Bring the t-shirt if it is warm

P was wearing a long sleeve shirt and asked me to tell C to bring him a t-shirt to soccer practice "if the day got hot". I called here, in front of him, and told her to bring him a t-shirt. No conditionals. P said, she only needs to do it if it gets hot. I told him that it was better if she simply brought the shirt. Otherwise, what if he thought it was a hot day but she didn't? In general, it is a bad idea to agreements based on subjective criteria, like whether the weather is hot or not. And even some apparently objective criteria van become subjective. For example, is it night or day during the sunset hours? Is it raining or not when it is drizzling?