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Through the global ocean conveyor belt and the world's atmospheric circulations from the three-cell model, there is a pretty clear picture on how climate is regulated around the globe through the transport of heat, momentum, and moisture through pressure and temperature differences caused by the unequal heating of the sun and Earth's varying seasons. This is on a planetary scale. On a synoptic scale, as mentioned previously, there are your weather maps associated with pressure systems and fronts that distribute heat across the mid-latitudes in order to attempt restoration of balance. However, this process begins with air masses.
Air Masses
Air Masses
How Air Masses Form
How Air Masses Form
Air masses are large bodies of air with uniform temperature and moisture, and they form when air sits over a region long enough to take on that region's characteristics. Their type depends on where they form (land vs. water) ad at what latitude (cold vs. warm). These air masses are a huge volume of air, usually hundreds to thousands of miles wide) with consistent temperature and humidity. They typically form over a source region, which is a broad, uniform area where the air can remain stagnant long enough to acquire surface properties. These source region requirements include: a large uniform surface (i.e. an ice sheet, desert, ocean), light winds so the air can remain stagnant, and several days to weeks of contact with the surface. In other words, source regions typically form under high-pressure systems since they are associated calm, stagnant conditions. Air remains over the region with minimal wind. Temperature and moisture transfer between the surface and the air. Air becomes cold/warm and dry/moist depending on the region. Once winds pick up, likely an influence from the Jetstream, the air mass travels, carrying its properties with it.
Classification of Air Masses
Classification of Air Masses
Air masses are named using a two-part code (shown in Table 1 below) based off of moisture and temperature characteristics. Usually, air masses are named first by using its moisture characteristic followed by its temperature characteristic. For example, c for continental and P for polar are combined to make cP or continental polar. Continental Polar air masses bring cold, dry air with clear skies and frigid outbreaks. See Table 2 below for all major types of air mass classifications, their characteristics, where they originate from, and what weather they can bring. However, as we know, the atmosphere is a chaotic system and the air is mostly never stagnant. Typically, these air masses interact and clash with one another figuring out who is the alpha male. This defines a front, a transition zone between two contrasting air masses that clash with one another.
Table 1: Classification of Air Masses
Explains how the different types of air masses around the United States.
All major types of air masses classified with their characteristics, meaning, and source regions.
Episode 54.mp4A song diving into the four types of fronts.
Cold Front formation.
Warm front formation.
Stationary Front formation.
Occluded Front Formation.
Fronts
Fronts
Cold Fronts
Cold Fronts
A cold front forms when a cold, dense air mass pushes underneath a warmer air mass, forcing the warm air to rise rapidly. Recall that cold air is more dense and warm air is less dense. Since cold air is denser, it does most of the work and hence is the lead player in this battle. Some key features include steep slopes causing a rapid lift of warm air, its fast moving, and usually produces a narrow band of intense showers and thunderstorms. Ahead of a cold front is typically warmer with partly cloudy skies with a higher dewpoint and southerly flow. If the airmass is hot and humid enough, clouds will tower into thunderstorms. Behind the cold front, cooler and drier air move in with typically westerly to northerly flow pulling in cooler air from the north. Clear skies and gusty winds are typical after its passage. On a weather map, these are blue with triangles pointing in the direction it is moving.
Warm Fronts
Warm Fronts
A warm front occurs when warm air glides up and over a retreating cold air mass. Whereas a cold front was due to an advancing cold airmass, a warm front forms when the cold air retreats. Some key features include gentle slopes and gradual lifting leading to more widespread rain, rather than narrow intense showers like a cold front. Its movement is typically slower than cold fronts, which is important to keep in mind when occluded fronts are discussed. Typically, warm fronts are associated with extended periods of light rain or drizzle, but can be very temperature dependent. For example, in the winter, south of a warm front there would be rain, along the warm front would be freezing rain since there is a shallow freezing layer, just north of the warm front transitions to sleet. The further north of a warm front you go, the surface cold air deepens and once the air column is deep enough, there can be snow. Southerly flow, partly cloudy skies, and higher temperatures is typical south of a warm front with easterly to southeasterly flow north of the warm front along with clouds, precipitation, and cooler conditions. On a weather map, these are red with half semi-circles pointing in the direction the front is moving.
Stationary Front
Stationary Front
A stationary front is exactly what the name speaks, and forms when neither air mass is strong enough to displace the other, so the boundary stalls. This can persist for days and if there is abundant moisture and rain for days, it can lead to flooding problems. Usually, these fronts are associated with cloudy, damp conditions, periods of rain that can linger, and sometimes a breeding ground for low-pressure systems to form in its initial stages. On a weather map, these are alternating blue triangles and red semi-circles pointing towards each air mass.
