The Low Down

Part One: Predicting the Weather by Richard Fox


A shelf cloud, from a thunderstorm moving through the mid-western planes (Near Konorado) in the USA bring heavy rain and hail.  Canon EOS 6D, Canon 16-35mm f/4L IS USM at 16mm, 1/5 sec at f11, ISO 100. A shelf cloud, from a thunderstorm moving through the mid-western planes (Near Konorado) in the USA bring heavy rain and hail. Canon EOS 6D, Canon 16-35mm f/4L IS USM at 16mm, 1/5 sec at f11, ISO 100.

When photographing the outdoors we are all slaves to the weather. Good or bad conditions can make or break a shot (or even an entire trip), so wouldn’t it be great if we could predict what that conditions would look like in advance? Is it possible? To a degree, says Richard Fox.

Chasing the weather is probably one of the most fundamental issues in landscape photography. But is being in the right place at the right time simply a matter of luck? Or can the ideal conditions be predicted in advance?

In reality, the type of weather we seek depends on the type of shot, mood and composition we want. Some photographers like to capture more subtle textures with long exposures, which requires an abundance of cloud  and ‘flat’ diffused light. Others prefer a slit of clear sky on the horizon at sunrise or sunset, giving vibrant warm tones and deep colour saturation. Not all of these conditions are easy to anticipate – for example while the first example is easy to see coming, the latter is not (which is perhaps what makes it so special).
 

It’s important to realise too that all weather forecasting is about probability. For instance, a good sunset needs both cloud in the sky and a clear horizon, and this scenario can be predicted fairly consistently. It’s harder, though, to predict whether the correct levels of cloud will be in the right places at the right times to really light up the sky like it’s on fire. 

Nevertheless, by looking at some of the most common aspects of weather, and how they can be predicted and related to photography, we can determine whether it’s worth getting out of bed at 4:30am or trekking to the top of a hill with your tripod and bag.

Weather forecasts and satellite imagery

Our local and global weather systems are pretty chaotic, in fact we all have come to learn that weather forecasting is often pretty inaccurate. Much forecasting is based on computer models these days, and traditional local forecasters are now few and far between. And while weather forecasting services such as those from the Met Office, BBC, Clear Outside or Weather Underground can certainly help you decide which day to venture out, I wouldn’t rely on them in isolation.
 
Hourly weather forecast for Rippon Tor Dartmoor. Image copyright Met Office www.metoffice.gov.uk.

I tend to regard forecasts as a trend predictor, with only the next 24 hours being relatively accurate. As I write this, the Met Office has started to forecast 100% cloud coverage throughout the first part of the day, becoming sunny for most of the afternoon, with skies clearing in the evening. But low clouds are particularly difficult to predict, and could spoil the party at the last moment.
With forecasts only taking us so far, the use of satellite imagery is probably the most useful tool for predicting cloud cover. Using sites like www.Sat24.com (also available as a smartphone app) will give you the last two hours of cloud cover, in both visible and infrared (good for the darker hours), broken down into 15 minute intervals. 
 
NASA satellite imagery taken from https://worldview.earthdata.nasa.gov/ showing a recent cold front moving though from the west. The clear skies behind would give some nice light and cloud cover in the west of England.NASA satellite imagery taken from https://worldview.earthdata.nasa.gov/ showing a recent cold front moving though from the west. The clear skies behind would give some nice light and cloud cover in the west of England.

When these images are animated, you can see where clouds are moving, how many of them are present, and if they are forming or disappearing. With time, you’ll start to see low, medium and high-level clouds as they move across at different speeds and directions, depending on wind direction. As we’ll see later on, the presence of cloud at different levels is absolutely key to predicting dramatic skies.

As well as the high-tech methods, it’s important to try to understand your own local weather patterns on a basic level throughout the year – in a similar way to gaining knowledge of a favourite location. You’ll begin to understand the prevailing winds, when rain is more likely, where clouds tend to break above the edges of high ground, and where mist forms. 

An example, here on Dartmoor, we have a prevailing westerly wind (like most of the UK), so cloud fronts come in from the Atlantic. Depending on timing, these fronts can clear to reveal the horizon for sunset in the west, or can come in for sunrise, giving cloud cover but a clear horizon.

Surface (synoptic) Pressure Charts

You’ll recognise synoptic weather charts from the forecasts on TV. They show the predicted surface pressure pattern using lines of equal pressure called isobars, and indicate areas of high (H) and low pressure (L) along with a central pressure value. High pressure is usually associated with settled weather while low pressure is normally associated with poorer storm-like conditions. 
 
