World Rugby offers a detailed guideline in regards to exposure when playing/exercising in various different weather conditions. Please visit the following links to learn more.
Hot & Humid Weather: World Rugby Hot Weather Guideline
Cold & Rainy Weather: World Rugby Cold Weather Guideline
Note: Regarding lightning and thunder we follow the 30/30 safety rule. When you see lightning, count until you hear the thunder. If it's under 30 seconds take shelter and then wait 30 minutes after thunder was heard last to go back out.