When starting a new application, it’s critical that you determine the environmental conditions your end device will live in. Your environmental conditions will affect how easily your product might reach maximum case temperature. Or maybe your user is in contact with your device and you need to design a safe touch temperature.
In many cases, the application is clear on what temperature range your device may be subject to. It may only require straightforward lab conditions (23-25°C at sea level) to perform well. Other times the application may need to accommodate extreme or harsh environments, which will drastically influence the end user’s experience with your product.
Most of the time you, the thermal designer, don’t have the capacity to control what the end user will do. Think of how you treat your poor smartphone all day; you constantly use it throughout the day, so you are generating heat and draining the battery. You may store it in your pocket next to your own body heat and no airflow to help cool it down. While your end application may not have such high demands as a smartphone, the environmental conditions your device will experience are critical to the overall performance and longevity of your end product.
After you have taken a moment to consider what temperature and altitude you expect, you might find that you have a large range of conditions.