A nice low pressure system moving northwest of Scotland. Image copyright Met Office http://www.metoffice.gov.uk.A nice low pressure system moving northwest of Scotland. Image copyright Met Office http://www.metoffice.gov.uk.

The pressure we see in these charts comes from the Earth’s atmosphere pressing down on the planet’s surface. We measure it in hectoPascals, which are also known at millibars. Standard pressure at sea level is 1013hPa, and a pressure system is defined as high or low relative to this value. The isobars on a weather chart – which join areas of the same pressure – are useful because they identify features like a high-pressure anticyclone (light winds, little cloud) or a low-pressure depression/cyclone (clouds form, rain is likely).

As well as pressure systems, synoptic charts also show weather fronts, which are labelled either ‘cold’ or ‘warm’. These are useful because they show different fronts coming in or moving away, or when a low-pressure system is on its way, which may bring rain and high winds (a morning to stay in bed perhaps?) The arrows give the direction in which a front may move, and the shape of the symbols the type of front: triangles for cold; semicircles for warm.

The importance of the sun

The direction of the sun changes through the year. Winter solstice, for example, sees the sun rising and setting much further south than the summer solstice, where it rises and sets further north. In the summer, the sun arcs high in the sky and sets in a more vertical direction, meaning the golden ‘hour’ (when sunlight changes from white to yellow then orange and red) is shorter. Here in Devon, I expect a golden hour lasting 61 minutes in the winter, but in the summer it’s just 53 minutes. This also changes with latitude too, getting longer as we shift further away from the equator, and with our height above sea level.
 
Burnt Ayrmers, Ayrmer Cove, South Devon at sunset in March. Between late spring to late autumn the sun sets behind the headlands. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 30mm 0.3 sec at f11, ISO 100.Burnt Ayrmers, Ayrmer Cove, South Devon at sunset in March. Between late spring to late autumn the sun sets behind the headlands. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 30mm 0.3 sec at f11, ISO 100.

For example, an observer at sea-level with their eye level at 5’ 7” (1.70m) off the ground will see the horizon at 2.9 miles (4.7 km) away when looking out to sea, for example. But for an observer standing on a hill or a tower 100 feet (30m) in height, the horizon is 12.2 miles (19.6 km) away. At 500ft (152.4m) it’s 27 miles (44km) off.

It stands to reason, then, that the higher up you are, the earlier you’ll see the direct light from the sun at sunrise, compared with standing at sea level. The same principal applies at sunset, giving you longer to catch those rays before they vanish. In fact, sunset is 1 minute later for every 1.5km you gain in altitude.

Plus at higher altitudes (and in the winter) the air tends to be cooler, containing fewer particles and less water vapour and haze – all of which means the landscape will be lit by comparatively stronger ‘cleaner’ light, the so-called ‘mountain light’. This is probably one of the most fundamental pieces of knowledge we need as landscape photographers, especially when you are on unfamiliar territory or in foreign climes.

However, on the flip-side of the coin and where low strata clouds are concerned, the higher you climb the bigger the gap in the horizon you need for the sun to shine through. So unless you have a clear horizon, you run the risk of being in the cloud base, rather than under it.

However, in this modern age we have some great resources for planning a shoot. Online services like Suncalc and The Photographers Ephemeris (TPE) are excellent, and smartphone-based apps, like TPE and Photopills, let you predict where the sun will be at different times while you’re in the field, and use GPS to accurately determine your location. Photopills and TPE also now come with augmented reality, so you can even line up your composition.

Cloud formation

Possibly the most important aspect of weather prediction for photographers is the formation of cloud. From ground level, clouds might all look the same, but in fact there are distinct layers of clouds – low, medium and high – which can all move independently in different directions and at different speeds. The highest ‘strata’ clouds can also break up into multiple sub levels.


 
By Valentin de Bruyn / Coton (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
There are 10 basic cloud types, excluding sub-categories and the more weird-and-wonderful types.

• High clouds are broken down into: Cirrus, Cirrostratus and Cirrocumulus 
• Mid-Level clouds comprise: Altostratus, Altocumulus and Nimbostratus 
• Low-Level clouds consist of: Cumulus, stratus, stratocumulus
• Multi-level clouds are called Cumulonimbus

The type of cloud formed can give rise to different conditions for landscape photography. Low-level cloud is closer to the photographer, so at sunrise or sunset any gaps that will let through the sunlight need to be much wider than the gaps in medium or high-level cloud. You might have seen this phenomenon at work if you’ve ever been standing waiting for the light to arrive, but the gap closes, leaving you watching stunning light about a mile away (usually we conclude we’re in the wrong place at the right time!). 
 
