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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Part Two: Rain vs Showers

When you hear showers on the forecast do you think, 'oh we won't get much' and when you hear rain on the forecast do you think, 'ah, a soaking'?

Lets clear that up once and for all.

By themselves, the term rain or showers doesn't tell you anything about how much precipitation you will see, instead it tells you how its going to fall.

You could get 1mm from rain, and 35mm from showers, or the other way around. You could also get 0mm when there are showers in the area.

Jane Bunn Rain vs Showers and will I get wet?

SHOWERS

Showers are hit and miss, start and stop. 

Cumulus cloud with shower

They usually begin and end suddenly, so we go from dry to downpour to dry to downpour, and repeat.

Showers are pushed around by the wind, so you only experience a particular shower if you are in its path - while surrounding areas are dry, often with blue sky.

Showers come from cumuliform cloud - the puffy ones that look like they are bubbling up. Sometimes described as looking like a cauliflower. 

You can tell how likely you are to see one or more of these, by how they are described over the area. 

The traditional way is by using isolated showers or scattered/widespread showers.

Isolated showers over a forecast zone = a shower or two for one spot in that area.

Scattered showers over a forecast zone = a few showers or showers for one spot in that area. 

For example: The Melbourne metro area could have a forecast of isolated showers, and the suburb of Ringwood can expect a shower or two

But even if there is lots of activity, ie widespread showers, they are still hit and miss. You can still miss out completely, while other suburbs have a rather wet day.

Isolated vs scattered showers

Since 2016, the likelihood is described by your percentage chance of precipitation. The Melbourne metro area could have a forecast of a medium (40%) chance of showers, and the suburb of Ringwood can expect a shower or two.

And that's why "shower or two" covers all bases, either:

  • no wet weather
  • one shower, or
  • several showers

(and it used to be that when BoM issue a forecast of a shower or two, they get it right, no matter what happens ;) )

In summary, showers come from puffy clouds, often separated by blue sky. They are hit and miss as the wind pushes them around, and the percentage chance shows you how likely you are to get hit by one or more.

And thats why you will never see 100% chance of showers.

RAIN

Rain comes from a grey, widespread sheet and it affects everyone.

Rain from stratus cloud

Its longer lasting and steadier, so it can start slowly, last for a while, then slowly taper off. 

Rain affects the whole area, usually in a uniform way, so all suburbs experience the wet weather.

Rain comes from stratiform cloud, widespread sheets of grey, with no blue sky in sight.

The steady, persistent precipitation is described as areas of rain, or in a rainband

Areas of rain for a forecast zone = rain at times for one spot in that area. 

But doesn't that just mean showers? 

No! :)    Showers are short in duration, whereas rain should last for a longer period of time. If you are stuck without an umbrella, if its showers you would only need to wait minutes before you could go outside, while rain may have you stuck for an hour or more. 

Lets see this in action:

INTENSITY

The term by itself doesn't indicate anything about the intensity - the showers or rain could be light or heavy.

You could end up with 0mm, 1mm, 35mm or 100mm, from either rain or showers.

Virga is rain (so, a large area of steady precipitation) that evaporates before it reaches the ground. This is how you can end up with 0mm, or 1mm from rain.

Showers can come from cumulus cloud that is short, producing small amounts, or very tall, with the potential for large totals. 

The intensity generally relates to how much moisture is in the air, to be turned into precipitation. 

One indicator of this moisture is the dew point temperature. I'll be covering that in the next part of this series. 

A good guide to intensity is the precipitation range on the BoM forecast. 

Shower or two precipitation range
Showers precipitation range

This post was just showers versus rain, and light versus heavy - but it doesn't touch on thunderstorms, drizzle, showers tending to rain, frost, fog, snow and everything in between. Sign up for my Weather Resources for a more in depth guide to the weather.

Stay tuned for the next part of this series: Part Three - dew point and feels like.

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Part Three: The Dew Point and Feels Like

The Weather Bureau uses a range of temperature readings to properly describe the air where we live. This part of Understanding the Weather should let you know more about the apparent temperature, and the dew point temperature, and why we use them.

BoM Weather Observations

BoM Weather Observations

The first four observations:

The temperature (Temp) is the actual air temperature outside, measured out of the direct sunlight. This is the one we most often hear about.

The apparent temperature (App Temp) is what it 'feels like', when you take the wind and moisture in the air into consideration.

The dew point temperature (Dew Point) tells us how much moisture is in the air. Technically, its is the temperature at which water vapour in the air will condense into liquid water. Or, what temperature you would need to lower the air temperature to, if you wanted saturation - ie. a cloud, dew or fog. 

Relative humidity (Rel Hum) also describes the saturation - so why don't we just use that? I prefer the dew point, because the relative humidity is exactly that, its relative. Relative to what the air temperature is doing.

Let's look at this in real life:

Its 7am in Melbourne, and the temperature is 5C while the dew point temperature is 4C. This airmass produces a relative humidity of 90%. That's really high humidity, but it doesn't feel warm. In fact, when you take the wind and moisture into account, it feels like only 3C!
Its 7am in Brisbane, and the temperature is 25C while the dew point temperature is 24C. This produces a relative humidity of 95% and yes, it feels warm, in fact it feels even warmer than it actually is. The apparent temperature is 28C!

Both situations were 'humid' but one felt frosty, while the other felt oppressive. 

DEW POINT AND FEELS LIKE

The dew point can tell us more about what it should feel like:

Jane Bunn Dew Point guide

Tropical regions may use 20+ = humid, and 24+ = oppressive. But southern regions will notice a dew point of 20+, as its not a regular occurrence.  

And this is why an air temperature of 25C can feel so different from city to city. Typically:

  • Melbourne - dew point of 5 to 10C = feels like 19C
  • Sydney - dew point of 15C = feels like 24C
  • Brisbane - dew point of 24C = feels like 28C

WIND CHILL

When you consider both moisture and the wind, it can make it feel very different to what the air temperature states.

A cold outbreak day in Melbourne may have a temperature of 13C, but the wind is a southwesterly blowing at 35km/h, so it feels like only 5C.

Melbourne Airport has often had a temperature of 9C with a southerly blowing at 50km/h, so it feels like -2C - welcome to Melbourne! 

Meanwhile, up in the mountains, a temperature of 2C with a wind of 80km/h, can make it feel like -16C. This is dangerous, and shows us why we should consider not just the temperature but also the wind speed.

FROST

Another handy hint with the dew point is the potential for frost tonight.

If nothing changes with the weather (ie the airmass remains the same, typically under a high pressure system), then the dew point at 3pm may show how cold the air temperature will drop overnight.

3pm: the temperature is 16C and the dew point is 2C. If nothing changes, the temperature should fall to a minimum of 2C by 8am the next day - the minimum.

 This is because the temperature can't go any lower than the dew point temperature, or the air would be more than 100% saturated. 

"Nothing changes" relates to a high pressure system overhead, with clear skies and light or calm winds. 

And yes, 2C can produce a frost, because that's 2C as recorded by the weather station which is 1.5 metres above the ground. The air forms layers overnight as the days heat escapes out to space, so its 2C at chest height, but -2C at the ground. Frost can form with an air temperature as high as 4C.