Snark Sky guide
How to read a weather forecast: rain chance, feels like, UV and air quality
Read a forecast in three passes. First the headline: temperature and the chance of rain for the hours you'll be out. Then the details that change what you wear: the feels-like temperature, wind gusts and the UV index. Finally the confidence: the next two or three days are usually reliable, while days 7 to 10 show a trend, not a promise.
Chance of rain
The percentage is the most misread number in any forecast. A 40% chance of rain means that on days set up like this one, it rains at your spot in about 4 out of 10 of them. It doesn't mean it will rain for 40% of the day, or over 40% of the area.
- Look at the hours, not just the day. A day marked 60% may be dry all morning with showers after 3pm.
- Check the amount too. A high chance of a light shower (under 1 mm) is a coat decision, not a cancel-the-picnic decision.
- Showers are patchy. On showery days, one street can stay dry while the next gets soaked, so percentages stay middling.
Temperature and feels like
The main temperature is measured in the shade, about head height. Feels like adjusts it for how the air acts on your skin:
- Wind makes cold days feel colder. 4° with a brisk wind can feel close to freezing.
- Humidity makes hot days feel hotter, because sweat dries more slowly.
- Sunshine isn't included, so a still, sunny winter day often feels warmer than the number.
Dress for the feels-like figure at the time you'll be outside, and look at the hourly curve: temperatures usually bottom out around dawn and peak mid-afternoon.
Wind and gusts
Wind speed is an average; gusts are the short bursts on top. Gusts matter more for umbrellas, cycling and anything that can blow over.
| Speed | What it feels like |
|---|---|
| Under 20 km/h (12 mph) | Light: leaves rustle, flags lift |
| 20–40 km/h (12–25 mph) | Breezy: umbrellas start to fight back |
| 40–60 km/h (25–37 mph) | Strong: hard work cycling into it |
| Over 60 km/h (37 mph) | Gale: branches break, take care outside |
The UV index
The UV index measures the strength of the sun's burning rays, and it is highest around midday. It isn't linked to temperature: a cool, clear spring day can have a high UV index.
| UV | Level | What to do |
|---|---|---|
| 0–2 | Low | Little protection needed |
| 3–5 | Moderate | Hat, sunglasses, sunscreen if out for a while |
| 6–7 | High | Seek shade around midday, use sunscreen |
| 8–10 | Very high | Extra care: cover up and limit midday sun |
| 11+ | Extreme | Avoid the midday sun where you can |
Air quality
An air quality index (AQI) rolls pollution such as fine particles and ozone into one number. On the US AQI scale, used in many apps:
- 0–50, good. Enjoy it.
- 51–100, moderate. Fine for most people.
- 101–150, unhealthy for sensitive groups. If you have a lung or heart condition, take it easier outdoors.
- Above 150, unhealthy or worse. Everyone may notice effects; cut down long or hard exercise outside.
How far ahead to trust it
- Today and tomorrow: usually reliable, especially for temperature.
- Days 3 to 5: good for planning; timings of rain can still shift.
- Days 6 to 10: a trend. "Turning colder" is useful; "rain at 2pm next Thursday" isn't.
Checking it on a phone
Snark Sky puts all of the above on one screen, with a line of commentary from a robot that has opinions about the weather:
- Now: temperature, feels like, wind and gusts, humidity, pressure and visibility.
- A 48-hour temperature curve with the chance of rain, so you can see when showers are most likely.
- A 10-day outlook with highs, lows, rain chance, UV, sunrise and sunset.
- Air quality, UV and the moon phase, each with a plain-English label.
- Personalities: Professional, Friendly or Snarky. The forecast is the same; only the tone changes.
- Share card makes an image of today's forecast and the robot's line.
Search for a place or use your location. Snark Sky is free with no account and no ads, and saves up to 3 places. Forecasts come from Open-Meteo, so it needs a connection to refresh. Snark Sky Pro ($2.99 once) adds the Overkill personality (with mild, bleeped swearing), unlimited places and widget images.
Common mistakes
- Reading 30% rain as "it won't rain". Three in ten is quite likely to catch you once in a while.
- Dressing for the temperature, not the feels like, on windy days.
- Skipping sunscreen because it's cool. UV and temperature aren't linked.
- Planning around day 10 as if it were tomorrow.
- Only checking the daily icon. A single cloud with rain can hide a dry morning.
Questions
What does a 40% chance of rain mean?
It means that, on days with a forecast like this one, rain falls at your spot in about 4 out of 10 of them during that hour or day. It says nothing about how heavy the rain will be or how long it lasts.
What is the feels-like temperature?
It's an estimate of how warm or cold the air feels on your skin once wind and humidity are taken into account. Wind makes cold days feel colder; humidity makes hot days feel hotter.
What UV index needs sun protection?
From 3 (moderate) upwards, cover up and use sunscreen when you're out for a while. At 6 and above (high), take extra care around the middle of the day.
How far ahead is a weather forecast accurate?
The next two or three days are usually reliable. Up to about a week is good for the general picture. Beyond that, treat the 10-day forecast as a trend and check again closer to the day.
What is a good air quality index?
On the US AQI scale, 0 to 50 is good and 51 to 100 is moderate. Above 100, people who are sensitive to air pollution should take it easier outdoors.
Where does Snark Sky's forecast come from?
From Open-Meteo, a free weather service that combines national weather models. The robot's comments change with the personality you choose; the forecast itself doesn't.