First, someone in the company must have encountered these temperatures before. Ask them.
Also Environment Canada should have historical records that will help you decide the level of protection you will need and a reasonable temperature value.
You, and your customer will have to come to an agreement on the level of protection required and how to achieve it.
"ABILITY is what you are capable of doing. MOTIVATION determines what you do. ATTITUDE determines how well you do it." -- Lou Holtz