Our ambulatory assessment study explores the impact of the weather on the mental well-being of people with increased susceptibility. Participants with hay fever ( = 28) were assessed three times a day over a period of two weeks. Self-reported assessments covered different indicators of mental well-being, including momentary affect, subjective health as well as symptom burden. Based on tracked time stamps and location information, the data was matched with concurrent observation data from weather stations. We applied multilevel analysis to identify the main effects of selected environmental parameters (temperature, precipitation, wind power, sunshine duration and relative humidity) on all indicators of subjective well-being. Results confirm the main effects of sunshine duration, relative humidity and temperature on momentary affect as well as of sunshine duration, relative humidity and precipitation on subjective health and symptom burden. However, influences of environmental parameters on momentary affect were quite small and do not differ from effects documented in previous research in healthy samples with non-increased susceptibility.