This article will cover:

I: Heat exchange at mid-latitude

II: Fronts

II.1 Cold front

II.2 Warm front

II.3 Occluded front

II.4 Stationary front

III. Cyclogenesis at the surface

IV. Cyclogenesis at upper level

IV.1 Trough

IV.2 Ridge

IV.3 Trough and ridge combined: the Wave pattern

IV.4 Trough and ridge at mid-level

IV.5 Jet-stream and upper level divergence

V. Cyclogenesis in 3 dimensions

At the end of the previous article about Wind, we showed the global circulation of the wind around the globe. For each hemisphere, the global wind circulation define 3 areas:

Equatorial and Polar regions are very specific in terms of weather and out of scope here.

Mid-latitude is governed by the Ferrel cell. The mid-latitude region is taken in sandwich between cold/dry air and warm/humid air, which makes this area a place where most of the heat transport happens and weather systems occur. This is also where most of the human population live on the globe and we will concentrate on the weather in the mid-latitude in this article.

For the Northern hemisphere, the northern part of the Ferrel cell is a place very dynamic where a lot of the weather occurs. Around 60° North, there is a imaginary line called the Polar front. On both side of this line there are very different weather conditions:

Those 2 very different air masses need to mix. The air mixing happens with figuratively by big fans pushing warm air to the north, and cold air to the south. The big fan that turn counter clockwise represent depressions in mid-latitude that sailors experience when sailing (eg. North Atlantic crossing from the USA to Europe, or the Vendée Globe sailors in the southern oceans while rounding Antartica).

This article will focus on explaining the mechanics of those depressions and how some of them intensify into powerful storms.

But before doing that, let's do a brief presentation of what is a weather front in the next section.

The above sketch shows 2 different air masses on both side of the polar front at 60° North. Meteorological symbols are used to easily identify what air mass is warm and what is cold, and also the movement of those air masses.

The cold air is replacing the warm air. The blue triangle points in the direction of the movement of the front.

The warm air is replacing the cold air. The red half circle points in the direction of the movement of the front.

An occluded front forms when a faster-moving cold front catches up to a slower-moving warm front, causing the warm air to be lifted off the ground between two cooler air masses. This process leads to a mixing of air layers and typically results in complex weather patterns such as cloud formation, steady rain, or even thunderstorms. After the occluded front passes, the weather often stabilizes and becomes clearer.

Stationary front is not moving as its name indicate.

The symbols used are a combination of the cold and warm front.

Respective of the above 2 fronts, the cold air is on the other side of the triangle, and the warm air is on the side of the half circles.

As seen in part I, the Depression / Big Low experienced by sailors sailing in mid-latitude are responsible for mixing cold and warm air at the polar front. Depression are experienced by sailors in the sign of falling barometric pressures; which is often accompanied by precipitation and strong winds.

This section will explain the physics of the formation of those depression. The reader can go further and read the material from https://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-13-extratropical-cyclones/

There are 5 stages of the cyclogenesis.

Stage 1: At the polar front around 60° North, cold air to the north and warm air to the south are separated by a stationary front. This is the starting phase or equilibrium

source: https://slcc.pressbooks.pub/

Stage 2: the formation of a frontal wave

A disturbance happens that will disturb this equilibrium and push the cold air to the south and the warm air to the north.

Stage 3: a newly developed cyclone

Stage 4: Mature cyclone, formation of an occluded front and triple point

The cold front catches with the warm front.

Stage 5: Dissipating phase

The above model describes what is happening at the surface. The air converges at the center of the Low and rises, like air in a chimney. But the rising air needs to find favourable conditions at upper level of the atmosphere to keep rising and then evacuate. This can be compared to a fire in a chimney. The chimney needs to help the rising of the air otherwise the fire will choke.

We will then move in the next chapter at upper level to find what are the favourable conditions for a depression to intensify and become a powerful storm.

For readers in the Southern Hemisphere, the images above may be confusing, as low-pressure systems rotate clockwise in that region. To save you from getting a stiff neck, we’ve flipped and combined the images below.

At upper level we use geopotential height rather than isobar. Those maps look a bit different and Low and High are represented by Troughs and Ridges.

A trough relates to low pressure. Readers of weather maps know the general Low system which shows a closed circle of the isobars. The Trough is an elongated area of lower air pressure, without a closed isobaric pressure contour. It can be imagined as a valley.

The picture above represents 2 things:

Image: pressure lines can be imagined as height on a trail map. If you are hiking on the trough line northward, you would be walking down a valley you will be surrounded by terrain on both sides.

A ridge relates to high pressure. The ridge is an elongated area of higher air pressure, without a closed isobaric pressure contour. It can be imagined as a hill.

The picture above represents 2 things:

Image: If you are hiking on the ridge line southward, you would be walking up a mountain with cliffs on both sides.

Trough and Ridge do not happen on their own. They happen next to each other and they somehow combine to make a wave pattern.

The wave pattern clearly shows that air is mixing, like the big fans we mentioned in part I.

Ridge is associated with with warm air, high pressure aloft, and a more stable atmosphere. This tends to result in dry, fair weather conditions underneath the ridge.

Trough is associated with cool & deteriorated weather and the ridge with warm & mild weather, as illustrated below.

Trough and Ridge happen both at the surface and in altitude. The ones in upper level of the atmosphere are the ones who drive global weather patterns which in turn influence what happens at ground level.

Forecasters are like detectives (see the TV show "the rookie" 😀) and they use the middle of the atmosphere (500 mbar, or around 5 kilometre up) as the place to find clues of where a storm will develop at the surface. Storms develop if the air is rising, see class 3 about Clouds. So the forecaster looks at the 500mb Geopotential map for areas where the air is rising. One may ask how can a weather map indicate where air rising?

Air rising occurs where air converges, which in turns happen when the air is turning or spinning counter clockwise in the North hemisphere. The turning and spinning is called Positive Vorticity. So basically all the forecasters need to do is to look for areas of positive vorticity to know where the storms will develop. .

more information at https://www.weather.gov/source/zhu/ZHU_Training_Page/Miscellaneous/vorticity/vorticity.html

Vorticity occurs due to 3 things listed:

The above theory is complex but really it can be broken down into a simple picture. The area of maximum overall positive vorticity is in front of the trough (blue area below). This is the danger zone and needs to be paid special attention. The back of the trough has vorticity but less so less important (orange area below).

Below see the example of real 500 mbar of geopotential.

We have added below on the same map, the areas in front of the trough where storms/depressions will generate.

Storms develop where air is rising as seen in the section above.

The air will rise until a certain point (top of the troposphere at 10 kilometre high; which is like a wall the air can not go through). The air needs to evacuate/diverge so the vertical rising of the air can continue and the depression can strengthen. So looking at the upper level atmosphere (300 mbar / 9 kilometer high) is the place where the forecaster/detective will look to find indication of diverging air as ideal conditions for storms to strengthen.

The jetstreaks are where the wind is really strong above 200 knots. The Right Entrance and Left Exit regions of jetstreaks are areas where winds aloft diverge, allowing air below to rise. Those 2 areas are marked in red below.

More information can be found in : https://skepticalscience.com/print.php?n=1967

If we combine everything we saw before, here is where a depression will intensify into a powerful storm: