Sourmist

Sourmist particles were a substance produced in Open Sky. The concentration of sourmist in the air was generally quite low, but a rise in the concentration of particles would usually herald the imminent arrival of a Great Storm, although they could very rarely lead to freak weather conditions such as Mind Storms or Endless Winters. Sky-scholars in Sanctaphrax studied them because it allowed them to know when to send Knights Academic on stormchasing voyages. 

Sourmist particles were toxic, although only harmful in extremely high concentrations. They were also acidic, which gave them their distinctive sour smell.

Detection

There were many ways of detecting sourmist, which meant that it was studied by four of the seven major schools of Santaphrax. If the concentration was high enough, such as before a Great Storm, you could simply smell them on the air.

Raintasting

If there was an increase in sourmist particles in the air, they would increase the acidity of the rain. Sourmist was also anti-magnetic, which meant that rain-gauges could detect them based on the magneticity of the rainwater.^[1]

Cloudwatching

Sourmist-rich steam was stored in vats on the roof of the College of Cloud. The opacity of the steam would change depending on the concentration of sourmist.

Mistsifting

The twin mistsifting towers of the School of Mist were able to capture mist from clouds coming in from Open Sky. The mist could be analysed to work out the concentration of sourmist particles.

Windtouching

During Stormchaser, the Professor of Darkness says he must consult the Windtouchers and Cloudwatchers, although it is not clear how windtouching can detect sourmist particles. 

References

1. ↑ Midnight Over Sanctaphrax, Chapter 3:The Loftus Observatory