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.
Showers are hit and miss, start and stop.
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.
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 comes from a grey, widespread sheet and it affects everyone.
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:
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 cover that in the next part of this series.
A good guide to intensity is the precipitation range on the BoM forecast.
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. See my Weather Resources for a more in depth guide to the weather.
The next part of this series is available: Part Three - dew point and feels like.