Greenhouse Effect
It turned out even the area around Kaer Morhen, a place which - one would think - Geralt knows by heart, can hide a great many interesting and undiscovered phenomena. Not far from the keep, Geralt came across an area where the weather differed remarkably from that of its surroundings. He decided to investigate what sort of forces were at work.
Map

Pin numbers match the objectives below. Open it on the interactive map →
Objectives
5- 1Investigate the weather anomaly using your Witcher Senses.Iron Mine
- 2Use the obelisks to restore the place's proper microclimate.Iron Mine
- 3Kill the chort.Iron Mine
- 4Return to the site of the anomaly in a few days.Iron Mine
- 5Read the mage's letter.
Journal
After a brief investigation, the witcher was able to conclude that the weather anomaly was the doing of one of the resident mages of Kaer Morhen, who had created a sort of outdoor greenhouse in which to cultivate the herbs used for the Trial of the Grasses. Though you might not know this, Geralt is quite the handyman, something which came in handy (pun unintended) at this time. He banged and nudged the broken magical apparatus and returned it to working order in no time. Before long, rare herbs were growing outside Kaer Morhen once again.