Hi Simon,
I know you're not being funny, it's a fair question.
I am sure rainwater isnt pure enough not to make windows dirty
Here I disagree. Rainwater has effectively been "distilled" by the water cycle, and usually has less than 0.7ppm TDS. Have a look at these results of a study done near Essex.
http://www.thegrid.org.uk/learning/hals/docs/triwaternotes.docSanitary Examinations of Water, Air, and Food
A Handbook for the Medical Officer of Health
By Cornelius B. Fox MD, MRCP, LOND.
Medical Officer of Health of East, Central & South Essexsince water up to about 10ppm TDS can be used for window cleaning, this is well within acceptable limits. It is the wind that makes windows dirty, and also rainwater dribbling down from above the windows, having dissolved salts and dirt, and then evaporating on the glass. Not the actual rain itself.
do you think your customers will believe you
Good question. I don't intend to make a big deal about it, just to give us the option of working in the rain if we really need to, and prepare them for the possibility. There is certainly no technical reason why you couldn't work in the rain.
Silly