## 2 Data and Methods
### Data
#### 2.1.1 Radiosonde Measurements  
The vertical profile data were obtained from the radiosonde measurements. A radiosonde is an expendable meteorological instrument package, often borne aloft by a free-flight balloon, that measures, from the surface to the stratosphere, the vertical profiles of atmospheric variables and transmits the data via radio to a ground receiving system [23]. The radiosondes are operated by the Slovenian Environment Agency (SEA). They are launched once per day from the Ljubljana-Bezigrad station that is part of the SEA observation network (altitude 299 m, longitude 14.5124\({}^{\circ}\), latitude 46.0655\({}^{\circ}\)). The set of data contains 4420 vertical profiles from October 2004 to May 2019. Each vertical profile consist of measurements of seven variables: altitude (\(z\)), temperature (\(T\)), dew-point temperature (\(T_{d}\)), relative humidity (\(RH\)), pressure (\(p\)), wind speed (\(|\textbf{v}|\)) and wind direction (\(\theta\)).  
The radiosondes can ascend to an altitude exceeding 25 km; however, oftentimes, the maximal altitude reached by radiosonde is significantly lower. To homogenize the data and not exclude too many measurements, we decided only to use data below 12 km for the analysis (this at least guarantees that the whole troposphere is included in the measurements). Quality control was performed to remove the profiles that contained non-realistic values or missing data. We excluded all profiles that met any of the following criteria: (i) the altitude of the lowest measurements is larger than 299 m (the altitude of the Ljubljana-Bezigrad station), (ii) the altitude of the highest measurement is smaller than 12 km, (iii) there exists a vertical gap without measurements that is larger than 100 m, (iv) the relative humidity of the lowest measurement is smaller than 10%, (v) the temperature of the lowest measurement is smaller than \(-\)35 \({}^{\circ}\)C and (vi) there is at least one non-numeric value present (e.g., infinite, not a number) in the measurements. Thus the final set that was used for the analysis contained only 3800 profiles. The majority of days without measurements are before 2011, as shown in Figure 1. The measurements were performed in the early morning hours: 46.8% of measurements were performed at 03 UTC, 41.7% at 04 UTC, 10.3% at 02 UTC, and the remaining at other times.  
During the ascent, the radiosonde performs measurements every second, and the typical vertical resolution of the profile measurement is about 5 m. The vertical speed of the radiosonde depends on various meteorological parameters as well as the amount of helium in the balloon. Thus, the rate of ascent can be slightly different each day, and the altitudes at which the radiosonde records a measurement are also different. Further, the values of the measured meteorological parameters typically do not change much in 5 m, so it makes sense to use a coarser vertical resolution; therefore, we decided to interpolate the vertical measurements to 118 predefined altitude levels at every 100 m, with the lowest level being at 300 m and the highest one at 12 km (linear interpolation was used).