| Thermodynamic Basics and How to Understand Them on Upper-Air Charts | |
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| Potential Temperature (Theta) | |
| Definition: | The temperature a parcel of air would have if brought dry adiabatically to a reference level of 1000 mb. |
| Procedure: | Follow parcel's dry adiabat to 1000 mb. |
| Wet Bulb Temperature (Tw) | |
| Definition: | The lowest temperature to which a volume of air at constant pressure can be cooled by evaporating water into it. |
| Procedure: | Find the LCL, then follow the moist adiabat down to the original pressure. The intersecting temperature is Tw. |
| Wet Bulb Potential Temperature (Theta-w) | |
| Definition: | Same as Tw, except represents the lowest potential temperature. |
| Procedure: | Same as with Tw, except follow moist adiabat to 1000 mb. |
| Equivalent Temperature (Te) | |
| Definition: | The temperature a volume of air would have if all the moisture were condensed out by a pseudo-adiabatic process and brought back to its original pressure dry adiabatically. |
| Procedure: | Find the LCL, then follow the moist adiabat upward until it is parallel to a representative dry adiabat. Follow this dry adiabat back down to its original pressure. The intersecting temperature is Te. |
| Equivalent Potential Temperature (Theta-e) | |
| Definition: | Same as with Te, except the parcel is brought to 1000 mb. |
| Procedure: | Same as with Te, except follow the dry adiabat to 1000 mb. |
| Virtual Temperature (Tv) | |
| Definition: | The temperature at which a parcel of dry air would have the same pressure and density as the parcel of moist air. |
| Procedure: | Tv is found (approximately) by taking the value of the mixing ratio curve that passes through the dewpoint curve at the same pressure and multiplying by 1/6, then adding this value to the temperature. |
| Vapor Pressure (e) | |
| Definition: | That part of the total atmospheric pressure which is due to the presence of water vapor. |
| Procedure: | From the dewpoint curve at a specific pressure follow the isotherm up to the 622 mb level. The value of the mixing ratio line through this point is the vapor pressure in mb. |
| Saturation Vapor Pressure (es) | |
| Definition: | Same as for e, except corresponding to the saturation value. |
| Procedure: | Same as for e, except follow the isotherm up from the temperature curve. (Note: Relative Humidity = e/es x 100) |
| Convective Condensation Level (CCL) | |
| Definition: | The height to which a parcel of air heated from below will rise adiabatically until it is just saturated. It is the height of the base of a cumulus type cloud if convection is caused by surface heating. |
| Procedure: | Follow the mixing ratio line corresponding to the surface (or lowest layer) dewpoint upward unitl it intersects the temperature sounding. This intersection is the CCL. |
| Convective Temperature (CT) | |
| Definition: | The surface temperature that must be reached to start the formation of convective type clouds. |
| Procedure: | Determine the CCL, and follow the intersecting dry adiabat down to the surface pressure. The temperature that intersects this point is the CT. |
| Lifting Condensation Level (LCL) | |
| Definition: | The height at which a parcel of air lifted dry adiabatically would become saturated. The LCL is always found at or below the CCL. |
| Procedure: | Follow the saturation mixing ratio line from the surface dewpoint and the dry adiabat from the surface temperature upward until they intersect. |
| Note: | The average mixing ratio line from the surface dewpoint and the dry adiabat for the layer beneath the inversion can be substituted for the surface values. |
| Level of Free Convection (LFC) | |
| Definition: | The level at which a parcel of air lifted dry adiabatically until saturated (LCL), then moist adiabatically becomes warmer than its environment. |
| Procedure: | Find the LCL, then follow the moist adiabat upward until it intersects the temperature sounding. This is the LFC. |
| Equilibrium Level (EL) | |
| Definition: | The level at which a parcel of air lifted moist adiabatically from the LFC becomes cooler than its environment. This is an estimate of the cloud height of convective type clouds. |
| Procedure: | Find the LFC, then follow the moist adiabat upward until it intersects the temperature sounding. This is the EL. |
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Written by: Matt
Hartman
July 12, 2001
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