The Answer Really IS Blowing in the Wind

#2083 "Moisture Conveyors"

We need to back up and re-establish again why the wind really blows. You will only need your imagination and your Coriolis Hand for this. I will keep this simple with no mathematics and some home-made graphics. 

The pressure and volume of any gas varies directly with the temperature. Tall mountains of hot air can be found over the equator. The cold air found over the poles, makes more of a valley in the atmospheric ocean.  Pressure results from the weight of the ocean of air that lies above a particular point. The pressure is higher at the ground beneath the hot mountain of air and lower over the cold poles. 

The air under pressure naturally flows from the higher pressure found at the equator toward the lower pressure at the poles. This pressure gradient force (PGF) is the fundamental reason for air to move. Lines of constant pressure on the surface are called isobars. Another way to look at the variations in pressure over the globe is to map the height above sea level at which a particular pressure is measured. A line drawn on a weather map connecting points of equal height for a particular pressure is called a height contour. These isolines and are completely analogous to terrain height contours. The 3-D shape of the atmosphere can be depicted on a flat sheet in the same way that the geography of the land can be contoured on a map. 

Contour Isolines can map a 3-D Physical Entity
for Display on a Flat Piece of Paper

But this ocean of air is on a spinning globe. Your Coriolis Hand with the thumb pointing upward at the pole illustrates the sense that the Earth is rotating. For the Northern Hemisphere, your Coriolis Hand is your right hand and every moving thing is deflected to the right by the Coriolis force. The Coriolis force seems real enough but it exists only to explain that we live on a non-inertial, spinning frame of reference. In the Southern Hemisphere your left hand is the Coriolis Hand and everything is deflected to the left. 

Air parcels moving in the free atmosphere flows from high pressure to lower pressure but must be deflected by the Coriolis force. In the simplest geostrophic wind, only two forces are balanced – the PGF and the Coriolis force. If you put your left hand in the cold air and your right hand in the warm air, you are looking in the direction this wind is blowing. We will keep all of our graphics in the Northern Hemisphere in order to keep the explanation a bit simpler. 

Wind rarely blows in a straight line. A wind blowing in a curve introduces a third force - the centrifugal force. The centrifugal force is what keeps the water in the pail as you swing that bucket quickly overhead. The centrifugal force always points away from the center of curvature.  Air moving in a cyclonic trough has the centrifugal force pointing away from the PGF. As a bonus, if you curl the fingers of your Coriolis Hand cyclonically in that trough, your thumb must be pointing upward which encourages ascending air and cloudy weather. The real gradient winds are weaker in the trough. 

The opposite effect happens in an anticyclonic ridge. The centrifugal force points in the same direction as the PGF and your Coriolis thumb points downward (less cloud in a ridge). The real gradient winds are stronger in the ridge. Meteorologists call the wind that includes the centrifugal force with the geostrophic wind, the gradient wind and it is one step closer to explaining what we observe in nature.

The gradient wind is what we observe in the free atmosphere that lies above the planetary boundary layer (PBL). Gravity waves are exactly perpendicular to the gradient wind so now we have a better understanding of both the gradient wind and those wave clouds - as I painted in #2083 "Moisture Conveyors". 

Today's Weather Maps

The attached image is an actual series of maps depicting the pressure contours and isobars... They reveal just how complicated weather can be... The map below will only focus on the 500 mb pressure height contours (on the left) and the surface sea level isobars (on the right). This will not be on any exam. 

A real weather map explained... 

We now understand the wind in the free atmosphere. To understand the winds in the planetary boundary level (PBL), we need to add in a fourth force - friction. But let us leave those for next Science Tuesday. If you understand the wind, you can understand the weather. Good things are never easy. 

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick

Cloud Streets and Langmuir Streaks

#2573 "Halloween Sunset on Fire" 11x14 oils

In Making Chili Science, we used hand waving to describe how long lines of cloud form in the atmospheric planetary boundary level, the PBL. The cloud streets only required some wind, a boundary and a slight amount of instability perpendicular to the boundary. By the way, the Chili was delicious and heated on the wood stove! 

