Temperature extremes can wreak havoc on a bow or an instrument; effort should be made to minimize the rate of temperature change it is subjected to. For instance, warm up or cool down the car before putting your bow and instrument in it; keep it in the case until it has warmed up to room temperature, etc. Do not leave your case in direct sunlight, in a closed car during hot weather, next to a heater and so on.
Humidity should be controlled as much as possible. Too much can cause warping; too little can cause cracking. 50% relative humidity is optimal; a comfortable range is 40-60%. Ideally you should focus on humidifying or dehumidifying the room your bow and instrument lives in (a wall mounted hygrometer can be used to measure the room’s relative humidity). Secondarily you may want to employ a case or instrument humidifier, keeping in mind that these implements only work in a closed case — once the case is opened, the humidity dissipates.