this lecture discusses atmospheric stability atmospheric stability helps us calculate the likelihood of vertical motion in the atmosphere if we can get a parcel to rise it might cool to the dewpoint temperature that means a cloud might form a stable atmosphere prevents vertical motion and an unstable atmosphere allows vertical motion when you force a parcel to move vertically in the atmosphere its internal temperature will cool because the volume of the parcel increases think about what happens when you let go of a helium balloon as it rises higher in the atmosphere there's less pressure on the balloon which allows it to expand eventually the balloon pops an adiabatic process is where the temperature changes without adding or subtracting heat with the environment when the parcel Rises it expands the volume of the parcel increases which decreases the kinetic energy a decrease in kinetic energy means the internal temperature drops so the parcels temperature cools by expansion have you ever used a can of air to clean your keyboard as you use the air the can gets cold that's because the air is compressed inside the can and expands when release from the can that's cooling by expansion since we understand the physics of adiabatic cooling we can calculate the exact temperatures inside the parcels but first we need to see what kind of parcel we have if the parcels air temperature is greater than the dew point temperature that's a dry parcel a lapse rate is a change in temperature with height with a dry parcel we use the dry adiabatic lapse rate and subtract 10 degrees Celsius for every kilometer in height when the air temperature cools to the dew point temperature the parcel is saturated that means it's condensing and releasing some latent heat the parcel still cools off but it doesn't cool off as fast as a dry parcel we switch to the wet adiabatic lapse rate at saturation the wet adiabatic lapse rate cools off at around 6 degrees Celsius for every kilometer in height we need to calculate the internal temperature change inside the parcel if the parcels temperature is greater than the dew point temperature we use the dry adiabatic lapse rate an air temperature of 25 is greater than a dew point temperature of 5 so we subtract the dry adiabatic lapse rate of 10 degrees that cools the parcel to 15 degrees at one kilometer then we do the same thing again the air temperature of 15 is greater than the dew point of 5 so we subtract the dry adiabatic lapse rate and cooled to 5 degrees air temperature at 2 kilometers once we cooled to the dew point temperature we switch to the wet adiabatic lapse rate now let's transfer that conceptual idea to how we perform these traces in meteorology the diagram with the lines shows the changes in our parcel temperature as a plotted graph the red line shows when the parcel is cooled using the dry adiabatic lapse rate and the blue line is where we switch to the wet adiabatic lapse rate the green line shows the dew point lapse rate let's add that concept now if you can force your parcel to a height where it can cool to the dew point temperature you'll be saturated that height where you'd have lifted your parcel high enough to begin condensing is the lowest height where a cloud could form we call that the base height of a cloud it's the bottom of the cloud this is reported as the lifting condensation level or LCL and it's a height above the surface like 2 kilometers it's also where we switch over to the wet adiabatic lapse rate after all that work we still need to determine stability what we do now is compare the calculated 80 vac temperature of the parcel to the environments temperature the environmental lapse rate or ELR is what we measure with weather balloons the average environmental lapse rate is usually around 6 degrees Celsius but it changes because the ELR is affected by the mixing in the atmosphere the black line on our diagram is the ELR or environmental lapse rate a stable atmosphere is where your parcels temperature is always colder than the environments temperature at every height the surface temperatures always start out the same but your parcel is what you're calculating using your adiabatic lapse rates and the environment is what is actually occurring look at a thousand meters the parcel is at 15 degrees which is colder than the environment at the same height same thing at 2000 meters the parcel is still colder than the environment this means that even if you forced your parcel up to 2 kilometers it will still be colder than the environment that means it will sink if your parcel sinks it will warm and if your parcel warms there's no chance you can cool to the dewpoint temperature the weather forecasts for stable atmospheres is usually sunny and dry an unstable atmosphere is where your parcels temperature is always warmer than the environments temperature at every height what that means is that your parcel will be able to rise vertically in the atmosphere unstable atmospheres are rare but the kinds of weather that happens in an unstable atmosphere is extreme because the clouds can grow to great vertical extent unstable atmospheres almost always bring severe weather more often we have conditional atmospheres where the atmosphere starts out as stable then moves to becoming unstable if we could lift the parcel up to the unstable area we'd have a chance for clouds and maybe storms the forecast for conditional atmospheres are always hedging on finding a lifting mechanism so you'll hear forecast like chances of rain later in the afternoon