At midday it can sap energy, deliver headaches and contribute to dehydration. In hot weather, try to get out in the early morning or late afternoon. And drink lots of water to avoid heat stroke and heat exhaustion. Rain Rocky Mountian weather is notorious for its volatility. Sunny summer mornings are often followed by afternoon showers or thunderstorms producing hail or even snow.
Rain can chill the rider or hiker, contributing to hypothermia and can impair visibility.
Dirt trails can become slippery. Start early and adjust your plans to conditions. Always travel with a waterproof, breathable jacket, even on short trips. Rain pants and plastic bags to wear under your shoes are also good insurance for longer trips. Cold Cool mornings are common even in midsummer.