Lesson 2

The atmosphere is always changing from one location to another, but in order to fully understand the atmosphere, it is important to know the different scales of motion before describing some atmospheric patterns that change our daily weather. Let's say you are roaming the city on a Sunday afternoon feeling a breeze as it is a windy day. The breeze is very localized. Pan out overhead of the city to a news reporter reporting a football game above the stadium. The reporter notices swaying trees and streetlights among the entire city. Zoom all of the way to the top of the troposphere to a man enjoying his book passing the time on a United Airlines flight. The plane shakes back and forth, front and back and the captain explains ongoing turbulence as they pass through the city. He mentioned that a cold front, a front with advancing cold air, was passing through causing widespread wind to much of the eastern United States. How that pattern shapes and changes in one location could be different in another location across the globe. The following described above are various scales of motion that all work together to shape our daily weather. The various scales of motion are as follows from top to bottom: 

Planetary Scale

Think of an entire globe across the entire country. This is the largest scale of motion that oversees all around the globe or hemispheres (>2000 km) and typically lasts from weeks to months. These atmospheric circulations typically control different climate zones across the globe. Large-scale wind and weather patterns may influence the scale below this known as the Macroscale or Synoptic scale. 

Macroscale or Synoptic Scale

This is the scale of motion where most meteorologists tend to forecast longer-range patterns in the weather forecast on a scale from days to weeks. Instead of a hemispheric or global scale, zoom into a country, per se the United States. Synoptic Scale is on a national scale from 1000 to 2500 km. Think of the weather map your broadcast meteorologist displays on the TV. Areas of low-pressure, high-pressure, and frontal systems typically are examined at this scale as well as patterns higher up in the atmosphere such as the jet stream, which is later discussed in this lesson. Think of the airplane example above. 

Mesoscale

On a much smaller scale, now zoom into a place such as a reporter looking overhead of a city to even as large as an entire state or region like the Midwest. Mesoscale is typically on a length scale of 5 to 500 km and typically lasts tens of minutes to hours and even up to a day. Weather phenomenon that occurs on a regionalized scale fall into this category such as Lake Effect Snow that are conducive to the Great Lakes, a dryline as mentioned previously, the sea breezes you feel on a hot summer afternoon at the beach in Florida, or even the thunderstorms that form in Tornado Alley. 

Microscale

The smallest atmospheric scale that is typically less than 1 km in length and lasts seconds to minutes is called the microscale. This is you walking in the city feeling a powerful breeze, or as meteorologists would say a wind gust. Variations in wind within the city or location that is very localized is one example. Have you ever witnessed a dust deveil? These phenomenon typically occur in a very localized spot of intense daytime heating with parcels rising and beginning to spin. Kind of like a pile of leaves being lofted in the air and spinning around. Friction, surface heating, and obstacles tend to drive these very localized scales of motion. 

In the previous lesson, you learned that temperatures normally decrease with height in the troposphere, but saw that temperatures can increase with height in the stratosphere due to the ozone and in the thermosphere due to the proximity to the sun. However, the troposphere temperature profile can also increase with height. While there are normal, typical characteristics of the atmosphere that is the general rule of thumb, there are sometimes special cases that deviate from the norm. How do temperature inversions occur in the troposphere? 

Nocturnal Inversion

During the day, the sun heats the Earth. The Earth absorbs the sunlight's warmth making the ground warmer than the surrounding air. That is normal. However, as the sun sets, the Earth loses that heat and it radiates back out to space, which warms the surrounding air. Therefore, the surface becomes colder than the air aloft, so the temperature increases with height. This is called a nocturnal inversion, "nocturnal" meaning it occurs at night. 

Warm Air over a Cold Surface

A second way for a temperature inversion to occur is when warm air travels over a cold surface. For example, let's say it is the winter in Canada and there is a snowpack on the ground. The snowpack is allowing cold air to stick at the surface, but then warm air travels over that cold surface. This also creates warm air over cold air allowing temperatures to increase with height. 

Warm Front

A warm front is when cold air retreats and warm air slowly glide over the cold air at the surface. Again, with warm air aloft and cold air at the surface, this also creates a temperature inversion. 

Subsidence Inversion

Previously, it was discussed when there is surface divergence, air sinks creating high-pressure at the surface. Now, let's take that same high-pressure from the surface and place it around 5,000 feet above sea level (850 mb level). The air column has expanded because warm air rises and expands an air column, allowing pressure to increase aloft as well. However, high-pressure only occurs aloft if there is pre-existing warm air at that layer of the atmosphere. No thanks to parcel theory, if there is high-pressure, then the air parcel sinks. As an air parcel sinks, it warms and compresses to match the characteristics of the surrounding environment. The stronger this subsidence occurs, the stronger the warming, the stronger the high-pressure system is aloft. This is typically a scenario where you see a heat dome set up over your region creating a summer heat wave. This inversion can also act as a lid preventing air to rise above it and is typically why pollutants become a problem as they are trapped near the surface. 

The atmosphere is like a fluid and like a cocktail, it can be mixed. There are trillions of invisible gas molecules in the air. Wind, which is caused by the PGF, and turbulence act to mix the gases of dry air in equal ratios from these pressure differences. However, where do these pressure differences come from? The sun. The sun heats the Earth, but since the Earth is a sphere, its rays distribute across the globe unevenly causing unequal heating. This unequal heating from the sun causes imbalances within the atmosphere and thus leads to pressure differences and temperature contrasts that drive our weather patterns to act and restore balance. 

Typically flow above 2 km in the free atmosphere, flow is freely flowing and smooth due to the absence of friction, which is why the LLJ exists. The boundary of the atmosphere between the surface to 2 km that causes turbulence and mixing is called the Planetary Boundary Layer. Otherwise known as the friction layer. Due to obstructions like trees, buildings, and mountains, the flow is no longer smooth and becomes chaotic. Swirling, chaotic pockets of fluid motion that form when flow becomes irregular, energetic, and no longer smooth are called turbulent eddies. These eddies mix heat, moisture, and speeds. Warm air rises and cold air sinks, which creates these convective cells and loops of air with different properties. The irregular flows only exist from the sun and unequal heating, which is why friction is more prominent during the day. If there is a lot of hot air and the parcel is warmer than the environment, then it will break into the free atmosphere and thunderstorms will begin to form. These small-scale motions are within the microscale. While most turbulence occurs near the surface, it can also occur near the level of the JetStream as there are temperature and pressure differences aloft as well. This is why airplanes can also experience turbulence.