The Ultimate Introduction To Weather

What makes weather? High pressure at the surface? Cyclones? Low pressure and clouds? What does it mean and how does it happen?

Preface

This page is designed to start from the beginning and cover the major factors that create and affect weather. Not just the basics, but the deeper explanation ‘why’…all while keeping the concepts easy to understand. This is not an easy task – for me as the author…or you as the reader! The topics on this page span physics, fluid dynamics, thermodynamics, and more. It’s not perfect, but I’m glad you’re here. Take your time reading, but please send me feedback if any part was especially helpful or difficult to understand! Hope it helps! -Eric

Earth and Sun

Most of us have learned the basics of the solar system, but here is a quick refresher since this is the foundation for our journey.

• The Earth orbits around the Sun (1 full orbit = 1 year)
• The Earth rotates counter-clockwise on it’s axis (1 full rotation = 1 day)
  • Due to rotation, something called the Coriolis Effect happens
• The Earth is made up of mostly water, but almost 70% of land is located in the Northern Hemisphere
• The Earth’s rotation is tilted on an axis relative to that orbit. Due to this tilt & orbit…
• The Equator experiences the most time ‘close’ to the Sun – this is why it’s always warmer near the Equator
• The poles experience the most time ‘distant’ from the Sun – this is why it’s always colder near the poles
• The Earth experiences times of the year where similar areas are closer to the Sun vs. other times (seasons)

Takeaway: The Sun disproportionally heats the Earth throughout the day and year. The strongest heating is near the Equator and the weakest is near the poles. More land being located in the Northern Hemisphere adds additional variability to surface heating.

Here is an image that shows the rotation of Earth on a tilted axis, the orbit around the Sun, and the seasons that result.

Rising & Sinking Air = Pressure Differences = Wind

As you recall from physics class, taking a hot shower, boiling water, or watching a hot air balloon…
Warm air is less dense and will therefore rises. Cool air is more dense and sinks.
When warm air rises, it creates a void at the surface – an areas of low pressure is created at the surface
When cool air sinks, more mass pushes down at the surface – an area of high pressure builds at the surface.

Pressure Gradient Force
As you blow up a balloon or tire, the air inside starts building pressure. This pressure is higher than the surrounding atmospheric pressure. If you create an opening in the balloon/tire, air from inside the balloon will try to escape to balance with its environment. If it’s sudden enough, the air will rush out; the balloon will pop. Air always wants to travel from areas of higher to lower pressure. This air movement is what we feel as Wind. It is controlled by something called Pressure Gradient Force which is basically the difference between two areas of different pressure (e.g. inside the balloon and outside).

In this case there are just two pressure values. But as we’ll look at later, high/low pressure systems in the atmosphere aren’t as fine-line and have a gradient (gradual) transition between them. The broader the gradient, the slower the movement of air (wind) is. The tighter the gradient (a balloon), the faster air (wind) will move.

Takeaway:
Warm air rises; Rising air creates a void: low pressure
Cool air sinks; Sinking air creates more mass: high pressure
Wind is air moving from high pressure to low pressure

Hypothetical: A Single Convection Cycle

Imagine a hypothetical scenario where the Earth was heated evenly at the Equator (and not at the poles). Warm air at the Equator would rise to the top of the atmosphere and spread out towards the poles. Just like steam in the shower hits the ceiling and spreads out. Eventually, it would cool and sink to the surface, finally traveling back towards the Equator. This heating/cooling loop is called Convection. Where air is rising near the Equator would create a single, large band of Low pressure that wraps the Earth like a belt. Where air is sinking (poles), would create two large areas of high pressure.

As described in this hypothetical scenario, the Earth would have one big Convection cycle. See image below.

Global Circulation (Convective Zones and Wind)

In reality, there isn’t just one convection cycle on Earth. Warm air does rise at the Equator, but it cannot make it all the way to the poles before it cools/sinks and returns to the Equator, making the first convective “zone”. This happens a second time, further away from the Equator, where air does make to the pole. A resulting third zone is sandwiched in-between the other zones. In the end, three separate zones (aka “cells”) are created in each hemisphere. This is often referred to as Global Circulation. These convective zones/cells are fairly permanent in location and work together to move air across the Earth. They are a major component to our climate (weather over longer periods of time – months/years.

As hot air rises, it travels away from the Equator at 0° latitude and eventually cools/sinks near the 30°-40° latitude. This area between 0° -> 30° is known as the Hadley Cells. Further away, air at the 60° latitude will rise and split in one of two directions. Some of that air goes towards the poles. This area between 60° -> 90° latitude is called the Polar Cells. The remaining air flows back towards towards the Equator, mixing with air from the Hadley Cells, cools, and sinks at 30°. This area from 60° -> 30° is called the Ferrel Cells.

