Warm air atop cold: Quantifying temperature inversions on the Kansas Mesonet
In 2017 the Kansas Mesonet underwent an upgrade that included adding a second 30-foot (10 meters) high temperature/humidity sensor to tower stations (Figure 1). This upgrade coincides with the already existing 6-foot (2 meters) temperature humidity sensor. With temperature measurements at different heights, the Mesonet is able to provide a small vertical profile of the lower atmosphere. This lowest layer provides great insight into the vertical mixing occurring over land which has substantial impact on smoke dispersal, spraying results, and temperature forecasting.
Figure 1. All 30-feet towers measure temperatures at 30 feet and observe inversions on the Kansas Mesonet.
What are inversions?
In the troposphere, the lowest layer of the atmosphere that reaches the earth’s surface, temperature typically declines as altitude increases. This rough estimation of temperature with height doesn’t always fit the situation due to weather, terrain, and solar radiation. Anomalies in the lowest layer of the atmosphere occur when temperatures increase with height due to these factors. These anomalies are called inversions. When cooler, higher density air, is in place under warm, less dense air the atmosphere can behave much differently than expected. These differences include poor air dispersion, light winds, and fog.
Using the Mesonet to determine presence of an inversion:
The difference between the 2-meter (2m) and 10-meter (10m) sensors will indicate the presence of an inversion as well as low-level stability. Keep in mind, inversions often differ greatly over very small distances. These observations provide a small snapshot of regional conditions but aren’t necessarily representative at your location.
When viewing the inversion data here: mesonet.ksu.edu/agriculture/inversion, the following data implies:
- Positive values infer that temperatures are warming with height and a surface inversion is in place. The more positive the value, the stronger the inversion at the surface. Inversion values 5F and higher are considered a strong inversion with values greater than 10F considered very strong. Oklahoma Mesonet has recorded inversions up to 20F, a very rare phenomenon that implies a significant inversion (Hunt et al., 2007).
- Negative (-) values indicate the temperature cools with height and the lower atmosphere is unstable. Therefore, no inversion is in place and mixing is effectively occurring. The more negative the value, the more cooling with height exists and the faster air parcels will rise.
- Zero values represent temperatures at 2m and 10m are the same. This indicates either stability or instability at the surface, dependent upon the time of day.
- In the morning: zero values indicate the inversion developed vertically to heights higher than 10-meters.
- In the afternoon: strong mixing of the layer has equalized the temperatures between the two levels and the atmosphere is unstable.
- In the evening: atmosphere is likely stabilizing and an inversion may soon develop.
- Winds at 2m are critical when an inversion is in place. They will vary and be mainly light and terrain influenced. However, these light winds will dictate the direction that spray, for example, may drift when it remains near the surface. The wind barb (represented by a line protruding outward from the value at a specific location) will indicate the direction the wind is coming from (direction wind would blow in your face if you were turned that way). Flags on the end of the wind barb indicate speed. A short flag = 5 mph, long = 10 mph, and a triangle = 50 mph. Add up all the flags on a wind barb to obtain the total wind speed. For additional description on wind barbs visit: http://mesonet.k-state.edu/about/windbarbs/
Figure 2. Data from August 30, 2017 at 9:00 pm. Strong inversions (values >5) are in place across much of western Kansas with weak to moderate inversions across central and eastern Kansas.
View data here: http://mesonet.k-state.edu/agriculture/inversion/
How can you tell in the field if an inversion is present?
More often than not, there is no simple way to determine the presence of an inversion. You absolutely must take temperature measurements at two different heights to determine the change in temperature with height.
Occasionally, there are some visual indicators of an inversion. A few of these indicators are:
- Low lying fog in valleys, low points, and over different ground cover.
- Poor dispersion from chimneys, industry, and dusty roads.
- Frost/dew on the ground.
- Sounds traveling large distances.
Want to learn more about inversions?
Visit the Mesonet Inversion page here: http://mesonet.k-state.edu/about/inversion/
Hunt, E. D., J. B. Basara, and C. R. Morgan, 2007: Significant inversions and rapid in situ cooling at a well-sited Oklahoma mesonet station. J. Appl. Meteor. Climatol., 46, 353–367, doi:10.1175/JAM2467.1.
Christopher Redmond, Weather Data Library/Mesonet Manager
Mary Knapp, Weather Data Library
Dallas Peterson, Weed Management Specialist
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist