Versione in Italiano QUI
Introduction
The Earth isn’t flat, unfortunately.
I say unfortunately because it would be a dream come true for every landscape photographer. With a flat Earth, clouds, which move parallel to the ground, would never dip below the horizon line and block sunlight at sunrise and sunset. Instead, there would always be a gap between the clouds and the ground where light would shine through, much to our delight.
The Earth isn’t flat, unfortunately
When we check the weather forecast, we usually focus on what’s directly above us and just a bit beyond. This is because we know, or at least have a sense, that a cloud above us will appear smaller and smaller as it gets closer to the horizon (whether the Earth is flat or not, lol), and so its impact on our composition will be visually less significant.
The perceived size of clouds gradually decreases as the distance increases (for example, a cloud 5 km away will appear smaller as it moves farther away).
This is intuitively and technically correct, but as is often the case in life, it’s the small things that make a difference. In this case, it could be that tiny, seemingly irrelevant cloud in your composition that determines whether the light will break through at sunset or sunrise.
So, I decided to create a (free) plugin for the popular portal Windy.com, allowing me to assess this possibility in the simplest way possible, and today I’m here to share it with you.
NOTE: You’ll find download instructions later as without reading the article, the plugin is absolutely useless
Disclaimer
Or rather, disclaimers—because there’s more than one, definitely 🙂
- This will be one of my typical long Frank-style articles, so don’t hate me—just grab yourself a nice cup of tea, coffee, or a glass of wine, and get comfortable.
- I’m not a meteorologist, and I might not even be a photographer—so take everything as the result of my practical experience and a bit of applied trigonometry. If you notice inaccuracies, glaring mistakes, or even conceptual errors, feel free to write to me!
- Using this plugin requires some basic knowledge of clouds and weather forecasting. If you’re unsure where to start, you might want to check out THIS article, which I wrote specifically on meteorology for landscape photography.
- Most importantly, let me clarify that this plugin won’t tell you whether you’ll get a colorful sunrise or sunset. Instead, it tells you if clouds will block the light when the sun is passing the horizon at sunrise or sunset (which isn’t necessarily the same thing). This means that the plugin acts like the Oracle of Delphi or the I Ching, where the answer or hexagram needs to be interpreted.
Let me reiterate: you should absolutely be skeptical of apps that promise (especially for a fee!) to predict the color of the sky. Except for very simple cases (which we could figure out ourselves), such as single cloud layers, it’s still impossible to make an accurate prediction. There are simply too many variables at play, many of which aren’t even tracked by the weather models we currently have access to. For example, meteorological models struggle to predict the base of cloud layers and non-cumulative cloud cover, let alone deliver specific data like cloud optical properties, vertical humidity and temperature profiles, solar radiation, atmospheric composition, scattering coefficients, and so on.
Of course, we hope this won’t be the case forever, and that AI might help us out in the future—but for now, we’ll have to rely on our oracles 😉
Clouds Horizon Distance Plugin in Windy.com
How the Plugin Works
The way the plugin works is very simple. Once it’s installed (we’ll get to that shortly), you just need to click on the map at the spot where you plan to take your photo.
At this point, the coordinates of the selected location are retrieved, and the elevation is determined through Open-Elevation (since Windy.com doesn’t provide that information).
With the elevation of the point and a bit of trigonometry, what I call “Cloud Horizon Distances” are calculated. This means where a cloud at a specific altitude meets the horizon for us, the observers at that selected spot (and elevation), due to the curvature of the Earth.
Windy.com then draws several concentric circles on the map, indicating the Cloud Horizons Distance for clouds at specific altitudes. Then, using SunCalc, the sunrise and sunset lines for the selected point on the current day are plotted. (Note: the sunrise and sunset lines aren’t infinitely accurate since they don’t account for the Earth’s curvature. Over large distances, you should see curves, not straight lines. For our purposes, this isn’t a problem at all since we only need them to tell us “where to look.” At the moment, I felt it wasn’t worth the time to focus on making this feature more precise. For more detailed sun position analysis, I always recommend using PhotoPills.)
At this point, by selecting the appropriate cloud layer on Windy, we can determine whether a cloud at a certain altitude will intersect the horizon along the sunset line, blocking the light.
