3.4. Morning Temperature Recovery and Fog Lifting

Let us say that this is at 2pm, what happens as we go closer to the sunset? During daytime the radiation heats up the soil or the land faster than it heats the air. So, the radiation directly heats the soil. And as a result, this temperature of the soil is very high. You can see that normally, when you are in summer or in the peak daytime, the land is very hot. You can see that in summers, it is much hotter than what the temperatures in the air are. So, as a result of which there is a positive temperature gradient in this direction.

During the day, especially around 2 PM when the sun is at its peak, the soil heats up much more quickly than the air above it. This is because the sun’s radiation directly warms the ground, which in turn transfers heat to the air just above the surface. This leads to a temperature gradient, where the air close to the ground is significantly warmer than the air at higher altitudes. This temperature difference causes vertical movement in the air, known as convection.

Think of a campfire: when you sit close to it, you feel much warmer than if you are a few feet away. Just as the heat radiates from the fire and warms the air around it, the sun heats the soil, which then warms the air above it.

What happens when there is no radiation, say at 7pm, sun is set, no radiation, what then starts happening? The soil then starts cooling; it cools very rapidly, it has given up all its heat. Then you see a certain small decrease, the air is still hot but the soil has started cooling so you start seeing this kind of behavior.

As night falls, the sun sets and there is no longer any radiation heating the soil. The soil cools down quickly during the night, losing the heat it absorbed during the day. Initially, the air above might still be warm from the day's heat, which creates a situation where the soil is cooler than the air. This can lead to a decrease in temperature near the surface. As the night progresses, the air also eventually cools down.

Imagine leaving a hot cup of coffee outside on a cool evening: the coffee cools down quickly while the surrounding air remains slightly warmer for a while. Eventually, both will reach a similar temperature as the heat dissipates.

Then when you have morning 6am or 7am sun rises, it starts going reverse because now you will start seeing things like this. The reverse is happening, the land starts heating up very quickly and it then starts heating the air on top of it.

When the sun rises in the morning, the cycle reverses. The land, which cooled overnight, begins to heat up quickly again once sunlight reaches it. As the land heats up, it transfers that heat to the air above, leading to warming of the air. This process can clear fog that may have formed overnight because the heat causes water droplets in the fog to evaporate.

Think about how a blanket of fog can hide the ground on a cool morning. As the sun rises, it’s like turning on a heater—soon the warmth drives the fog away, just like the way a sunny day clears away the mist.

In winter conditions, extreme winters when you see fog in places where it happens. You can see that in the daytime you don’t see any fog. In the nighttime, the land cools very rapidly and if the conditions are humid and it is amenable for fog formation, it will form fog near the surface.

Fog typically forms during the night in winter when warm, moist air cools down close to the ground, allowing tiny water droplets to condense. When the sun rises, the ground heats up quickly, warming the air and leading to the evaporation of these water droplets. This process is known as 'lifting of the fog.' As the warm air rises, it carries the moisture upwards, clearing the fog from the surface.

Consider a cold winter morning when you see a thick layer of fog in a valley. As the sun climbs above the horizon, you can often see how the fog begins to rise and clear away, similar to how a lid is lifted off a pot of boiling soup.

This profile is called as an environmental lapse rate. It is called a lapse rate because it is temperature profile as a function of height. The environmental lapse rate varies from place to place throughout the day, season to season.

The environmental lapse rate describes how temperature changes with height in the atmosphere. Typically, as you go higher up in the atmosphere, the temperature decreases. However, there may be conditions such as temperature inversions—a layer where the temperature increases with height—caused by factors like thermal radiation that affect pollutant transport and air quality.

Imagine a cake with layers of different temperatures; the top layer is colder than the one below. If you place a hot cupcake in the air below—in this case, the hot air parcel—it will rise until it reaches the top layer, but if the top layer is warmer, it won’t rise easily, trapping the hot air and any pollutants within.

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