Bringing back what we know about warm air risking/cool air sinking. The impact of these different cells creates bands of high and low pressure around different latitudes. The red arrows in the image below show the general surface wind that moves between areas of high to low pressure. However, due to the dynamics of the Earth’s surface, these bands are not perfectly uniform and winds do not flow perfectly between north-south either. We’ll discuss more of this later.

Takeaway:
Global Circulation describes three distinct convective zones in each hemisphere. Two of them are the result of heating.
The third is sandwiched in the middle and its rotation is a biproduct of the other two.
As a result, general bands of high and low pressure exist at different latitudes with winds that flow between them

Convective Zones are like gears in the atmosphere; Air is the chain.

The distinction between these three zones is important. The Hadley Cells and Polar Cells are similar because they are both “thermally-direct” and closed loop. Meaning, they are driven by the convection cycle: strong heating from the Sun, rising/spreading of air, finally a strong cool air sink/return.

However, the Ferrel Cells are not driven by surface heating/convection. They work like a gear, turning the opposite way, in-between the circulation of the other two gears (cells). Now, imagine a chain of air was connected between these gears. See orange arrows in the image below. The warm air that rises from the Hadley Cells follows the path of the orange arrows eventually making its way (through the United States) all the way up the North Pole. Frigid cold air that sinks at the pole, flows back towards Ferrel Cells where it is lifted and some of it is brought back across the the upper air of mid-latitudes.

If Ferrel Cells did not exist, weather on the whole planet would be different, but especially in the mid-latitudes. It plays a critical role in bringing areas like the US and UK warm moist air from the south and cool dry air from the north. The mixture of these different air masses is what results in different pressure systems, jet streams, fronts, storms, etc.

Takeaway: Ferrel Cells circulate in reverse, like a gear in-between two other gears.
They play an important role in weather of the mid-latitudes.

Coriolis Effect

As the Earth rotates, every place on the planet needs to collectively complete a full rotation within 24 hours (1 day). However, a sphere is not equally shaped top-to-bottom, like a cube. The Equator is the ‘widest’ part of Earth (on its axis). So, to complete a full rotation, a person at the Equator will need to travel much further (faster) than a person near the pole to complete a rotation. This is the same as going around a racetrack – the outline lane is the longest distance and would require you to travel the fastest. For racers, the inside lane of the track is desired because it is the shortest distance. For a sphere, this area is at the axis of rotation, for Earth – the poles. These areas rotate relatively slower.

As an object moves from one rotational speed to another, a deflection of its path is perceived from the surface.
This is known as the Coriolis Effect, which impacts the actual path wind travels.

Refer to the image below. Red lines represent wind moving from the Equator (fast rotation) towards the pole (slower rotation). Since the winds are moving to a slower area, their deflection must result in being ahead of their starting path. The green lines represents winds moving from slower to faster rotational speed. These winds will also take on a deflection, but the result needs to be behind the original path, since the destination will be moving faster. Both lines represent the movement of wind, but notice in the Northern Hemisphere, both deflections occur to the right (and to the left in the Southern Hemisphere).

Takeaway: The Earth’s rotation causes movement of air masses (winds) to take on a curved path. In the Northern Hemisphere, this path is always deflected to towards the right.

Convective Zones + Coriolis Effect = Prevailing Winds

As we learned, there are three distinct convective zones in each hemisphere. Now let’s add what we just learned about in the Coriolis Effect (deflection of winds), and you can see the impact to wind direction. These generally predictable wind patterns are known as the Prevailing Winds. The Ferrel Cells creates what are known as the Westerlies Winds, which aid in transporting warm moist air from the Equator towards the poles.

The Jet Stream

At the intersection of the three convective cells are large air masses of differing temperature. These prevailing winds merge together to form stronger flows of winds. Think like creeks coming together to form a river. These stronger rivers of faster moving winds are called Jet Streams. They are located in the upper-most layer of the atmosphere (200-300mb pressure or ~27k ft) and typically travel 70-150mph, and reach over to 250mph.

The two primary jet streams are the Polar Jet Stream (at the intersection of the Polar Cell and Ferrel Cell) and the Subtropical Jet Stream (at the intersection of the Ferrel Cell and the Hadley Cell). You can see their locations in the image below. Because of the United States’ latitudinal location, both Jet Streams play an important role in our weather; however, the Polar Jet Stream usually takes a more dominant role.

