What do we mean when we say the weather is “fine”?
The word fine is often used to convey the positive attributes of something. It is synonymous with good, well, enjoyable.
How are you? I’m fine!
How was the movie? It was fine.
This is a fine bottle of wine.
When we write weather forecasts we define the term fine to mean that the sun casts sharp shadows. If cloud is thick enough to stop the sun from casting sharp shadows then, even if it doesn’t rain, we don’t think that’s a fine day.
However, New Zealand isn’t known as the “Land of the long white cloud” for nothing, and only infrequently is the sky completely cloud free for a whole day. Cloud often comes and goes. So, when writing forecasts, there are a number of questions to be answered when describing the state of the sky:
- How much of the sky will be covered by cloud?
- How thick will the cloud be?
- How will the amount of cloud vary throughout the day?
- Is there going to be more or less cloud than the previous/coming days?
Our perception of fine weather also varies with the seasons. In the summer months the sun is stronger and even if there is a lot of thin or high cloud it can still manage to cast sharp shadows. Also, if cloud does block the sun for short periods of time we are less likely to notice because the air is warmer; in fact, it might feel like a relief for a short time! In the winter when the sun is weaker it may struggle to cast sharp shadows, and the day will feel cooler. If the sun is blocked by cloud, even for a short time, then it can affect the temperature more significantly and make you feel colder.
So, when it’s not a clear-cut blue sky day we consider all these things, as well as how the weather will make people feel. Will they feel it was a fine day? Or a cloudy day?
Auckland on a Fine Day. Photo by Joerg Mueller.
We’ve added a lot more detail to the forecasts for Auckland on metservice.com.
In addition to the existing Auckland forecast on the Towns & Cities page, we’ve divided the greater Auckland region into five sub-regions – each with its own characteristic rainfall, temperature and wind patterns:
To provide hourly predictions of air temperature, wind speed and rainfall for these sub-regions, we’re using
- Data from our own localised-for-New-Zealand weather modelling and statistical processing systems (for more about this, see the blog on MetService’s Investment in Forecasting)
- Observations of temperature and wind speed from representative weather stations within each of the sub-regions (see next point), which we’re blending with the modelled data for the first few hours of the forecast.
- The weather stations we’re using are Whangaparaoa (for North Shore), Whenuapai (for Waitakere), Auckland Airport (for Manukau), and Ardmore (for Hunua). For Auckland City we’ve created a “virtual weather station” near the Newton Interchange; this will do the job for now, but we want to replace it with a real station within Auckland City soon.
The index map at the bottom left of the map area links back to the Auckland Towns & Cities page you’re already familiar with, containing the overall Auckland urban forecast and max/min temperatures for the next 10 days.
This initiative was partly motivated by the great feedback we received about the ‘dust graphs’ of wind speed and rainfall added to the Christchurch pages in February, to alert people to the potential dust nuisance in the areas affected by liquefaction and, more recently, demolition of large buildings.
As always, we’re looking forward to hearing what you think! Tweet @MetService or drop us an email at email@example.com
This blog post is the second in a three-part series discussing verification of MetService forecasts. Here, we present the method used for verifying maximum and minimum temperature in city forecasts, along with some recent examples.
Four times daily (around 11:15am, 4:15pm, 10:50pm and 3:45am) MetService issues city forecasts for more than 40 locations. Currently, for 32 of these (and soon for most of the remainder), the forecasts of tomorrow’s maximum temperature and minimum temperature from today’s late morning issue are verified against observations from a nearby automatic weather station.
Over a given period (typically, a month), for each location, the forecasts of tomorrow’s maximum and minimum temperatures from the late morning issue of the city forecasts are compared with observed maximum and minimum temperatures. As with the verification of precipitation in city forecasts, “tomorrow” is the 24-hour period between midnight tonight and midnight tomorrow and the scheme operates automatically – that is, there is no input by MetService staff.
Results dating from the implementation of this particular scheme in March 2009 through to 28 October 2010 are below.
The graphs clearly demonstrate what every experienced forecaster knows:
- Temperature forecasting is generally most difficult over the east of the South Island
- It is much harder to forecast the minimum temperature than it is to forecast the maximum temperature.
MetService’s performance target for the forecast of maximum and minimum temperature in city forecasts is maximum temperature within 2° C and minimum temperature within 4° C 77% of the time for the 2010/11 financial year and 80% of the time for the 2011/12 financial year.
Even across small distances, there can be significant variations in temperature during the course of a day – just ask any horticulturalist. Thus, the observation point used to verify the forecast for a given place may or may not fairly represent the temperature there – or may be a good indication in some weather situations but not in others. MetService observation sites are commonly at airports, which in general are at least some distance from the city or town they serve.