Chili Science Graphic using Empty Bean Cans

Similar lines occur on a lake and they appear in many of my paintings including "Halloween Sunset on Fire". The surface of the water is the boundary and it is easy to surmise that there can easily be some vertical motion of the water in the lake boundary layer.  The lake boundary layer is called the mixed layer depth or the MLD in physical oceanography. Those lines in the lake form within 30 minutes of the wind shift. 

The comparison of the oceanic mixed layer depth (MLD) to the atmospheric planetary boundary level (PBL) is identical. When the stresses at the surface are large (wind, solar heating, friction) and the stratification is small, the fluid becomes well mixed. The properties of the fluid such as velocity, temperature, salinity and pollutants are nearly uniform in the vertical within the depth of mixed layer - whether the fluid be air or water. 

The science in both the atmosphere and the ocean are essentially the same. The available explanatory graphics are typically much better in the oceanographic literature but we are working to change that starting with the Chili cans. 

Irving Langmuir (1881-1957)

It is important to be naturally curious and Irving Langmuir (1881-1957) was such a man. Langmuir was an American chemist and physicist who won the 1932 Nobel Prize in Chemistry for his work in surface chemistry. While observing windrows of seaweed in the Sargasso Sea in 1927, Irving developed his own brand of hand waving chili and he gave those streaks his name. In my painting above, I associate the calm and bright, mirrored bands as lines of downwelling. The rippled bands are upwelling in the Langmuir Streak Conceptual Model. Think of the ripples as a shattered mirror in thousands of pieces primarily reflecting the larger area of dark sky overhead. The flat areas of calm water is a great mirror angled to best reflect the sunset on the horizon.  

The similarities between Langmuir Streaks and Cloud Streets are clearly not coincidental. Provide a boundary for a fluid, a dash of instability and stir briskly with wind … and nature will cook up a line within that well-mixed layer. The accompanying atmospheric example of turbulent stratocumulus cloud streets is based on a photo I took - artistic licence is not a factor in the photo. 

This picture reveals all of the important facts. The wind direction is revealed by the gravity waves on the lake. The long lines of backlit cumuliform cloud are parallel to those winds. The process that created those lines are the same as described by our friend Irving Langmuir back in 1927 before the Great Depression. 

#2468 "COVID Contrail Singleton Sunset"

Langmuir Streak circulations also shape snowsqualls but we can look even further afield. Long lines of cirrus cloud can also be see at the top of the troposphere! It was not a big step for me to also call those icy bands of cirrus, Langmuir Streaks. The tropopause is the stable layer at the top of the portion of the atmosphere which contains much of our weather. The "trop" is a very effective boundary in the atmospheric fluid. The jet stream provides the wind that mixes everything together in the layer beneath the tropopause. The relative jet speed maximum provides the vertical instability. Presto, lines of cirrus appear in what was previously just called a single mass of baroclinic zone cirrus. 

The dominant line on the left in the accompanying painting is a jet contrail but the rest of the banding in the cirrus is the result of Langmuir Streaks. The gravity waves within the bands reveal the wind direction. I also used this painting to describe the drifting contrail. That contrail post contains the original photographic inspiration that is free of any artistic licence . 

Art and science are the same thing and both are inspired by observing the nature of things - being naturally curious. Just my opinion of course. None of this stuff is going to be on any pop quiz anytime soon. 

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick

Making Chili Science

#0839 "Cold Air Mass Cumulus" 8x10

This is the next installment in knowing the weather through observing the wind. 

I would tend to dream on quiet night shifts playing thought experiments in my mind when examining satellite imagery. The simpler the explanation, the more likely it would be to bear fruit and to circulate among the team. Here is one of my favourites. 

Imagine large pencils rolling along a table. I used colloured pencils a lot when working the forecast desk. Friction provided by the table and even a gentle push at the top gets the pencils rolling. In today's reinvention of that nightshift, we were making Chili during a snowstorm, so I used cans – same idea but tastier. 

These hand waving thought experiments from night shift required both of my hands with me looking along the direction of the wind in the accompanying graphic. Bodies are biologically constrained so that the fingers must be curled in the same orientation as the rolling pencil or bean can.

Two Cans Rolling on the Floor

Now imagine any non-uniformity like a locally stronger push on one of the cans. That push is equivalent to a locally stronger wind. This situation is exactly analogous to what happens in the planetary boundary layer (PBL) of the atmosphere. Friction at the surface slows the wind while aloft, the stronger wind is never perfectly uniform. 

