3.5. Environmental Lapse Rate

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? So, which means that I need to know what is the temperature profile as a function of height.

So, let us say that this axis is temperature, it is also the ground. This is height Z. So, what will be the temperature profile on the Earth's surface? First, this is environmental data, normally if I go outside now, what is likely to be the temperature profile?

The temperature profile refers to how temperature changes with height in the atmosphere. Vertical convection occurs due to differences in temperature, which create movement in air masses. To understand this, we look at a graph where the vertical axis represents temperature and the horizontal axis represents height above the ground. Typically, the temperature decreases as height increases, especially within the troposphere, the lowest layer of Earth's atmosphere.

Imagine standing outside on a sunny day. The ground feels warm because it absorbs heat from the sun, while the air higher up remains cooler. This difference in temperature causes warmer air at the surface to rise, leading to vertical movement, much like a hot air balloon rising in cooler air.

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.

In the daytime, solar radiation heats the soil more quickly than it heats the air. This causes the temperature of the ground to be significantly higher than the air temperature above it. The result is a temperature gradient where the ground is warmer, and the air directly above is cooler, contributing to vertical air movement as the warm air rises.

Think of how a pavement feels much hotter than the surrounding air on a summer day. The heat from the pavement warms the air directly above it, causing that air to rise and creating movement in the atmosphere.

What happens when there is no radiation, say at 7pm? 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.

At night, when the sun sets and there is no incoming solar radiation, the ground cools quickly. Although the air immediately above remains warm, the temperature difference shifts, leading to a situation where the soil is cooler than the air above it. This results in a reverse temperature gradient and impacts air movement as well.

Consider how in the early morning, the grass may be covered in dew after a cool night, while the air still feels warm. As the grass cools faster than the air overnight, this heat transfer difference influences the early morning temperatures.

This profile is called as an environmental lapse rate. It is called a lapse rate because it is the 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 refers to how the temperature changes with height above the earth's surface. It can differ based on geographic location and seasonal conditions. This rate helps explain why temperature profiles can vary at different times and places, affecting weather conditions and air quality.

Imagine the difference in temperature when you climb a mountain. As you ascend, you feel the air getting colder—this is a practical demonstration of the environmental lapse rate as temperature decreases with height.

So this region is called as the temperature inversion. The temperature inversion means generally in the daytime temperature is reducing as a function of height, but here temperature is increasing as height.

A temperature inversion occurs when the temperature increases with height instead of decreasing. This is unusual and can trap pollutants near the surface because warmer air above prevents cooler air below, and thus pollutants, from rising. Such conditions can lead to poor air quality and smog.

Think of a lid on a pot. When cooking, the lid keeps the heat inside. Similarly, during a temperature inversion, warm air acts like a lid, trapping cooler air, and anything in it, leading to pollutants gathering at ground level instead of dispersing.

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.

When warm air is released into a cooler surrounding, it experiences buoyancy, which allows it to rise. As it rises, the pressure decreases, and the air parcel expands, causing its temperature to drop. This process is significant in understanding pollution dispersion and meteorology.

Consider a hot air balloon. The warm air inside the balloon is less dense than the cooler air outside, allowing the balloon to rise. As it gains altitude, it cools down, demonstrating the principles of buoyancy and temperature change.

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

The mean mixing height is where the environmental lapse rate intersects with the adiabatic lapse rate. Understanding this height helps predict how pollutants will disperse in the atmosphere, especially concerning stability conditions (stable, unstable, or neutral). Each condition influences how pollutants spread or accumulate.

Think of mixing a bottle of oil with water. At the point of mixing (mean mixing height), the oil and water share a boundary. Similar to how pollutants behave in the atmosphere, where certain conditions dictate how well they will mix and disperse.

--