Occluded Front
Occluded Front
An occluded front forms when a cold front catches up to a warm front, remember a cold front is much faster, lifting the warm air completely off the ground. This happens mainly in mature midlatitude cyclones; you're large weather systems associated with areas of low-pressure and frontal systems attached to it and are most prominent in the midlatitudes. These fronts are typically associated with heavy precipitation, widespread clouds, and strong winds, which are variable. There are actually two types of occluded fronts. The first one is a cold occlusion, which occurs when the air behind the cold front is colder than the air ahead of the warm front. So, the coldest air plows underneath both the warm air and the cool air ahead of the warm front. The warm air is lifted sharply and the cool air ahead of the warm front is forced upward too, but not as aggressively. This is typically more common in western and central North America where regions of continental polar (cP) air is colder than the maritime polar (mP) air. On the other hand, a warm occlusion occurs when the air behind the cold front is less cold than the air ahead of the warm front. So, the cold front approaches, but the air ahead of the warm front is actually denser and colder. The "less cold" air behind the cold front rides up and over the colder air ahead (like the behavior of a warm front). The warm air is still lifted, but the structure is more gentle. This is typically more common in the Pacific Northwest where areas of maritime polar (mP) are behind the cold front, which is warmer than the continental polar (cP) air ahead of the warm front. On a weather map, this front is purple with a mix of triangles and semi-circles (blue + red = purple).
Dives more into EST and EDT and how it relates to UTC in various time zones in the United States.
Universal Time Coordinate (UTC)
Universal Time Coordinate (UTC)
This is the time used to regulate clocks and time worldwide. It is also referred to as Greenwich Mean Time (GMT). This is the time at the observatory in Greenwich, England at the prime meridian (0-degree longitude). Most weather models use UTC as it is not adjusted for daylight saving or daylight endings, so it is a good reference year-round. Each 15-degree of longitude adds one hour of time ahead of UTC in the western hemisphere and one hour behind UTC in the eastern hemisphere, which is why we have time zones. Through daylight endings time where we set our clocks back one hour is associated with Eastern Standard Time (EST). For daylight savings, clocks are set forward one hour and is associated with Eastern Daylight Time (EDT). EST is five hours behind UTC while EDT is 4 hours behind UTC, but also depends on the time zone you are in. For example, in the United States, there is Pacific time, mountain time, central time, and eastern time that are the four time zones. Each time zone further west is one more hour behind UTC.
Episode 59.mp4Discusses the three types of cyclones and the terms used to describe them weakening and strengthening.
All stages of a midlatitude cyclone based off of the Polar Front Theory.
Typical structure of a midlatitude cyclone. Warm sector between the warm and cold front where severe weather typically occurs. Cool sector north of the warm front. Northwest of the low is the coldest air and can be associated with snowstorms.
Typical comma-like structure to a midlatitude cyclone.
Cyclones
Cyclones
Cyclones are areas of low-pressure centers that spin counterclockwise or cyclonic in the northern hemisphere and clockwise or anticyclonic in the southern hemisphere. There are three types of cyclones: Extratropical cyclones, which originate in the midlatitudes and are cold-core lows, Tropical Cyclones, which originate in the tropics and are warm-core lows, and subtropical cyclones, which is a hybrid of both warm-core and cold-core lows. Cyclones that are most associated with clashes of air masses that form fronts are midlatitude cyclones (also called extratropical cyclones) where cold, polar air meets warm subtropical air, which is a zone called the polar front. This boundary is inherently unstable because the two air masses have very different densities and temperatures. When a disturbance occurs along this front, the atmosphere begins to reorganize itself into a rotating low-pressure system.
Polar Front Theory (Norwegian Cyclone Model)
Polar Front Theory (Norwegian Cyclone Model)
Polar Front Theory was developed by the Bergen School, which describes how cyclones originate, intensify, and decay along the polar front. It is the foundational model for understanding midlatitude weather systems. There are six stages in the formation and decay of a midlatitude cyclone:
Stationary Front (Initial Stage)
Recall, that a stationary front is when neither air masses are stronger than one another and therefore, remain stagnant or stationary. There is cold, polar air and warm, subtropical air lying side by side waiting for a signal. Winds blow parallel to the front in opposite directions and typically high-pressure is situated north and south of the boundary. There is no movement yet, but the boundary is primed for instability forming clouds and precipitation.
