Meteorologist Dr Kyle Clem believes weather forecasting really just comes down to your perspective.
Crowded House famously sang about four seasons in one day, but sometimes when you search for a weather forecast, it can feel like there’s a different season every hour, depending on which app you look at.
But isn’t the weather … just the weather?
Wellington-based meteorologist Dr Kyle Clem explains why one weather app might tell you it’s sunny – while another warns of rain.
Why do different weather apps often give different forecasts for the same location?
Everybody starts with the same global data and from there, different agencies like MetService or Earth Sciences (formerly Niwa) run their own in-house models to get a more accurate local picture of local conditions.
So all apps [end up with] slightly different data. And they use different levels of technical or sophisticated techniques to extrapolate global weather model data down to an individual point.
What is the global weather model?
There are only two main global weather models, one from Europe (the European Centre for Medium-Range Weather Forecasts) and one from the United States (the Global Forecast System).
These ... run four times a day, every day, always. They produce this gridded [weather] data over the entire Earth, and [one] grid cell would be a 20 to 30 sq km box.
You can [imagine] how much the weather varies over 30km, especially over complex terrain.
Generic, basic weather apps just use that value of the closest grid cell [to forecast the weather]. Other, better apps have done a bit more research ... which give the most accurate forecast.
How do we get reliable data in New Zealand?
We do need to use more advanced techniques here in New Zealand than other parts of the world do to get reliable data. And this is why MetService and Earth Sciences run their New Zealand-specific, in-house models to turn the global model output into something that’s representative of New Zealand.
That’s unique to New Zealand.
If you were in the middle of the United States, where it’s just flat, you don’t need to ... do as much manipulating of the data. A point at an airport in the middle of Kansas is going to be representative of a much, much larger area than a point in New Zealand.
Where should I go to check the weather?
If you want an accurate forecast, your best bet is to go to the local national forecast service. So, in New Zealand, you would want to use MetService.
They take the same data that all the apps use (generated by the global weather model), but they run their own nested models within it to get a more accurate representation at a higher accuracy or a higher resolution. And then they even have humans who interpret it.
It’s still not perfect, but it’s going to be better [than the international or general alternatives].
What’s the downside of using another weather app instead?
If you use an [international or generic app, then you’re relying on something that is a standardised approach to the entire world. There’s nothing about it that is unique to your given location or that region of the world.
How far in advance can we trust a forecast?
It depends on the condition or forecast.
Slowly varying patterns are harder to predict because you don’t know when precisely things will break down and change. But if things are active and they’re constantly changing, those are actually easier to predict.
Does New Zealand’s unique microclimate make it harder to forecast than, say, in Europe or the US?
Yes. And this is why New Zealanders probably get so frustrated with weather forecasts, because it changes so much over such a short distance. And that’s just the reality.
For example, Wellington is very hilly and there are lots of buildings. So you might be sitting in one location that gets shade all day, or at different times of the day, or is exposed to the wind when it comes from a certain direction. And just a kilometre away, the weather will be completely different if that location gets sun at a different time of day and it’s not exposed to the wind in that specific direction.
So the complex topography and all the microclimates in New Zealand make your interpretation of a weather forecast much more uncertain.
What’s one thing you wish everyone understood about weather forecasting?
It’s all about your perspective. Do you want to take it with glass half full or glass half empty? Because the reality is that weather forecasts are, remarkably, quite accurate.
Do you think Kiwis are obsessed with the weather?
We are a country that is very interested in forecasts and maybe a bit obsessive. But it’s not bad. We just like to be outside. We want to know if it’s going to rain and we want to know what the temperature is going to be.
What kind of weather is the most difficult to forecast?
Pop-up showers and thunderstorms in summer.
Varsha Anjali is a journalist in the lifestyle team at the Herald. Based in Auckland, she covers travel, culture and more.