Longwaves (“Rossby Waves”)

The Polar Front is the name of the area where the air masses of Ferrel Cells and Hadley Cells meet up (seen around 60° mark on image above). Due to disproportionate heating, geographical features like mountains, and many other factors, the air masses that merge together here vary quite a bit. This is more prominent in the northern hemisphere where the majority of land is located. The cold outflow of air from the Polar Cell mixed with the warm tropical air being brought up by the Ferrel Cell create uneven boundaries separating the mixing of these pulsing air masses.

This results in very large boundary “waves” that span the Earth, somewhat resembled in purple in the image below. You can see the wave separating the blue (cold) polar air form the green (mid-latitude) air. Note: this green area would be approximately located under the Ferrel Cell (although in this image, not to scale).

These waves are known as Rossby Waves, but are also called Longwaves. They average between 3-7 full cycles across the Earth, where each individual wave ranges from 3500-5000 miles (wider than the United States). The waves themselves move slowly, but the air within them moves very quickly.

Jet streams follow along these waves, but not always perfectly. The jet streams meander a bit which plays a role in strengthening or opposing weather patterns at the surface. Rossby Waves and jet streams also migrate north and south (latitude) with the Sun throughout the year which have a great impact on our climate and seasonal weather patterns. They are also impacted by other factors like Ocean Temperatures (SSTs), ENSO patterns (El Niño and La Niña), etc.

Ridges and Troughs

All of this variability has a great impact on our weather. The meandering Rossby Waves and polar jet stream separates the cold polar air from the warm tropical air. But sometimes, the waves/jet stream begin to cut deeper into opposing air masses. This movement creates gradients of high and low pressure centers. Think of carving out a valley or building up a mountain…only not with physical land, with air pressure. When associated with high pressure, we call these Ridges. When associated with low pressure, we call these Troughs. Persistent ridges/troughs in Longwaves can remain in the same area for extended periods of time, causing prolonged weather events such as droughts or floods.

Shortwave

Within Longwaves, small kinks or disturbances can move through the flow causing weather to occur. They are caused by upper level fronts or pockets of cold air, very similar to the Longwaves themselves. These shorter-length waves embedded within the Longwave are called Shortwaves and usually last 3-5 days. They create smaller scale high/low pressure systems and are associated with the small scale “storms” when we think of “weather”.

Takeaway:
Longwaves = long waves that separate cold polar air and warm tropical air, spanning the entire earth
Shortwaves = smaller kinks embedded within Longwave and can be a major disturbance for weather

Analyzing Ridges & Troughs

Whether in Longwaves or Shortwaves, ridges and troughs separate different air masses of pressure. Sometimes theses ridges/troughs are shallow/gradual; sometimes they are steep. On land, you might look at a topographical map to tell how tall or steep a mountain is. On those maps, areas with the same height are connected with a line marking that height. With pressure, it’s very similar. However, it is common to look at maps with a constant pressure level (typically millibar) to see heights where that pressure exists. The closer the height contours are to one another, the steeper the slope, and the greater the Pressure Gradient Force is = wind is faster. The opposite is also true, the more spread out contours are, the shallower the slope is, and the smaller the Pressure Gradient Force is = wind is slower.

Upper-Level Atmosphere Analysis – Longwaves and the jet streams are commonly found on the 200mb (summer), 250mb (spring/fall), or 300mb (winter) constant pressure chart.

Mid-Level Atmosphere Analysis – Troughs and Ridges are commonly analyzed on the 500mb constant pressure chart.

Lower-Mid Atmosphere Analysis – Shortwaves can also be seen on pressure charts, although it may be easier to view them through a series of images in a loop. The 700mb chart is one of the most commonly used to analyze shortwaves.

Takeaway:
Constant pressure charts are used to analyze different atmospheric features such as jet streams, troughs/ridges, longwaves, and shortwaves)
The closer the contours = greater Pressure Gradient Force = stronger winds
The wider the contours = smaller Pressure Gradient Force = weaker winds

Convergence & Divergence

Imagine wind moving the along jet stream like cars do on the highway. For the sake of discussion, assume no cars enter or leave the road. In this situation, there are two reasons traffic would slow down: 1. fewer lanes ahead OR 2. encountering an obstacle, like toll booth (the old kind where you have to stop). In both situations, traffic will jam up and slow down. This is called Convergence. The same thing happens with wind, but due to Coriolis Force, we learned winds do not follow exactly in their lanes like cars do…they want to veer a little to the right (Northern Hemisphere).

As a result of the traffic convergence, cars slow down and create a backup. In atmospheric convergence, the winds can’t just jam up or stop. Due to Conversation Laws, the winds needs to keep moving or go somewhere. If they can’t be condensed any more side to side, they need to move up or down. Since winds in the jet stream are at the top of the atmosphere, the only place for them to go is down. So, when there is convergence aloft, it will force the excess air down, creating sinking air, resulting in High pressure at the Surface. Sound familiar?

