Stability continued Skew-T diagrams
On Friday, the subject of stability was introduced. This lecture will be used to reinforce the issue of atmospheric stability that can be confusing at first to understand. Much of this lecture will be material duplicated from Friday's lecture.
Lapse rates
The lapse rate is the rate at which temperature is changing with elevation. The magnitude of the lapse rate determines the type of bouyancy forces that a parcel of air will be subjected to upon displacement.
Stability vs. Instability
Exactly how fast the temperature falls as you move up in the atmosphere determines the atmospheric stability. If the atmosphere is stable, then a parcel of air displaced vertically will return to its original level. However, if the atmosphere is unst able, a parcel of air displaced vertically will continue to accelerate.
Upon closer examination, to understand this, we need to examine the Ideal Gas Law. The Ideal Gas Law defines how the pressure, temperature, and density of a gas are related. When determining if the atmosphere is stable or unstable, we are actually co ncerned with how density is changing with elevation, but how density changes with elevation is governed by how temperature is changing with elevation.
If we solve the Ideal Gas Law for density, we find that (rho) = P/RT (remember rho is the notation used for density). Look at the ratio P/RT which defines density. When we move upward in the atmosphere, we know pressure will always decrease. That will act to decrease the ratio of P/RT thus decreasing density. The rate at which pressure decreases with height can most of the time be considered constant. So it will be a given that as you move upward in the atmosphere, the P term in the ratio P /RT will decrease thus decreasing the density.
We also know that as we move upward in the atmosphere, temperature decreases. As T in the P/RT ratio decreases, that will cause the ratio to increase, thus increasing the density. So if the rate at which temperature is decreasing is greater than the rate at which pressure is decreasing as you move upward vertically in the atmosphere, density will be tending to increase as you move upwards. This would be a condition of instability. We would have denser air on top of less dense air. We know that mor e dense fluid will sink so in conditions of instability, we would expect vertical motion.
However, if the P term is decreasing faster than the T term, then density would be tending to decrease with elevation and the atmosphere would be stable. We would not expect significant vertical motion in this scenario.
The Dry Adiabatic Lapse Rate
If the rate at which temperature is decreasing with elevation and the rate at which pressure is decreasing with elevation is such that density is not changing significantly with elevation, the atmosphere is considered neutral. The lapse rate that woul d lead to this condition is called the Dry Adiabatic Lapse Rate. The value of the Dry Adiabatic Lapse Rate is 9.8 C / km. If we are not considering dry air, the value of the adiabatic lapse rate is different because as moist air rises, there is c ondensation which is a heat releasing process.
What good is this information?
If a meteorologist can predict and determine the lapse rate, he or she can get a better idea of what type of stability conditions can be expected. Knowing the conditions of stability can offer a better idea of what type of vertical motion will be expe cted. Knowing the degree of vertical motion can offer better hints when it comes to predicting cloud and precipitation development.
The big value of understanding the stability conditions of the atmosphere is using that information to forecast vertical motion.
Skew-T Diagrams
A Skew-T diagram is a chart employed by meteorologists to show how the atmosphere is behaving vertically. Another word for a Skew-T diagram is a sounding.
Study the Skew-T diagram below. This is a diagram of the atmosphere from Amarillo, Texas taken Sunday February 8 at about 11 AM.
Skew-T diagrams are complicated looking looking charts at first glance. But with practice, they are as easy to read as a times table. Along the y-axis, temperature and pressure are represented. Temperature is represented along the x-axis. There are two jagged lines that migrate up the diagram. The righthand line is the temperature profile, the lefthand line is the dewpoint profile.
This chart, from just a quick glance, can show the trained meteorologists how that temperature is changing with elevation to give an idea of which layers of the atmosphere are stable and which layers are unstable. In addition, it shows which layers of the atmosphere are dry and which layers are moist.
The closer together the two white lines are, the closer the temperature is to the dewpoint temperature and the more moist the atmosphere is.
The "flags" along the righthand side of the diagram are wind speed and direction indicators. This gives information about how the winds are varying with height. We will learn in coming weeks that this is very important to weather forecasting as well.
How do we get information to plot Skew-T diagrams?
Weather balloons, also called radiosondes, are launched at many locations across the country twice a day. They are equipped with weather instruments that take measurements at many different levels in the atmosphere.
Each time measurements are recorded, they are signaled back to a tracking station on the ground.
A weather balloon launch.