When you brew outside, there are a number of factors that are entirely dependent on the weather.  An important one is your rest temperature.  Your recipe/process may call for a rest at a given temperature for 15, 30 minutes, or even as long as an hour.  While brewing outside, the ability to hold that temperature is dependent on your weather that day.  Brewing in the summer on an 80 degree day, it might be a lot easier to hold 145-degree temperature than when  brewing on a 30-degree day.

The ability to hold temperature is largely dependent on the amount of grains being soaked (the more the grain, the more resistant the mash is to the weather outside).  The simplest way to stay in control is to wrap your mash tun in layers to keep the heat transfer down.  In the summer you may be fine simply turning off the propane heat and waiting the whole mash period, but come fall you probably need a wrap (towels work well), and come winter you definitely need a wrap.  An easy solution to this warmth is to get a good winter coat that’s big enough to fit around your mash tun and then throw that over it when you reach your rest temperatures.  It’s certainly a lot easier than turning on and off the heat on your burner to try to maintain a fixed temperature!