The Javascript calculator below was designed to calculate LCL conditions using only three atmospheric input parameters that are commonly measured and reported during most local televised weather forecasts: surface temperature, surface dewpoint, and surface pressure.
Though this LCL calculator was designed to calculate the lifting condensation level (LCL) for a parcel of air lifted from the surface, you calculate the LCL conditions for an air parcel at any other pressure level by substituting values appropriately.
When performing computations which require temperature as input, we normally must convert temperature to Kelvin degrees. Our Javascript calculator assumes Celsius degrees and makes the conversion to Kelvin for you. Therefore, you should enter the surface temperature in degrees Celsius. Refer to the temperature converter if you need to convert between degrees Fahrenheit, Celsius, and Kelvin.
The dew point temperature, like the observed temperature, is normally converted to degrees Kelvin. However, our LCL calculator performs this conversion for you as well. Enter the reported dew point temperature in degrees Celsius. Remember, dew point temperatures are always lower than or equal to observed temperatures.
The final input required by our LCL calculator is the surface pressure. Be very careful that you enter the reported surface pressure rather than the sea level pressure. If you are acquiring the input data from a surface map, it is likely the pressure provided is a sea-level pressure. Be sure to check! The surface pressure value should be entered in kiloPascals (kPa). The LCL calculator automatically converts the pressure to Pascals (Pa). Surface pressures commonly range from 92.0 kPa to 103.0 kPa. depending on the topography and weather conditions of the region being studied.
The pressure at the LCL is calculated and returned as a value in units of kilopascals (kPa). This computed pressure represents the pressure level in the atmosphere at which a lifted parcel with an initial temperature, T, and an initial pressure, P, would have a relative humidity of 100% (i. e., saturated). The pressure at the LCL is always lower than the initial pressure of the parcel unless the air is saturated initially, in which case, the two pressures are equal. Pressure always decreases with height.
The parcel temperature at the LCL is calculated and returned as a value in degrees Celcius. Do not be misled! This temperature is the temperature the parcel would have if lifted to the LCL and, in most cases, is not equal to the temperature of the surrounding air. In Module 6 we will learn how the difference in these two temperatures relates to the stability of the atmosphere.
The temperature of the parcel at the LCL is always lower than the initial temperature of the parcel unless the air is completely saturated. For saturated air, the surface temperature, dew point, and the LCL temperatures are all equal to each other, and the slightest cooling (lifting) will produce condensation.