Do cold showers cause rhinorrhea?

My nose secretes excess mucus (is runny) far too often, which annoys me and might disgust others. I always shower with cold water. Someone suggested that the latter causes the former, based just on having read that cold promotes rhinorrhea. We can do better.

Over 18 days, I randomised whether to shower with cold or warm water. I actually showered on 72% of those days. When I did, I consistently used the temperature chosen by spreadsheet. I recorded how much nasal mucus there was to irk me on each day, on a scale from 0 to 3. The mean annoyance was 1.9, and the stddev was 0.8.

When I had recently showered cold, rhinorrhea irked me, on average, at level 2.3. Rhinorrhea averaged at 1.7 after showering warm, and at 1.7 on days mostly not preceded by showers. On the one hand, that's an effect size (Cohen's d) of 0.94 that cold showers make my nose runnier than a warm shower or the lack of any. On the other hand, that effect size has SE 0.52, which leaves about a 4% chance that the real effect's negative.

Sith I recorded my showers in a spreadsheet anyway, this made a convenient opportunity to check if temperature or time-of-day affects how long my showers take. Cold showers take longer than warm ones, by an effect size of 0.35 — but with SE 0.56, which is to say that temperature doesn't matter. Morning/midday showers take longer than evening/night ones, by an effect size of 0.75 with SE 0.55.

So she who suggested this whole inquiry was probably right. If I care enough to fix my runny nose, I should stop showering cold. But the warm weather these days, in the northern hemisphere, make it less of an issue anyway.