Now imagine the opposite. Let’s say more lanes of traffic are added or you are leaving the toll booth. In either case, traffic starts speeding up again, and cars start spacing out from one another. This is called Divergence. When there is divergence aloft, air spreads out and/or begins accelerating. But again, due to Laws of Conservation, something needs to ‘replace’ the mass of air that is trying to move faster away (think of it like creating a void or vacuum). To replace this void, air is drawn up from the surface, creating rising air motion, resulting in Low pressure at the surface. Sound familiar?

Takeaway:
Convergence aloft = pushes down Sinking air = High pressure at Surface
Divergence aloft = pulls up Rising air = Low pressure at Surface

An Atmosphere Trying to Stay Balanced

The atmosphere is always trying to stay balanced between different layers. This balance is not just upper to lower layers, but side to side as well. This creates a matrix-like environment of high/low pressures winds and convergence/divergence occurring all the time around the Earth.

Gradient Wind Balance

Unlike cars on the road, winds do not actually flow perfectly straight or the same speed along the contours of ridges and troughs. There are a number of forces impacting winds and their speed/direction of flow. We’ve already discussed two of them, but there’s an important third.

Gradient Wind Balance is a balance of three forces that impact winds.
1. Pressure Gradient Force (force moving winds from high to low pressure)
2. Coriolis Force (force imposed from the Earth’s rotation, causes deflection to right in Northern Hemisphere)
3. Centrifugal force (force on an object that is being rotated, force is opposite to the center of rotation)

As winds move along, they don’t just pass by the the low/high pressure centers associated with the trough/ridges. They actually circle around them. The third force is introduced because winds rotating around a trough or ridge will have a Centrifugal Force acting against it in the opposite direction of rotation (pressure center). Just like spinning around on the merry-go-round, you feel Centrifugal Force wanting to pull you away from the center. All these forces work to balance out, but results in different wind speeds in different situations.

If you want more information on how these forces work, check out this video. Otherwise, just remember that winds will slow down and flow slower through a trough and speed up and flow faster through a ridge. We already learned that this slow down will mean convergence and a speed up will mean divergence. Now, we can place on a chart relative to the trough and ridge.

Takeaway:
Winds slow down before going into Trough – this creates Convergence before the trough)
Winds speed up after going away from a Trough – this creates Divergence after the trough

Low Pressure + Convergence + Coriolis = Cyclonic Rotation

As air moves towards areas of low pressure, a number of forces will act upon it. The image below shows the broader impact of these forces. Coriolis Effect with Convergence around a Low Pressure. As winds initially travel straight from high to low pressure (blue lines), they “Converge” on the low pressure. Influenced by the Coriolis force (red lines), winds will veer to the right, resulting in the actual wind path (black lines). Adding these all together will form a counter-clockwise rotation around the are of low pressure.

Putting the pieces together

When I was initially learning weather and looking at this image below, some parts of it made sense, but I had a difficult time actually seeing things like convergence/divergence or the connection with This is because the image is ‘idealized’ (perfect situation) and in reality, multiple forces will play a role in the direction and speed of the wind.

In the altered image below, I’ve drawn “Streamlines” which are green lines commonly found on 200-300mb height charts and represent how the jet stream winds actually flow. Here, it’s easier to see Convergence marked by the red circle as well as Divergence marked by the blue circle.

As we’ve covered already, the resulting impact of convergence aloft is sinking air, causing high pressure at the surface. The result of divergence aloft is rising air, causing low pressure at the surface.

High & Low Pressure Impacts on Weather

Surface High Pressure – mild, warmer temps
The sinking air associated with high pressure at the surface will suppress any rising air in that area, and thus prevent clouds/precipitation/storms from forming. While cooler air aloft is sinking, the sun typically has clear skies which allows for a lot of surface heating.

Surface Low Pressure – cloudy, precipitation, storms
If the rising air associated with low pressure is warm and moist, it will eventually condense and form clouds. Depending on other environmental factors, precipitation and storms can occur.

Fronts
Fronts are what happens when air masses of different temperatures collide.

Here’s one example: When cooler, sinking air from the Convergence aloft, flows down, it creates high pressure at the surface, This cooler air spreads out from the center of the high pressure and flows towards areas of low pressure. The counter clockwise spin of winds around a low pressure pulls this cooler air around and a cold front is created. This is a common ingredient for thunderstorms in the United States. Also notice in the images above, the surface low pressure center is actually ahead (east) of the trough axis.

Additional Reading

Weather Topics Landing Page (Menu)

Thunderstorm Ingredients