Atmospheric Thermodynamics:Making Predictions on a Skew-T

Reading a Skew-T DiagramA Skew-T diagram is a very useful thermodynamic

diagram that is used in meteorology to analyze the weather and make forecasts. The diagram is as complex as it looks and contains plenty of physics behind each of its components. The Skew-T diagram consist of: Isobars- Lines of constant pressureDry Adiabats- Shows the trajectory of an unsaturated (dry) parcel of air risingIsotherms- Lines of constant temperatureSaturation Mixing Ratio- Lines that show the ratio of water vapor and air in a parcel of air.Moist/Saturated Adiabats- Lines that shows the trajectory of saturated (wet) parcel of air.

By thinking about a volume of air, the ideal gas law, and the laws of thermodynamics, it is possible to get a general understanding on how to read a skew-t diagram.

Clifford FrancisColorado College

Skew-T Components• Lines of constant

temperature

• Skewed at 45 degrees

for calculating

parameters

• Measured in Celsius

• Horizontal lines of constant

pressure

• Plotted on the Y-axis

• Pressure decreases much

slower with height

• Pressure is plotted

logarithmically

• Adiabatic Process: no heat

exchanged with the outside

system

• Unsaturated parcel cools

while rising

• As unsaturated air parcels

rise, any air parcel sinks

• Ratio of water vapor and

air (in kg)

• Once saturated, no more

water vapor can be held

• Line should match the

phases of water as

temperature changes

• When air reaches

saturated, water vapor

condenses

• Latent heat is releases in

the atmosphere

• Moist adiabatic lapse rate

is always less than the dry

adiabatic lapse rate

A Parcel of Dry AirIf we imagine a small volume of air, let’s say a

cube, then we can think of what will happen to that volume of air using the ideal gas law. As our parcel of air increases in altitude, it would: Expand, air pressure will decrease as altitude increases, and its temperature would decreases.

A Parcel of Moist AirIf we consider a parcel of air that also contains water molecules,

we will have to rethink about the ideal gas law and how the water molecules would effect our parcel. We must note that water has different phases that are dependent on its temperature. Temperature, however, will be dependent on the pressure and location of our parcel in the atmosphere. Hydrogen bonds in our parcel of air will continuously condense and evaporate.The Clausius Clapeyron Equation is used to look at the change in states of water. It shows that the saturation vapor pressure increases with temperature. A moist air parcel reaches saturation when it obtains a balance of condensation and evaporation.

It is important to think about Clausius Clapeyron Equation when it comes to cloud formation. When we look at an actual skew-t diagram with all the components together, we would want to think in terms of our parcel of air to get an understanding of how clouds will form and how much humid the atmosphere will be. When looking at a skew-t, it will read the temperature and humidity collected from a weather balloon.

Skew-T Diagram ExampleData on a Skew-T is collected by a weather balloon that measures

pressure, humidity, and temperature as it rises in the atmosphere. That data is then displayed on a skew-t diagram.Here we have a skew-t diagram from a weather balloon from Lamar, Colorado. The Dewpoint profile and the temperature profile head towards each other at around 650 hPa. Dewpoint is the temperature at which water vapor is present. When the temperature and Dewpoint lines are near each other, it tells us that there is a lot of humidity. High humidity means we have clouds and chances of precipitation.

https://www.ospo.noaa.gov/Products/atmosphere/soundings/sfov/s

kewt/den.html

https://www.e-education.psu.edu/meteo300/node/7

https://www.e-

education.psu.edu/meteo300/node/7

https://weathertogether.net/weather-101/how-to-read-

skew-t-charts/

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Earth’s atmosphere is at

hydrostatic equilibrium, so

we can write out the forces

-$ − -) − /0Δ2 3 = 0 ; let’s say changes are infinitesimally small dp and dz

45 = −/0 424542 = −/0

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5@ = /@A@B ; Ideal gas

law for dry air