How It Works - SunWhen Sun Visibility Forecast

The Simple Version

SunWhen answers one question: "When will I see the sun?" We combine weather forecasts with sunrise/sunset data to predict clear sky periods at your exact location.

Just open the app, allow location access, and instantly see when the next sunny period will be.

Step-by-Step Process

1

Location Detection

When you open SunWhen, we request your location using your browser's geolocation API. This gives us your precise coordinates (latitude and longitude) needed for accurate forecasts.

2

Weather Data Retrieval

Using your coordinates, we fetch hourly weather forecasts from Open-Meteo, a reliable open-source weather API. We retrieve 7 days of hourly data including cloud cover percentages and daylight information.

3

Sun Visibility Analysis

Our algorithm analyzes each hour to determine if the sun will be visible. We consider two factors: Is it daytime? Is cloud cover below your threshold? Hours that meet both criteria are marked as "clear."

4

Clear Period Detection

We group consecutive clear hours into "clear periods" and calculate their duration. This tells you not just when the sun will appear, but how long you can expect it to last.

5

Results Display

Finally, we present the information in an easy-to-read format: the next clear period, upcoming sunny times, current conditions, and a 48-hour visual forecast.

Understanding Cloud Cover

Cloud cover is measured as a percentage from 0% (completely clear) to 100% (completely overcast). Here's what different levels typically mean:

0-20%: Clear skies - excellent sun visibility
20-40%: Mostly clear - sun visible most of the time
40-60%: Partly cloudy - intermittent sunshine
60-80%: Mostly cloudy - occasional sun breaks
80-100%: Overcast - little to no direct sunlight

You can adjust your personal threshold in the settings. A lower threshold means you want clearer skies; a higher threshold means you're okay with some clouds.

The 48-Hour Chart

The visual chart shows cloud cover for the next 48 hours:

Yellow bars: Clear conditions (below your threshold during daylight)
Light gray bars: Cloudy daytime hours
Dark gray bars: Nighttime hours

The height of each bar represents the cloud cover percentage. The dashed line shows your current threshold setting.

Data Sources

Weather Data: Open-Meteo

We use the Open-Meteo API, which aggregates data from national weather services worldwide. It provides:

Hourly forecasts up to 16 days ahead
Global coverage for any location on Earth
Updates every hour with the latest data
High accuracy for short-term forecasts

Astronomical Data

Sunrise and sunset times are calculated based on your coordinates and the current date. This ensures we only show sun visibility during actual daylight hours.

Accuracy and Limitations

Weather forecasting is inherently uncertain. Here's what to expect:

Next 6 hours: High accuracy (85-95%)
6-24 hours: Good accuracy (75-85%)
1-3 days: Moderate accuracy (65-75%)
3-7 days: General guidance only (50-65%)

We recommend checking back regularly for updated predictions, especially for activities planned more than a day ahead.

Tips for Best Results

Allow precise location: More accurate coordinates mean better forecasts
Adjust your threshold: Find the cloud cover level that works for your needs
Check the chart: Look for patterns in the 48-hour forecast
Refresh before activities: Get the latest data before heading out
Consider the duration: Longer clear periods are more reliable

How SunWhen Works