Part One: Mostly Sunny vs Partly Cloudy

Partly Cloudy is a forecast used by the Weather Bureau a lot of the time (so is Shower or two, but I'll do another post on that). But what does Partly Cloudy actually mean? And is Mostly Sunny the same thing? 

What is the difference between partly cloudy and mostly sunny, and what exactly is fine?

Is Mostly Sunny the same as Partly Cloudy? No, not really. 

When you hear me say Mostly Sunny, you can expect heaps of sunshine, or heaps of sunlight.

When you hear me say Partly Cloudy, or 'a mix of sun and cloud', you can expect exactly that, half cloud, half sun. 

But first of all: lets start with wet or dry. 

What is Fine? Is the answer Sunny or No wet weather?

When I ask this as a starting question at my weather workshops I'll usually see most of the hands going up for sunny. But that's not correct, fine actually tells us nothing about the cloud, it just means that it is dry. It can be 'cloudy but fine', ie grey overhead but completely dry.

The misuse of the word fine was one of the reasons I decided to go into the media. I was a forecaster at the Weather Bureau in Sydney, and there was one presenter on radio that used to drive me crazy. 

She would take a forecast of:

     FINE AND MOSTLY SUNNY

and - because its so long - shorten it to mostly fine. 

This completely changed the meaning! 

The public - if they knew what fine meant, thought we would have some wet weather, it would be mostly dry: dry for much of the time, but not all of it. 

Now, when its a dry forecast, the Weather Bureau just tell us about the cloud, and leave the 'fine' off.

But for clarity, I like to write:

     DRY AND MOSTLY SUNNY

to reduce the confusion as much as I can.


Hopefully I've cleared that up - fine = dry - and nothing about the cloud.

So, lets go through the difference between sunny, mostly sunny, partly cloudy, mostly cloudy and cloudy.

SUNNY

Sunny is less than 2/8ths of the sky covered in cloud. It's essentially a clear, blue sky. Its your picture perfect summer's day, crisp autumn pearler, relief in winter after so much grey, or a hint of warmer weather in spring. 

MOSTLY SUNNY

Mostly sunny is 2/8ths of the sky covered in cloud. Most of the sky will be clear. Or it could mean that most of the day will be sunny - starting with fog or grey, low cloud, and turning into bright sunshine by mid-morning. Or it could be one of those days with wispy high cloud - but there is plenty of sunlight coming through it, so its mostly sunny.

Sunny or mostly sunny is what you think of when you remember an awesome summer's day from the past.

PARTLY CLOUDY

Partly cloudy is the half way mark - half sun, half cloud (4/8ths of the sky covered in cloud, 4/8ths of blue sky). I often refer to it as 'a mix of sunshine and cloud'. Partly cloudy could mean: 

  1. morning grey skies, followed by bright sunshine (ie. sunny then cloudy = partly cloudy overall); or
  2. the sort of weather we see with showers, where cumulus clouds are interspersed with clear sky; or
  3. bright morning sunshine, before cloud thickens in the afternoon ahead of some wet weather or a weather change; or
  4. thicker sheets of high cloud covering the sky, but sunlight coming through.

Partly sunny is sometimes used instead - but they both mean the same, half sun, half cloud.

MOSTLY CLOUDY

Mostly cloudy is a fairly grey day. There is more cloud than sun, or 6/8ths of the sky covered in cloud. We see a fair few of these in Melbourne and southern Victoria! Grey for most of the day, with some sunny breaks.

CLOUDY

Cloudy is the last one - either hardly any sun at all, or completely overcast (8/8ths). But despite that cloud cover, it can still be completely dry. The sky looks ominous, but doesn't produce anything, so there is no need for an umbrella :)


What's with the eighths?

The sky is technically observed using oktas. This might help visualise what I detailed above:

Image by BBC

Image by BBC

(but not as helpful with high cloud that covers most of the sky, with lots of holes to let sunlight through). For more on the different types of cloud, see 'What cloud is that, and does it mean it will rain?' in my Weather Resources.

The next installment is available now - Part Two: Rain vs Showers.

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