5.2. Influence of Mechanical and Thermal Forces on Dispersion

Okay, 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?

In this chunk, we introduce the concept of temperature profiles in the atmosphere and their relation to vertical air movement. When there is a difference in temperature between the ground and the air above, it can create a force called thermal force that causes air masses to move. Hot air rises because it is less dense than cooler air, leading to vertical convection. This means as the temperature of the air near the ground increases, it pushes the warmer air up, creating convection currents.

Imagine a pot of water being heated on the stove. As the water at the bottom heats up, it becomes less dense and rises to the top, while cooler water descends to take its place. This circulation of warmer and cooler water is similar to how warm air rises and cooler air sinks in the atmosphere, creating convection.

During daytime the radiation heats up the soil or the land faster than it heats the air. Therefore, the air closer to the surface is hotter than air above. What happens when there is no radiation, say at 7pm? The soil then starts cooling, and you start seeing this kind of behavior.

This chunk discusses how temperature varies between day and night. During the day, the ground heats up faster than the air above it due to solar radiation. As a result, the air just above the ground is warmer than the air higher up. However, as night falls and the sun sets, the ground cools quickly while the air above remains warmer for some time. This leads to a situation where cooler soil causes heat to transfer upwards from the warmer air, creating an inverse temperature gradient.

Think of a cozy blanket on a cold winter night. The blanket retains heat longer than the surrounding air, much like how warm air remains above cooler ground overnight. Just as the warmth from the blanket slowly rises until the room temperature stabilizes, warm air also rises until it cools as the night progresses.

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.

Here, we introduce the concept of lapse rate, which describes how temperature changes with altitude. The environmental lapse rate varies based on location, time of day, and weather conditions. It’s crucial for understanding how air parcels will behave as they move through the atmosphere. For instance, in warmer regions, the lapse rate may be higher because the air near the surface is heated more than at higher altitudes.

Consider climbing a mountain. As you ascend, you often notice that the temperature drops significantly. This is similar to the lapse rate: as you go higher into the atmosphere, you experience cooler air, demonstrating the principle of how temperature decreases with altitude.

Normally if I release a parcel here, what happens is, if its temperature is higher, it wants to go up. There are two things at play here. One is buoyancy which is making it go up, but as it goes up if there is no exchange of energy, its volume also expands like this and it cools.

This chunk explains the behavior of air parcels based on their temperature. When a parcel of air is warmer than the surrounding air, it becomes buoyant and rises. As it lifts, it expands and cools, following the concept of dry adiabatic lapse rate, which describes how quickly temperature decreases with altitude in a dry air parcel without heat exchange.

Think of a balloon filled with hot air. As long as the air inside the balloon is warm, it rises because it is less dense than the cold air around it. However, as it rises and expands, the air inside cools down, eventually becoming denser and causing the balloon to descend, similar to how air parcels behave in the atmosphere.

So, what we look at is the following. So, this is adiabatic lapse rate... and this environmental lapse rate... with some mechanical turbulence aids here, I push it up a little bit and that force that is pushing it up is mechanical force.

In this portion, we cover the stability of the atmosphere based on the temperature profile. We differentiate between stable, unstable, and neutral conditions. In a stable environment, an air parcel that is displaced will return to its original position, while in an unstable environment, it will continue to rise. Neutral conditions occur when buoyancy has little effect due to similar temperatures in the surrounding air.

Imagine building a stack of books. If you push a book slightly to the side and it falls back, that’s a stable situation. If you push it just right and it keeps toppling over, that’s unstable. If it wobbles but stays balanced, that’s like a neutral situation. This visual helps us understand how different atmospheric conditions affect the movement of air parcels.

This is the definition of what people call as the mean mixing height which is the intersection of the adiabatic and environmental lapse rates.

This section introduces mean mixing height, a crucial concept in estimating how pollutants disperse in the atmosphere. It refers to the altitude at which the temperature profiles of the adiabatic and environmental lapse rates intersect. This height indicates the level where mixing of pollutants can occur, influencing air quality and pollution dispersion modeling.

Consider a glass of water with a few drops of food coloring. When you stir it, the color spreads throughout the water. The height at which the color disperses the most effectively can be likened to the mean mixing height. By knowing this height, scientists can predict how pollutants will spread in the air, just as you can visualize how the color will evenly mix throughout the water.

--