Does this sound complicated? Don’t worry! Just think of it as a new tool for planning, and we’ll go through how to use it effectively together.
Clouds Classification in the Plugin
As you may already know (or should know), clouds are typically divided into three levels: High, Medium, and Low. For us photographers, this simplified classification works perfectly for many reasons, which we won’t delve into here to avoid making this article even longer and more tedious.
Classic classification of cloud layers (image from vecteezy.com)
The altitude classification varies slightly depending on where you look it up. Unfortunately, Windy.com doesn’t provide an exact altitude breakdown, so I decided to divide the three groups as follows:
- Low Clouds: clouds between 400 meters (1200 feet) and 1200 meters (3900 feet), represented by Blue circles.
- Medium Clouds: clouds between 2000 meters (6500 feet) and 4000 meters (13000 feet), represented by Purple circles.
- High Clouds: clouds above 6500 meters (21300 feet), represented by a single Red circle.
(The conversions to feet are approximate, sorry, long live the MKS system!)
The choice of this division and the fact that there’s no continuity between layers is due to the fact that what matters to us is the base altitude of the cloud, as it’s the part that will meet the horizon first (and thus block the light at sunrise or sunset), not its vertical development. (For those more experienced, I know this can be debated since the upper part of an already “set” cumulonimbus could pass over the Cloud Sunset Horizon of another cloud and block the light, but why be masochistic and complicate things even further?)
For Low and Medium Clouds, two main rings are drawn at the altitude limits of the group, along with a series of dashed intermediate rings at intervals of 200 meters (650 feet). This gives us better accuracy when we have precise altitude data.
I’ve chosen to have just one ring for High Clouds, which I consider the “Minimum Horizon Distance.” Essentially, high clouds never block the light for us but instead help create interesting colors after sunset. I believe this ring is useful to understand whether we have good cloud cover at least to the horizon, which could correspond to beautiful colors after sunset.
How to Install the Plugin
Now that we know what it does and why, let’s move on to how to install the plugin and use it for our photography planning.
As of the time I’m writing this, even though Windy.com has an app for both smartphones and tablets, plugins only work on the web version. You can’t use plugins on smartphones at all, but some tablet browsers do allow plugin usage. This isn’t a limitation of the plugin but rather a current limitation of Windy.com.
Once you reach the Windy.com website, all you need to do is open the menu at the top right and click on “Install Windy Plugin.” You’ll then see a list of plugins in a column, and among them, you’ll find “Clouds Horizon Distance.” Click on it, then press “Install Plugin,” followed by “Open Plugin.”
You’ll notice a text box appearing above the area where the weather model is usually selected, meaning the plugin has been loaded correctly.
You can now close the menu and start using it by clicking on any point on the map.
When you click on the map, you’ll see the circles drawn along with the sunrise and sunset lines, and the text box will be populated with information about the selected point.
Now, all that’s left is to understand what to look for and how to interpret it.
How to Use the Plugin
As we’ve already mentioned (but I’ll happily repeat because it’s the key to understanding everything), when you click on a point on the map, the Clouds Horizon Distance are drawn. These represent the distance at which a cloud starts to dip below the horizon line due to the curvature of the Earth. This depends on the altitude of the observer (the higher up we are, the farther the Cloud Sunset Horizon will be) and, of course, the altitude of the clouds themselves.
If, at sunrise or sunset, I see a cloud at the intersection between the Cloud Horizon Distance and the sunrise or sunset line (depending on the case), it means that cloud will block the sunlight.
In practice, here’s what you need to do:
- Activate the plugin (once you’ve installed it, it’ll always be in the menu, ready to be activated).
- Click on the map where you plan to take a photo and wait for the Cloud Sunset Horizons to be drawn.
- From the text box, note the time of sunrise or sunset (depending on which you’re interested in).
- Move Windy.com’s time slider to the hour of sunrise or sunset (again, depending on which you’re interested in).
- Select the weather model you’re interested in (I recommend revisiting THIS article).
- Select the Windy.com layer corresponding to the first cloud layer you want to analyze (I usually start with the low clouds).
- Check if there are clouds on the map at the intersection of the sunrise or sunset line (depending on which you’re interested in) and the Cloud Sunset Horizon for the selected cloud layer.
- Repeat the same process for the remaining cloud layers.
