After a break of a few days and snowfall in late August, heatwave conditions returned to the Alps in mid-September.
By HansPeter Roesli (Switzerland)
In the valleys of the Italian part of Switzerland the daily maximum temperature stayed well above 25 °C for more than a week in the first half of September, Usually such values are only observed on isolated days in this season.
On 12 September at Jungfraujoch (3580 m) a new record for the daily maximum temperature was set: 10.8 °C, the former maximum temperature was 9.7 °C on 9 September 2000.
The VIIRS Natural Color RGB 11 September (Figure 1, top right, click to expand) shows the Alps and their surroundings under mostly clear skies in this second heatwave of 2018. A number of elements are visible on the image:
- Thin cloud in the eastern part of the image — remnants of a weak warm front that had moved further east off the image.
- Some low-level (summer) haze over the Po Valley, in particular over its western parts where cirrus clouds were absent (seebullet above) — no haze was visible over the most western big lakes (e.g. compare Lago Maggiore left to Lake of Garda right) and in the Alps, see photo. Also the small cumulus clouds that decorated the south-alpine foothills were indicative of the hazy, more humid air (pushed uphill by thermal winds).
- Extended brown areas indicating early loss of chlorophyll.
- Pale-green rice fields in the Italian Piedmont.
- Only sketchy snow and ice fields — no traces left from the late-August snowfall as shown by the VIIRS Natural Colour RGB of 27 August (Figure 2, right)
Figure 2 is a comparison of the VIIRS images from 11 September and 27 August, showing the dry conditions versus the earlier snowy conditions.
Previous case study
August snowfall in the Alps (27 August 2018)