Leathered. Low strata cumulus clouds and fog break as the sun warms the clouds and land. Leather Tor, Dartmoor. Canon 6D, Canon 17-40 f/4L USM at 17mm, 0.3 sec at f11, ISO 100.Leathered. Low strata cumulus clouds and fog break as the sun warms the clouds and land. Leather Tor, Dartmoor. Canon 6D, Canon 17-40 f/4L USM at 17mm, 0.3 sec at f11, ISO 100.

It’s down to luck whether that gap in the cloud is in the right place as the angle of the sun is low to the horizon. And overall the behaviour of low-level clouds, such as cumulus, are harder to predict. They usually form mid-morning and disappear again after sunset, and may also form over the sea when the sea-surface temperature is warmer than the air passing over it (as they are classed as convective clouds).
 
Peak Lighting. Cumulus clouds drifting over at sunset looking towards Tre Crim in the Italian Dolomites. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 35mm, 1/40 sec at f11, ISO 100.Peak Lighting. Cumulus clouds drifting over at sunset looking towards Tre Crim in the Italian Dolomites. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 35mm, 1/40 sec at f11, ISO 100.

Larger confluent high-level cloud, like altostratus and cirrostratus, are often a bit more stable, but the light breaks they provide tend to be less dramatic. They also develop at sunrise and sunset. It’s not uncommon to be waiting for a sunrise only to see upper or mid-level cirrostratus or altostratus clouds form and block the intensity of the light.
 
Sligachan Port Ruiage. Red cirrus clouds as the sun rises over the Sligachan river behind the Red Cullins. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 35mm, 2 sec at f11, ISO 100.Sligachan Port Ruiage. Red cirrus clouds as the sun rises over the Sligachan river behind the Red Cullins. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 35mm, 2 sec at f11, ISO 100.

Clouds can be different colours too, depending on their constituents and the light falling upon them. A cumulonimbus cloud that appears to have a greenish/bluish tint is a sign that it contains extremely high amounts of water; hail or rain that scatters light in a way that gives the cloud a blue colour. 

A green colorization occurs mostly late in the day, when the sun is comparatively low in the sky and the incident sunlight has a reddish tinge that appears green when illuminating a very tall bluish cloud. A yellow colour is due to the presence of pollutants like smoke, dust and pollution and can display as an orange or pink colouration during the day or way before sunset or after sunrise.  
Red sky at night? 

A dramatic red sky is something we all like in our landscape photography, and is actually one of the easier phenomena to predict, as there are a few tell-tale signs. When we say, ‘red skies’, we are really talking about higher level clouds. In fact, the single-most important factor in forecasting a red sky is the gap between the clouds and the horizon. No gap; no red clouds – simple as that.
 
Altostratus clouds at sunrise with some distant cumulus/stratus. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 20mm, 1/25 sec at f5.6, ISO 100.Altostratus clouds at sunrise with some distant cumulus/stratus. Sony A7R Mk II, Canon 16-35 f/4L IS USM at 20mm, 1/25 sec at f5.6, ISO 100.

So high-level cirrus clouds can often become pink in an otherwise clear sky at sunset, and the spectacular sunset photos we all want, but this is less likely with mid-level cloud and much less likely with low-level cloud.

It’s not impossible though, and you won’t have long to wait after the sun dips down to wait and see. Cumulus clouds at low level will turn red just as the sun sets, medium-level clouds come next, then the highest strata, which can turn pink up to 30min after sunset. These timings are reversed at sunrise, of course, with high cloud turning pink first, before mid-level, then finally low-level.
So if you are really lucky – and the gap is in the right place – you can see how it’s possible to observe low-level clouds as grey, those at mid-level lit up rosy-pink, and the highest level remaining white. Spectacular stuff!
 
Long wave length red light penetrates the atmosphere much more than the short wavelength blue spectrum light. At sunset as it passes through more of our atmosphere as sunset but when overhead the light is less scattered and so more white (full spectrum).Long wave length red light penetrates the atmosphere much more than the short wavelength blue spectrum light. At sunset as it passes through more of our atmosphere as sunset but when overhead the light is less scattered and so more white (full spectrum).

And where does the red colour come from in the first place? As the sun dips lower in the sky it passes through many more miles of air than it does when it’s directly overhead at noon, for example. It’s dust and other particles in the air which scatter the blue light in the spectrum. Leaving only the remaining red light to give the sky its notable appearance. 

• Next month: Richard explores how to predict mist, fog and haze.


Article By Richard Fox