Over and over: if clouds are at these intersections, the light won’t pass. (NOTE: I use these absolute terms with poetic license, as we all know there’s never absolute certainty in weather forecasts.)
Let’s look at a couple of practical examples, starting with a simple scenario and considering a single cloud layer.
Let’s assume we’ve reached point 6) in the list above and want to evaluate the behavior of Low Clouds for a sunset situation at the selected shooting location (in this case, my beloved Butt of Lewis Lighthouse in Scotland).
Let’s say we see something like this:
The most dreaded situation: a clear patch overhead with a cloud block at the horizon
As the photo’s caption suggests, this is the most commonly dreaded situation: a blocked horizon with a clear patch overhead.
By selecting the forecast from our chosen weather model at the indicated sunset time and selecting the Low Clouds layer on Windy.com, we can see that low clouds intersect the orange sunset line at their Clouds Horizon Distance (the blue ring zone) regardless of their exact altitude. (Note: I added the red crosses to the screenshot to make it clearer for you, but they don’t actually appear on the map.) This means that, if the forecast is accurate, the clouds will block the light at sunset.
To top it off, there’s a clear patch directly above us. Assuming for the sake of this example that this is the only cloud layer forecast for sunset, we might as well head to the pub. (Also, from the graph earlier in this article, we remember that the few low clouds visible before their Clouds Horizon Distance will be almost microscopic for us.)
Now, let’s imagine the sunset scenario looks like this instead:
Our dream situation
In this case, the situation is exactly the opposite. Along the sunset line, we have low clouds that seem to cover the sky from directly overhead to the first Clouds Horizon Distance (400 meters). However, from the first to the last Clouds Horizon Distance, there appears to be nothing, except perhaps something at the Clouds Horizon Distance for clouds at 400 meters. If this layer of low clouds isn’t as low as it seems (so, slightly above 400 meters), and if the forecast is correct, and assuming for this example that this is still the only cloud layer present, we might expect to see the sun set in front of us (as it sets over the sea).
While this could be considered an ideal situation (and it is), let’s remember that seeing the sun set doesn’t necessarily mean the clouds will color. In fact, this case is a perfect example, because even if you physically see the sun setting, the low clouds in front of you will never color. This is a situation where I would take advantage of the composition using side light with the sun at least 6 degrees of elevation, essentially at the start of the golden hour.
If we now wanted to step away from this ideal single cloud layer scenario, we would need to analyze the situation for Medium Clouds and High Clouds in the same way, and then put together everything we observe.
If you’ve understood how the plugin works (and I’m sure you have), this opens up endless possibilities for analyzing and interpreting the situation. Just remember that there’s no right or wrong situation, only one that’s more or less favorable for what you want to capture.
To make things a bit easier and spark your creativity, let’s now go through a few examples that take into consideration not just a single layer but all the cloud layers, so we can try to piece everything together to understand what we can expect.
Example 1: Medium Cloud Block
You know those moments when you’re at sunset, everything seems perfect, like the dream evening you’ve always wanted, and then suddenly a streak of something appears on the horizon, blocking the light at the last moment? More often than not, it’s a (not so friendly) layer of medium clouds, so far away they seem almost invisible and harmless when the sun is still high enough, but they turn into a dense, impenetrable wall as the sun approaches sunset (the same applies to sunrise, of course).
Let’s head over to capture a sunset at the beautiful Boscastle in the UK and see what’s on the menu for the evening:
Analyzing the individual layers, we can make these observations:
- There’s an optimal layer of low clouds that doesn’t seem too dense, which is ideal for a shot with frontal light, and the Clouds Horizon Distance appears relatively clear (blue rings).
- We have medium clouds only at the Clouds Horizon Distance for medium clouds (purple rings).
- There’s excellent high cloud coverage, ready to reflect colors even after sunset.
If it weren’t for the medium cloud layer, we could expect a very interesting sunset in my opinion. However, the medium clouds have decided to crash our party, and although they won’t contribute anything to our composition, they are crossing right at the intersection between the Clouds Horizon Distance and the sunset line (the orange line). If the forecast is accurate, it means they won’t let the sunlight pass, neither directly to us nor to the high clouds that were ready to act as reflectors.
What to do in such a case? Should we go home?
