Why Mountains and Highlands are Colder ?

Highlands are an interesting climate zone with chilly weather and sometimes even snow capped mountains. Higher the elevation, colder the climate. But why does it feel colder in the mountains? How exactly does it gets colder with altitude? It's an age old question and the answer is not quite straight forward. Let's see how it may happen.

 

First off, Our experience of hot and cold when we walk outside is mainly determined by the air surrounding us. In other words, we are immersed in an ocean of air all around us which is what we refer to as the atmosphere. The atmosphere is composed of many gaseous molecules (nitrogen, oxygen, carbon dioxide, etc.). It is held in place by the gravitational pull of the Earth.

 

 

Bridge between the Hot and the Cold

Let's consider the boundaries of our atmosphere. Its lower point is the lands and the oceans. At its upper limits is the empty space.

The space is notorious for its extreme negative temperatures.  In contrast, the ground is roughly about 200 degrees (Celsius/Kelvin) hotter. This is a stark temperature difference across the atmosphere. By taking into account the negative temperature of the space and the positive temperatures recorded on the surface of the Earth, it is plausible to assume that the atmosphere would record a gradient of temperature across itself. 

 

Surprisingly, this is not what we observe in reality. While the temperature drops as we ascend for the first 10km, it reverses to get hotter beyond that and then again repeats to drop again a several times. It appears to behave differently at different levels of altitudes. In other words it is non-linear and we may encounter higher temperatures than the ground level at higher elevations. However, at least for the first 10km of our journey, there is an consistent decrease in temperature. This region is known as the troposphere and we will limit our observations to it for the sake of discussion.

The major driver of thermal energy is solar radiation from the Sun. Earth receives sunlight in day time and slowly loses its warmth at night. This fluctuation is reduced by atmospheric effects resulting in a more or less average ground level temperature.

 

 

Thermal Capacity Gradient of Air

The air molecules in the atmosphere exert its weight on each other leading to highest air pressures at the surface level. Here, more air molecules are packed densely together than at higher elevations. This density directly correlates with the capacity to contain thermal energy. Therefore, lower the level, the more resistant it is to temperature changes. The lower regions gains heat from the ground and remains closer to ground temperature as opposed to the higher altitudes. The higher we go, the farther it is from the heat source and hence colder.

Wind Gradient

Another interesting consideration we can take into account is the differences in wind patterns as we move up. Air molecules at the surface are densely packed (in other words: more viscous) due to atmospheric pressure as discussed earlier and are significantly harder to move than air higher up. Furthermore, air close to the earth's surface exhibit air drag, resistance to move. Therefore, in general terms the higher we go, the air becomes easier to move. Resulting winds could better cool the higher altitudes.

Changes in Day-Night Temperature Fluctuations with Altitude

The surface of the earth warms up during the day time due to solar radiation. Air molecules closer to the heated surface of the earth absorb some of the heat and ascend by way of convection. Such heated gases rise up to higher elevations where they are cooled by expansion. This convection process brings heat from the surface to higher elevations.

 

In contrast, at night time, the ground cools off and the convection is reduced. Thermal energy transferred to higher altitudes is reduced resulting in a greater cooling effect at night time. Therefore, temperature changes with a higher magnitude at higher elevations.

Summary

Temperature drops with increasing elevation within the bounds of troposphere. The surface of earth stores thermal energy which is slowly transferred to atmosphere by convection. Air at the lowest levels are the most dense, viscous, resistant to move and with highest capacity to store heat. This results in reduced temperature fluctuations. Closer to the surface means hotter climate. In contrast, higher altitudes have less dense air with low thermal capacity that is easily swept away. Convection is strong during day time and promotes heat transfer to higher levels. Absence of it during the night allows for greater drop in temperature in highlands.