Venturi Meter and Bernoulli's Principle 

Now let’s invoke Bernoulli's principle which states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure. Bernoulli said a lot more but without getting into the math, the local decrease in pressure encourages air inflow towards the locally stronger wind speeds. Air must flow from higher to lower pressure. The pressure gradient directs air inward toward my thumbs where the wind speed is stronger. 

What happens when the stronger wind speed is applied to the rolling cans over a period of time?

.

If I then turn my thumbs to align with the stronger wind of the speed increase, the thumbs must also align. The sense of rotation of the fingers from both hands, complement each other and a local updraft is created as you gaze down on your fingertips. Air convergence also creates an updraft – the air has to go somewhere! If the resultant lift is sufficient to reach the lifted condensation level for the air mass, a cumulus cloud is born. 

If we try to immerse ourselves in the atmospheric frame of reference, the local speed maximum must be shouldered by local speed minima, at least in a relative sense. On that night shift, I flipped my thumbs around to point in the opposite direction in order to simulate the relative speed minimum. This is a simple slight of hand. Do not rotate your body or move your arms.  Being biologically constrained once again, the fingers of my hand now circulated downward. The air in the local minimum was descending. If there was cloud, it was dissipating in the sinking air. 

The final step was to add the circulations formed with the local speed maxima to the speed minima. The circulations from my fingers merged together perfectly. The resultant pattern could be replicated as far as the wind regime permitted. 

A Locally Stronger Wind
Shouldered by Locally Weaker Winds

But why a line? On that night shift, I invoked the continuity equation. Air converging to a pressure decrease caused by a locally stronger wind speed can be self-sustaining. In an unstable environment that encourages the growth of cumulus cloud, the initial cumulus cloud will be guided by the wind. The conditions that caused the first cloud to form, will also result in a second and a third cloud. Before one can say “a mix of sun and cloud”, a line of cumulus guided by the wind has formed. On the bright side, a line of clear skies shoulders the street of cumulus. This pattern is simply copied and pasted across the landscape. Cloud streets happen every day... 

Snowsqualls are great examples of cloud streets

The wind rolling pencils or cans on a table top can create cloud streets parallel to those winds. This movie played out in my mind but was confirmed by the satellite image. More on these concepts to follow. 

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick

Know the Wind

 

#2221 "Singleton Squally Snow" 6"X 8" oils

Know the wind and you know the weather!

Understanding the winds in the free atmosphere was where we started a couple of years ago with "Cloud Shapes and Lines in the Atmosphere". The patterns in the sky are created by winds within the atmospheric frame of reference. The relative winds do the sculpting and the shapes are the same around the globe. Those patterns reveal the wind and those winds lead to a better understanding of the weather through conceptual models like the "Conveyor Belt" and "Isentropic Surfaces". You will find several entries on each top if you search my Art and Science Blogs. Each Blog tried to add another piece to the atmospheric puzzle. 

It is also important to appreciate the wind in the not so free atmosphere! The planetary boundary layer (PBL) is the lowest part of the atmosphere directly influenced by contact with the earth. Heat, moisture and momentum are interchanged constantly within the PBL. Friction turns the free atmosphere winds (20 to 50 degrees) while dragging the speeds down (sometimes by 50%). The surface winds you experience veer (turn clockwise in direction) and increase in speed in order to transform to the winds of the free atmosphere at the top of the PBL. 

Cumulus clouds within the PBL reveal that the air is positively buoyant. Such a happy PBL is referred to as the Convective Planetary Boundary Level (CBL). The turbulent mixing and interaction between the earth and the atmosphere can be very deep and even extend up to the top which is called that tropopause. A larger percentage of the free atmospheric winds can get to the surface in a CBL. 

Stratus clouds indicate that there is negative bouyancy within the PBL. A stably stratified planetary boundary layer (SBL) means that any exchanges of characterisitics between the earth and atmosphere occur within a shallow layer. The stable profile of a SBL limits how much free atmosphere energy can get to the surface. 

A quick look at the lowest clouds in the atmosphere will tell you what is happening in the PBL - this is the take home message. The painting from 6:00 pm on Friday February 9th, 2019 was in the wake of a very strong cold front. Multi-lake snowsqualls originating from Lake Michigan were making it as far as Singleton Lake. It was a cumulus PBL!

