how the lapse rate will affect sinking or rising motion is why stability considerations are so important to weather forecasting.
To give you an idea of how important the lapse rate is to determining the weather, in prolonged periods of atmospheric stability, there is virtually no vertical mixing or motion of the atmosphere. As a result, pollutants often get trapped in the lowest layers of the atmosphere (where we live) and cause large-scale health and environmental problems. Once the lapse rate becomes less stable, the atmosphere begins to become more well-mixed and the pollution problem abates. As we'll learn more when we study thunderstorms in mroe detail, the lapse rate also plays a huge role in determining the magnitude of thunderstorms.
So what we have learned is that how the temperature is changing with elevation plays a large role in determining how much vertical motion will or will not exist. We know how we measure temperature on the surface. But how do we figure out how temperature changes with height? With weather balloons.
Weather balloons
Weather balloons, also called radiosondes, are utilized to determine the atmospheric lapse rate. They are helium filled and are quite large. The inflated weather balloon portion stands about 6 feet with about 15 feet of cord that dangles to a radiosonde. A radiosonde is an instrument that measures temperature, pressure, and humidity as it travels upward through the atmosphere. The radiosonde has an antenna that radios the meteorological information back to a tracking station on the ground. Computers there plot the data graphically so meteorologists can visually get a better sense of what is occurring vertically in the atmosphere.
There are approximately 100 weather balloon launching stations across the country. These stations are called upper air stations. Tallahassee is a weather balloon launch site. Launches occur at 7 AM and 7 PM each day.
Charts that are compiled from weather balloon information are called Skew-T diagrams, or soundings. Soundings show how dewpoint, temperature, pressure, and wind speed are varying vertically in the atmosphere.