We get a fair number of questions about the forecast dotted blue freezing level line you’ll find on the Route Profile or Meteogram views in WeatherSpork. When there’s only a single freezing level (more accurately, the melting level) forecast, you’ll see just a single dotted line as is depicted in the image below. This identifies the expected freezing level along the route. Simply put, the atmosphere above the blue dotted line is forecast to have an environmental temperature colder than 0°C and the temperature below the blue dotted line is forecast to have an environmental temperature warmer than 0°C. The image below depicts a freezing level forecast over the Dalton Municipal Airport (KDNN) to be 10,762 feet MSL at 1642Z as shown along the SporkPole (vertical line cursor).
However, you may notice that in some cases on the Route Profile or Meteogram views there may be two dotted lines over any particular location like you see depicted below. This is very common when there is a temperature inversion aloft. That is, the environmental temperature increases with increasing altitude. Shown below on the SporkPole over the Purdue University Airport (KLAF), the highest freezing level is forecast to be 6,036 MSL and the lowest freezing level is forecast to be 1,934 feet MSL at 1733Z. This implies that the environmental temperature crosses over the 0°C isotherm more than once, hence, multiple freezing levels exist. In most cases where only two freezing levels exist, the atmosphere between these two altitudes will be above 0°C. However, it’s also possible the environmental temperature may cross the 0°C isotherm more than twice. Consequently, the temperature could also be colder than 0°C between the lowest and highest freezing levels depicted on the Route Profile view.
In the end, there’s no way to tell how many times the environmental temperature crosses the 0°C isotherm using just the WeatherSpork Route Profile view. Instead, you will need to drill down using a thermodynamic diagram (Skew-T) in the WeatherSpork Airport view and examine the forecast temperature profile aloft. Shown below is the Skew-T log (p) diagram from the Rapid Refresh model showing the two hour forecast valid at 1700Z for the Purdue University Airport. You can see that the environmental temperature crosses the 0°C isotherm a total of three times (designated by the three blue arrows) between the lowest and highest freezing level. This corresponds fairly well to the forecast in the Profile View.
Why are multiple freezing levels important? Well, in many cases, it’s not all that critical and may not suggest adverse conditions. However, during precipitation events, having multiple freezing levels may be indicative of a freezing rain (FZRA) event. That is, snow falling from aloft is melted into rain when falling through the layer of air that is warmer than 0°C and that rain falls into a subfreezing layer and becomes freezing rain.
Moreover, you will also see multiple freezing levels depicted on the Airport Meteogram view as shown above for Purdue University Airport. All the same rules documented above also hold true for the Meteogram view.
Lastly, you can also find a forecast for multiple freezing levels in the Imagery view under the Graphical AIRMETs (G-AIRMETs) selection. The freezing level G-AIRMET depicts areas of multiple freezing levels with a violet dot-dot-dash line as shown below. Within this polygon, you will also find the potential of two or more freezing levels at the valid time, in this case 1800Z. Purdue University Airport is located within the green circle below and is within the multiple freezing level polygon. The label on the left (outlined in blue) that points to this area indicates that multiple freezing levels exist (0°) from 1,000 feet MSL (010) to 6,000 feet MSL (060) in this area. Similar to the Route Profile view, this isn’t a forecast for freezing rain, however. It simply identifies where multiple freezing levels exist.
So using a combination of the Route Profile view, Meteogram view, freezing level G-AIRMET along with the Skew-T diagram will keep you warned about a potential nasty icing encounter along your proposed route of flight when multiple freezing levels exist along with the potential for precipitation.
Weather Systems Engineer