Predictability and popcorn

Among the most spectacular of meteorological phenomena are those associated with convection, particularly thunderstorms.

Convection is all about the vertical motions that I mentioned in an earlier blog post, but confined to pockets or cells of strong up and down motion. These motions transfer heat through layers of the atmosphere on a local scale; this is why many showers and thunderstorms affect only small areas. But when the temperature differences are large, these storms can be very powerful.

Below is a dramatic photo of a convection-related cloud that occurred during May in Horowhenua – a roll cloud

Ohau (Horowhenua) Mon 17 May 2010, looking north. Photo courtesy Terry Fitz

The roll cloud occurred ahead of a strong convective downdraught associated with a heavy shower in the background of the photo. As the downdraught reached the ground it spread out and, at the leading edge, a band of turbulent ascent and cloud formed. This example of convection was very short-lived and affected only a small area.

As I stated in my previous  blog post, thunderstorms are of particular interest to pilots because of the many hazards associated with them, such as turbulence (bumpiness), airframe ice, lightning and hail. The movie “Twister” showed some spectacular examples of convection, including tornadoes and funnel clouds that accompany the most severe forms of convection. Many of these powerful storms occur in the United States but we do get a few in NZ from time to time – you can keep an eye out for them using the Severe Thunderstorm link on

If you watched the movie at a cinema, perhaps you were munching popcorn at the time. There’s a neat analogy between making popcorn and the development of convection that I thought I’d describe …

To make popcorn you need to put the popping corn onto a hot surface, then eventually the ears of corn will explode and puff out. A question: could you predict which piece of corn will explode first and when?


I doubt it … the only accurate prediction you could make is that they will generally start to explode once the surface reaches a certain temperature. And if the surface of the pan isn’t heated, there’s no chance of any popping.

In a similar way, when the land is heated it isn’t usually possible to predict precisely where a cell of convection will form. But we are able to diagnose those situations most likely to initiate convection, and also the possibility of severe convection.

So trying to determine where a cell of convection will start is difficult or impossible. In stable situations, if we know that there won’t be any significant heating of the land (analogous to not heating the pan), then there won’t be any convection, or not much anyway.

Forecasting convection in the atmosphere is more complicated than just knowing where surface heating will occur, because you also have to assess things like:

  • heating and cooling in the upper levels of the atmosphere (related to atmospheric stability)
  • 3-D wind flows
  • the moisture distribution
  • triggering mechanisms
  • topography (the lie of the land).

But I hope you get the basic idea. However all is not lost in terms of prediction … once a cell of convection has formed it is sometimes possible to estimate its future movement and development. And this is where our growing network of weather radars is increasingly valuable, as the radars send us information about convection in its initial stages, allowing useful short-term predictions to be made.

Rain or Showers

We had an enquiry recently from an astute member of the public asking about the comings and goings of rain.  

They had noticed that in southerly weather the rain has a tendency to “come in bands (e.g., 20 minutes rain, 20 mins dry, 20 mins rain etc.) rather than as a more constant rain that comes with northerlies”. They were wondering why this was. This is a good question and I will try to answer it here.  

Radar examples

First, let’s have a look at examples of northerly and southerly precipitation. Below are radar images that show rainfall-sized drops as explained in an earlier post on the storm of late May. As before, light falls are yellow and heavier falls blue.  

1.  Rain approaching Taranaki from the north on 5 June 2010. In this animation the precipitation is relatively uniform, suggesting more continuous rain:  

New Plymouth radar imagery, 6pm to 10:30pm NZ Standard Time, 5 June 2010

As an aside, the semi-circular area of yellow over the North Taranaki Bight (towards the end of the animation) is caused by cluttering reflection off the sea.  

2.  Showers moving onto the Canterbury coast and plains from the south on 8 June 2010. Here the precipitation is speckled indicating showers (with breaks in between):  

Christchurch radar imagery, 6pm to midnight NZ Standard Time, 8 June 2010

The key to unlocking the cause of the difference between rain and showers lies in the nature of vertical motion. In the post on The Structure of Highs I explained the importance of up and down motion of air. Even though vertical motion is usually much weaker than horizontal motion of air (wind), it really dictates the state of the sky:  

  • upward motion of moist air favours the formation and maintenance of cloud, and possibly precipitation too,
  • downward motion of air inhibits cloud, favouring clear skies.

Relating this to our original question, northerly flows over New Zealand occur ahead of approaching depressions or troughs of low pressure. These flows are characterized by gently rising warm moist air covering a large area. In these situations you’re more likely to get continuous rain.  

In contrast, southerly flows occur immediately behind departing depressions or troughs, where there tend to be pockets of more vigorously rising air surrounded by generally sinking clear air. In addition, if the air flows over the sea, the colder drier air picks up extra moisture and becomes unstable so that the rising air gets extra buoyancy. In these situations the precipitation is likely to be punctuated with breaks, i.e. you’re more likely to get showers.  

Combinations and exceptions

There are some complications to this. Occasionally within the gently rising northerly airstream there are pockets or bands of vigorous rising air that can even generate embedded thunderstorms. These are caused by subtle instability mechanisms. They are of great interest to pilots, because the surrounding rainy areas can make the embedded and hazardous thunderstorms difficult to detect.  

It’s also possible to have an approaching front generating a broad area of rain as it flows over the top of an older shallow flow generating light showers. I can show you an example if you’re interested – just leave a comment below. 

Topographic effects can lead to preferred places for showers in southerly flows. For example, have another look at the radar animation of showers above. Note the band of showers over the Canterbury Plains at the western edge of the radar echoes. This is caused by a low-altitude convergence effect that favours upward motion there. There are no showers farther to the west due to sheltering from the eastern Otago ranges. If you are beneath that band of showers, then it will seem more like continuous rain than passing showers. 

Another common situation for showers to behave more like continuous rain is when an unstable airmass flows onto the West Coast – in these conditions the cloud is often continuous, and the precipitation is too, with alternating periods of heavier and lighter rain.  

If you look very closely at the second animation you may detect a slight change in the orientation of the band of showers – it turns a few degrees of angle clockwise and crosses Christchurch. This turning is caused by a small clockwise shift in the direction of the wind flow. Subtle changes like this are a real challenge for our forecasters, but our growing network of weather radars is increasing our ability to forecast them.