You can set an early alarm and hope for fog. Predicting it is still tricky, though. Even if I put a green label next to it. A number alone isn’t enough to get me out of bed. I want to know what it means, too.
The free Fog Forecast tool shows you the chance of ground fog at sunrise for your chosen location over the next 5 days. You don’t need an account. Each morning has a score alongside the weather conditions behind it. That gives you a way to judge which morning is worth a closer look.
Predicting fog around sunrise
Ground fog, also called radiation fog, forms most often on clear nights with little wind and high humidity. The ground cools to near the dew point. Those are the conditions Fog Forecast looks for.
When you’re planning a morning’s photography, it’s the combination that matters. A still night alone isn’t enough reason to expect fog. Neither is high humidity. The tool therefore looks at cloud cover, humidity, wind and the difference between temperature and dew point.
The forecast is specifically for ground fog around sunrise. It doesn’t cover the whole morning. The score tells you how favourable the conditions are at your chosen spot around sunrise.
Choose the place, then the morning
When you open Fog Forecast, it immediately shows the forecast for Wijchen in the Netherlands. I live there, and the page had to start somewhere. If you’re planning to photograph somewhere else, search for that place or address. You can also use the location button to check your current location.
You’ll see the next 5 days for your chosen location. Each day shows the date, sunrise time and a score out of 100. Alongside those are cloud cover and humidity as percentages, and wind speed in km/h.
I’d start by looking through the days. A morning that looks promising and fits your diary is a useful starting point. The highest score won’t do you much good if you have to be somewhere else that morning.
What does the score mean?
The score sums up the forecast. The labels make it easy to compare the days:
- High: a score of 70 or above, highlighted in green.
- Moderate: a score between 40 and 69.
- Low: a score below 40.
If you see ‘Chance of fog: 72/100’, read it as a score, not a percentage.
A high score is a reason to look more closely at that morning. A low score means the calculation suggests less favourable conditions. At 69, the label says Moderate. At 70, it says High. I wouldn’t let that single point decide my whole morning.
Look at the values below the score as well. You might see light winds alongside plenty of cloud cover, for example. That tells you the conditions aren’t all equally favourable.
Cloud cover, humidity, wind and dew point
The calculation starts with four factors. Each gets a score out of 100, then those scores are averaged. These are the thresholds for favourable and unfavourable conditions:
- Cloud cover: 35 per cent or less is favourable. At 70 per cent or more, it counts as unfavourable. That reflects the clear nights when ground fog most often forms.
- Humidity: 92 per cent or more is favourable. At 75 per cent or less, this factor gets an unfavourable score.
- Wind: 8 km/h or less is favourable. At 18 km/h or more, wind counts as unfavourable.
- Temperature and dew point: a gap of 2 degrees or less is favourable. A gap of 5 degrees or more gets an unfavourable score.
The score changes gradually between those thresholds. A value just outside the favourable range doesn’t immediately get the lowest score. Useful to know when you’re looking at the numbers: these aren’t four on-off switches.
Cloud cover, humidity and wind are shown in the forecast. The gap between temperature and dew point counts towards the score, but isn’t shown alongside those three values. So you can’t fully work out the final score from the figures displayed under each day.
The calculation checks two points in time: the hour before sunrise and sunrise itself. It uses whichever score is higher. The displayed score therefore reflects the more favourable of those two moments. It doesn’t tell you how long any fog will hang around.
How reliable is the forecast within 60 hours and beyond?
Fog Forecast uses weather data from MET Norway, the Norwegian Meteorological Institute. For roughly the first 60 hours, MET Norway also provides its own fog forecast: the proportion of the area expected to have fog. When available, that is averaged in with the four factors.
Further ahead, the tool relies on the four-factor calculation alone. A score for the near future and one for later in the forecast therefore aren’t calculated in quite the same way.
Fog is difficult to predict using model data alone, especially more than a few days ahead. Use the forecast as a guide to your chances. It’s no guarantee.
Choosing a morning for photography
If a promising morning is still a few days away, keep your plans loose for now. Check the forecast again closer to leaving. On the first check, you’re choosing a possible morning. On the next, you’re deciding whether it still makes sense.
Work backwards from sunrise. Allow time for the journey and for looking around your spot before the sun comes up. Remembering the date and leaving the rest at ‘sometime early’ is a bit vague for a forecast calculated around a specific moment.
If you also want to check where the sun will rise, use the Sun and Moon Planner. The fog forecast helps you choose a morning. The sun’s position helps you plan for that spot.
And think about whether you’d still want to photograph there if the fog doesn’t turn up. That would be part of my decision to go. You can check the conditions and plan when to leave, but a score doesn’t make a photo.
Check Fog Forecast, look up your spot and see which morning stands out. Then comes the bit I haven’t made a button for: getting out of bed.