If the situation is exactly as described, since the medium cloud layer seems to break up a bit further south, I would definitely stay on location, especially if Windy.com shows that the wind is pushing them northward (clearing the Clouds Horizon Distance). In that case, I’d hope the forecast isn’t entirely accurate and take my chances.
Otherwise, if it’s not looking good, I highly recommend the excellent pub “Cobweb Inn” in Boscastle 😉
Example 2: High Clouds Save the Day
Let’s now head to my beloved Gulf of Poets and analyze a situation that shows just how crucial it is to evaluate each cloud layer individually, and how what seems like a total disaster can turn into a success.
Suppose we want to evaluate a classic sunset from Lerici or Tellaro, looking out over the Gulf toward the sunset line.
Analyzing the individual layers, I believe we’d agree on the following:
- There’s a fairly dense layer of low clouds covering us (57% cumulative cloud cover), which also crosses the Clouds Horizon Distance (blue), but doesn’t go too far beyond (important!).
- There are no medium clouds.
- There’s abundant high cloud cover, extending from the minimum Clouds Horizon Distance (red) all the way overhead.
Looking at just the low cloud layer, we might conclude that light will have a hard time breaking through at sunset, as the clouds are passing over the Clouds Horizon Distance and the sunset line (orange). We wouldn’t expect to capture bold light, so perhaps we’d aim for a more dramatic image of dense, low clouds.
However, the combination of no medium clouds and a plentiful supply of high clouds makes us think that once the sun sets, the high clouds could color up, since there are no medium clouds blocking the light. If we’re lucky and the cloud cover predictions are correct, the low clouds will have gaps (remember the 57% cumulative cloud cover forecast!), allowing the colors above to shine through. This would create a beautiful contrast between the brightly lit high clouds in pastel hues and the defined, dark low clouds.
Honestly, this is one of my favorite situations, and I always hope to find it!
As you can see, once again, although the plugin doesn’t directly tell us if we’ll have a colorful sunset, it gives us all the tools to understand what will likely happen, allowing us to play our hand with an advantage!
Example 3: Medium Clouds Cooperating (“Kermorvan Effect”)
The statistics on medium clouds, unfortunately, don’t often work in our favor. Due to the typical extent of these layers, they frequently cross their Clouds Horizon Distance (remember, the purple rings), with the effects we’ve already discussed.
However, once in a while, the planets align, the gods of meteorology smile upon us, and the medium clouds are in just the right place.
Let’s move to Brittany, and head to the magnificent Kermorvan lighthouse for a sunset shoot. Here’s the breakdown of the cloud layers at sunset:
What you’re seeing is what Alessio and I lovingly nicknamed the “Kermorvan Effect,” as it was the first place where we had the luck to photograph these conditions. (The photo, HERE)
In essence, you can see that:
- There are no low clouds forecasted, neither above us nor at the Clouds Horizon Distance (blue rings).
- There is a layer of medium clouds that extends overhead but does not affect the Clouds Horizon Distance (purple rings).
- High clouds are present, though they play a minor role as they are largely covered by the medium layer.
What you can expect in these cases is for the medium cloud layer to light up with a lot of color, while the high clouds contribute to some color up to their Clouds Horizon Distance.
As you can guess—and have probably experienced—this is quite a rare scenario (since I’ve been shooting, I think it’s only happened to me three times). But when it’s forecasted, you’ll definitely want to be at the right spot!
Conclusions
As you’ve probably gathered by now, the idea behind this plugin isn’t to provide absolute answers but to offer an additional tool for analyzing our photography adventures.
The examples we’ve covered are just a few, and as you well know, the situations we can encounter are almost infinite. What matters is having a new method of analysis, and with time and experience, I’m confident you’ll be able to instantly recognize how to handle each situation at a glance.
As always, if you’d like to dive deeper into these topics, you’re welcome to join my WORKSHOPS in the field or book an ONLINE LESSON, where we can discuss all of this and work through real cases together!
In the meantime, remember that this is an open project, so I hope to keep adding more features over time. If you have any ideas or are knowledgeable in the field and think you can contribute to its development, don’t hesitate to reach out to me HERE!
This plugin was created as a free tool and will remain free because having fun in the field is a right everyone should enjoy <3