Frontal Wave
In order for the cyclone to form, there needs to be a small disturbance. Usually, this is accompanied by a shortwave trough aloft, a wave of energy that allows the two stagnant air masses beginning to clash. This creates a kink in the front where waves of air begin to form and a low-pressure begins to deepen from the temperature differences, and is usually supportive of upper-level divergence, for air to rise and the cyclone to deepen further. Warm air pushes poleward while cold air pushes equatorward and hence form the birth of a cyclone.
Open Wave
The open wave stage of a cyclone begins to form distinct warm and cold fronts along the area of low-pressure where the cold front orients from north to south of the low and the warm front extends eastward of the low, forming an open wave. The airmass between the warm front and the cold front is called the warm sector. The warm sector is warm, humid airmass that feeds energy to several thunderstorms and possible severe weather outbreaks depending on the time of year and the characteristics present. Precipitation forms ahead of the warm front and is widespread stratiform rain while along the cold front, a narrow band of heavier rain or storms develop. The cyclone now has a recognizable comma-shaped cloud pattern.
Mature Stage
The low-pressure center deepens as upper-level divergence strengthens called cyclogenesis due to the Jetstream. Winds increase due to the tightening pressure gradient and fronts sharpen as temperature contrasts strengthen. The cold front starts to move faster and begins catching up to the warm front. This stage marks when the storm is the strongest.
Occlusion
In this stage, the cold front overtakes the warm front, lifting warm air completely off the ground forming an occluded front. The cyclone reaches maximum intensity but begins losing its temperature contrast or its fuel. As a cyclone occludes, the original surface low begins to lose its instability because the warm sector is lifted off the ground. The temperature gradient weakens, and the primary low starts to fill. However, if there is still strong upper-level forcing and residual instability along the triple point, the point where all three fronts connect, a secondary low-pressure may develop and become the dominant area of low-pressure. The triple point is often the last place where surface warm air still exists, a strong temperature gradient remains, and deep lifting is maximized making it a prime location for secondary cyclogenesis. This secondary low can deepen rapidly, become the new dominant center, and shift the storm's track. The parent low often drifts and fills while the secondary low takes over.
Dissipation
Now, that the warm air is aloft and cold air at the surface wraps around the area of low-pressure, the cyclone lost its energy source and begins to dissipate known as cycloysis. Pressure begins to rise, winds weaken, and precipitation starts to fade. High-pressure fills in behind the low, but if there is still a residual temperature gradient and energy, a stationary front may form and initiate new cyclogenesis. If there is another shortwave trough that brings energy to the wave, then the process could restart all over again. Therefore, cyclones are the atmosphere's way of balancing temperature contrasts. They transport warm air poleward and cold air equatorward, helping to redistribute energy across the planet. Keep in mind Polar Front Theory only explains part of the story as it only describes surface characteristics and not the entire atmosphere as it is a 3-D structure, not a 2-D structure. This will be explained in a later course, but for now, we'll stick with the basics.
Air masses originate in source regions such as many world deserts, ice sheets, and oceans. When these air masses meet, it forms a boundary where energy is fueled between temperature gradients and forms clouds and precipitation. If there is a disturbance in the upper atmosphere such as a Jetstream bringing in a trough of cold air and it accompanies the warm air at the surface through daytime heating from the sun, a cyclone forms. Cyclones around the world act to redistribute heat toward the poles and cold air towards the equator, acting as one big engine for the atmosphere to restore balance. The key takeaways are as follows:
Air masses consist of uniform characteristics of moisture and temperature, which are classified by its moisture (whether on land or ocean), and its temperature (originating at what latitude).
When air masses mix and clash with one another, it forms a boundary known as a front.
There are four kinds of fronts:
Cold front, which forms when cold air advances and warm air is forced to rise.
Warm front, which forms when cold air retreats and warm air gradually rises up and over.
Stationary front, which forms when neither cold or warm airmass dominates and remains stagnant or stationary.
Occluded front, which forms when the cold front overtakes the warm front. There are two types:
Cold occlusion- When colder air is behind the cold front than ahead of the warm front.
Warm occlusion- When the coldest air is ahead of the warm front with the cooler, warmer air behind the cold front.
The Polar Front Theory is a model that explains the evolution of a midlatitude cyclone through six stages: Stationary Front, Frontal Wave, Open Wave, Mature Stage, Occlusion Stage, and Dissipation. A fundamental concept to restore balance in the midlatitudes.
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