Lapse Rates

Lapse rates are progression at which air temperature changes with increasing / decreasing height in the atmosphere.  The rate is considered positive when temperature decreases with elevation, zero when temperature is constant with elevation, and negative when temperature is increasing with elevation.  While there are two different class of lapse rates, normal and adiabatic. The difference between normal and adiabatic lapse rates determine the vertical stability, or instability, of the atmosphere. That is, an air parcel’s tendency to embrace or prohibit vertical motion.

Environmental Lapse Rate—non-rising air that is affected by radiation, convection, and/or condensation. It averages about 6.5°C/km.

Dry Adiabatic Lapse Rate—rate of cooling with increasing altitude. It is constant at about 10°C/km.

Moist Adiabatic Lapse Rate—air, saturated with water vapor, is not constant but is determined by the combined effects of expansion cooling and latent heating (LH) because saturated air cools slower than dry due to the heating produced by condensing water vapor. Is always less than the dry adiabatic lapse rate.

Due to the fact that density differences are affected by the differences between the adiabatic lapse rates and the environmental lapse rate, one may notice that absolute instability occurs when the environmental lapse rate (Г_E) exceeds the dry adiabatic lapse rate (Г_D) [i.e. Г_E > Г_D]. Whereas, absolute stability occurs when the environmental lapse rate (Г_E) is less than the wet adiabatic lapse rate (Г_W) [i.e. Г_E < Г_W]. However, when the environmental lapse rate (Г_E) falls between the wet adiabatic lapse rate (Г_W) and the dry adiabatic lapse rate (Г_D) [i.e. Г_W < Г_E < Г_D] the atmosphere is considered conditionally unstable, as you can see from the picture below.

Chinook, Santa Ana and Katabatic Winds

Chinook, Santa Ana and Katabatic winds are those that flow downslope in response to the distribution of high- and low-pressure systems over and near large mountain areas, where compressing of descending air leads to adiabatic warming.

Chinook winds, off the eastern slopes of the Rocky Mountains in North America, form due to air flowing across the range. Low-pressure systems east of the mountains cause strong winds to descend the eastern slopes. Although, sometimes the presence of a large mass of cold, dense air near the base of the mountain range may prevent a chinook from flowing all the way down the slope.

Santa Ana winds, contrary to what people believe, occur in response to a large area of high-pressure which descends toward lower elevations and warms by compression causing air to flow out of the Rockies, they are not warm because they pass over hot desert surfaces. When Santa Ana’s develop, the combination of hot, dry winds, low humidity, and an abundant source of fuel can set the stage for an extensive fire that destroys a great deal of land or property.

            Katabatic winds, on the other hand, originate when air is locally chilled over a high-elevation plateau, where the air becomes dense due to its low temperature and flow downslope. These very strong gusts and lulls of winds cover much of coastal Antarctica and Greenland. They also flow out of the Balkan Mountains towards the Adriatic coast, where they are called boras; whereas, in France, they flow out of the Alps into the Rhone River Valley and are called mistrals.

Physical Principles: The Varying Value of the Saturated Adiabatic Lapse Rate

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Physical Principles: Adiabatic Lapse Rates and the First Law of Thermodynamics

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Adiabatic Lapse Rates

What do you know about the Wet Adiabatic Lapse rate?

-       Less than dry adiabatic lapse rate

The more it slants = greater lapse rate 

Lapse rate = Rate of cooling

What’s the difference between the Dry and Wet Adiabatic Lapse Rate?

-   Unsaturated à reaches LCL à Saturated

-   Latent “Hidden” heat - The heat either released or absorbed as a result of a change of state.

-   When a cloud forms….

o   The water vapor reaches the LCL à Temperature decreases à It’s doing work à slows the rate of cooling

-   Water vapor condenses à latent heat is released

Mixing ratio (w) - A measurement of the amount of water vapor in the air of a given sized quantity of dry air. Grams of vapor per kilogram of dry air.

W = Water vapor in the box

-   Dewpoint Lapse Rate = As it goes up it decreases = 2 deg C / km

-   Dew point (Td) - The temperature to which air must cool at constant pressure in order for air to reach saturation (commonly dew to form); indicates moisture content.