It was a day after a major winter storm crossed Ontario. The winds were gusting to 45 knots and perhaps stronger. I do not own an anemometer but things tend to break with wind gusts of 50 knots. The Arctic air over the Great Lakes was cold enough to create classic lake effect snowsqualls. What was very unusual was that the squalls reaching Singleton were actually originating from Lake Michigan with a bit of a boost from Lake Huron too. The weather is never dull.

The next few Blogs will develop some more observations from the planetary boundary layer (PBL)

Warmest regards and keep your paddle in the water,

Phil the Forecaster Chadwick

Where is the sun?

 

#1650 "Cloud Signs"

Where is the sun? That's an appropriate question to ask, especially during winter solstice. We need the sun for vitamin D and to lift our emotions. In real time, it is easy to find the sun most days. Just look up and you will find the bright orb in all except the darkest of overcast skies. But how to you find the sun in an image if the sun itself is not in the picture? Here are some Creative Scene Investigation tips. 

You are in luck if there is a shadow in the scene. Simply follow the elongated shadow back to the source like a sun dial and you have your answer. After that it gets trickier. 

A plein air artist typically paints with the sun on their back. There are several reasons for this. The sun feels good on your back especially on those cold and blustery days. Those same rays of warmth also illuminate the scene revealing the rich colours that might be there. The sun also lights up the canvas and the colours on the palette so that one can see the subtleties of the pigments. I tint every canvas with a colour that is complementary to the scene. That complementary tint also tones down the stark, bright whiteness of the canvas. The reflected glare off a white canvas can be as difficult and hard on your eyes as staring directly into the sun. A plein air artist rarely paints looking into the sun. If you see a back lit canvas from me, it is almost certain to be a studio work. 

If there are clouds in the image like in this example, you will also likely find your answer as to where the sun is hiding. How the cloud is illuminated reveals where the sun must be and the interpretation is really quite elementary. 

Front lit Cumulus

A cloud that is illuminated from the front must have a brighter centre while its edges and the cloud bottom must be darker. An observer of a front lit cloud has the sun on their back. The cumulus cloud on the left above is front lit. The middle of the cloud is brighter than the edges. The bright middle portion of the clouds contains billions of cloud droplets that reflect and refract all colours of the rainbow back to your eye. These colours combine to make white light. The explanation is similar to that often used to describe why whole milk is whiter than skim. 

The cloud shape also reveals the wind direction and relative speed. The upwind, arch shaped edge of the cloud is on the left indicates that the wind has a component that is blowing from the left - simple vector addition.

Back lit Cumulus

A cloud that is back lit is the direct opposite of a front lit cloud. A back lit cloud has a darker centre with brighter edges. The observer is looking toward the sun when viewing a back lit cloud. The light from the sun is forward scattered by the smaller number of cloud droplets on the edges. The dark middle portion of the clouds contains billions of cloud droplets that block much of the direct light from reaching your eye. The upwind, arch shaped edge of the cloud is on the right indicating that the wind has a component that is blowing from the right.

I took these two cumulus cloud pictures seconds apart from my kayak. The sun does not have to be visible to deduce the direction of illumination or the wind. The wind direction was the same in both images. As a result, I was looking easterly to view the front lit clouds (above left) and westerly to photograph the back lit clouds (above right). If you apply Creative Scene Investigation (CSI) just to these images, you could deduce that the timing of the pictures had to be early to mid afternoon with a light northerly wind in advance of a ridge of high pressure.

This simple observation of cloud illumination clearly reveals the location of the sun. Most people probably realized this intuitively. The cloud shape also reveals the wind direction. 

If you know any two of the following, you can easily deduce the third: sun position; time of day; direction of north. Given sunrise or sunset and with astronomical tables, the time of day and direction of view can be determined quite accurately. 

I typically include a clue about the time of day and location in the title of my art - anything to make "Creative Scene Investigation" a bit easier and more accurate. I neglected to do both in painting number 1650 - sorry. I was painting in Rockport on the St Lawrence at 1:30 pm Saturday August 29th, 2015. Here is a link to this painting on Fine Art America where I describe the details behind this plein air work. 

Keep you paddle in the water and warmest regards,

Phil the Forecaster Chadwick