3.2. Daytime Temperature Behavior

So, let’s consider two things, first thing to be considered is what is called as the temperature profile as a function of height. So, we are saying that vertical convection happens as a result of thermal forces which means there’s a temperature difference. So, what is the temperature difference that will result in vertical movement of air masses? ... This height beyond that it will change is somewhere in the troposphere. Beyond that point the temperature profile changes for other reasons, it happens very far away and we are not interested in that region. We want what is happening very close to the surface of the earth.

The temperature profile of the atmosphere varies with height and is crucial for understanding how air masses move. Vertical convection occurs due to differences in temperature, meaning warmer air rises and cooler air sinks. This creates exchanges in air masses driven by thermal forces. At the Earth's surface, especially during the day, the soil heats up more quickly than the air above it, creating a temperature gradient where the ground is hotter than the air above. This profile is key to understanding weather patterns and pollutant transport.

Think of it like a pot of water on the stove. As the water at the bottom heats up, it becomes lighter and rises while the cooler water at the top sinks. Just as the heat from the stove creates movement in the water, sunlight heats the ground, causing the warmer air to rise and creating currents in the atmosphere.

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. ... The air is warmer than the soil, the soil has cooled down really fast and you have heat transfer is occurring in the reverse direction and this is slower process than radiation.

In daytime, the sun heats the ground more quickly than it heats the air. This rapid heating creates a positive temperature gradient, where the ground temperature is significantly higher than that of the air just above it. As the day progresses towards sunset, the ground begins to cool while the air retains its heat longer, reversing the temperature profile. This shift affects how heat and pollutants are transferred in the atmosphere.

Consider a blacktop road on a hot summer day. The asphalt gets very hot before the air does, and when you walk on it, you can feel the heat radiating from the ground. As the sun sets, the road cools down quickly, while the air above it takes longer to lose that warmth.

What happens when there is no radiation, say at 7pm, 6:30 or 7pm, sun is set, no radiation, what then starts happening? The soil then starts cooling,... it is taking time. So it’s happening slowly. ... 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.

After sunset, without sunlight, the ground cools quickly, cooling the air just above it slower than it cools itself. This can lead to temperature inversions where the air right above the ground is warmer than the air higher up. Such conditions can cause fog to form because condensation occurs when warm moisture-laden air meets the cooler air near the ground. The environmental lapse rate describes how temperature changes with altitude and can vary based on the time of year or weather conditions.

Think of a glass of cold water on a warm day. As the cold glass cools the air around it, condensation forms on the outside. In the same way, as the ground cools at night, the moisture in the warmer air condenses, creating fog near the surface.

Now, this is what’s happening in the system. So this region is called as the temperature inversion... we will see why this inversion is important from the point of view of pollutant transport.

Temperature inversion occurs when warmer air traps cooler air near the ground, preventing pollutants from dispersing. In this state, the usual pattern of rising warm air and sinking cooler air is reversed. This phenomenon is particularly concerning in urban areas as it leads to higher concentrations of pollutants at ground level, having significant implications for public health and environmental quality.

Think about a layer of warm syrup sitting on top of cold water in a glass. The syrup does not mix with the water below it, just like how the warm layer of air can trap pollutants in the cooler air beneath it, preventing them from rising and dispersing.

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