Hikers put on thick clothes when climbing a high mountain for protection from the weather as they go up high.
As we get higher up to the mountain, the temperature decreases because of the relationship of altitude or elevation, temperature and pressure. As the elevation increases, the atmospheric pressure that surrounds the object is affected. Atmospheric pressure is the weight of the air pushing down to the object from above. As the height increases, there is less air present on that level therefore, the pressure decreases. As the pressure decreases, the air molecules spread out further from each other (which makes the air expand more) and the temperature decreases.
Hikers put on thick clothes because as the they go up high, the temperature increases where it can reach a negative degree Celsius - temperature, depending on the height of the mountain. This is to prevent freezing their body while hikers are up in the mountain. If their bodies are exposed to extreme low temperature, it can lead to disastrous situations.
For more information:
Hikers needs to put thicker clothes when they climb a high mountain since temperature decreases as altitude increases in troposphere.
When we climb a high mountain, it is still part of the troposphere layer of the atmosphere in which temperature decreases as altitude increases. Lower troposphere is warmer compared to the upper troposphere because of the heat coming from the interior of the earth.
Hope it helps...=)