Negotiate on Days With Nice Weather

You feel happier in good weather, so you're more likely to help people.

A calendar of weather with a sunny day circled

Overview

Weather has a powerful effect on behavior.

Good Weather Triggers Positive Behavior. People give larger tips and gratuity (Cunningham, 1979).
Bad Weather Triggers Negative Behavior. People leave more negative reviews (Brandes & Dover, 2022).

Therefore, schedule your negotiation for a day that has nice weather.

If you need to negotiate during bad weather, discuss the weather beforehand. This discussion eliminates the negative effect because it orients people toward the reason behind their dampened mood (see Schwartz & Clore, 1983).

Cunningham, M. R. (1979). Weather, mood, and helping behavior: Quasi experiments with the sunshine samaritan. Journal of personality and social psychology, 37(11), 1947.
Schwarz, N., & Clore, G. L. (1983). Mood, misattribution, and judgments of well-being: informative and directive functions of affective states. Journal of personality and social psychology, 45(3), 513.
Brandes, L., & Dover, Y. (2022). Offline Context Affects Online Reviews: The Effect of Post-Consumption Weather. Journal of Consumer Research, 49(4), 595-615.