‘From a meteorologist’s point of view, showers come out of lumpy cauliflower-shaped towers of cloud known as cumulus, whereas rain comes out of flat layer clouds known as nimbostratus. But the two types of clouds can occur together so the distinction between rain and showers is easily blurred. Individual cumulus shower clouds are created by strong upward motion over a small area and usually go through their life cycle in less than an hour. Nimbostratus is associated with weak upward motion over a large area and can last for days.’
– Erick Brenstrum, in The New Zealand Weather Book
In other words, flat wide clouds produce rain and towering clouds produce showers. It all depends on the type of upward motion.
Clouds themselves are collections of water droplets or ice particles, or both. Sunlight shining on surface water provides the energy for water molecules to evaporate, transforming from liquid to vapour. When the atmosphere is buoyant enough this vapour bubbles or floats up into the thinner air aloft. It thus cools. When it is high enough so that air temperature and pressure are low enough, the water vapour condenses onto the surface of any dust or salt particles (aerosols) floating in the air or deposits onto ice particles, making or growing cloud droplets. This process releases some of the energy given by sunshine as heat into the air, so that cloud formation is a warming process.
Anyone who has walked up a hill knows that it takes a lot of work to defy gravity. The air also doesn’t rise without some sort of forcing. The two main forcing methods are called ‘solid object’ and ‘buoyant air’.
First, let’s look at the ‘solid object’ method. This is the one that produces gentle widespread rising motions and makes rain clouds rather than shower clouds. Both mountain ranges and areas of dense air present themselves as solid objects: the mountain range to any air mass; dense air to less dense air masses. In both cases an approaching air mass will, depending on its stability, flow around, as well as ride over, the obstacle. This produces a slope along which there is rising motion, or a whole layer of rising air.
The tropics are the Earth’s source of warm moist air. When this air is propelled pole-wards it rises gently as a layer, overriding the cooler denser air of the mid-latitudes. If this is the only upward motion taking place, the resulting clouds become flat and layered. The clouds of this type start off high up as icy Cirrus (CI) and then flatten out as Cirrostratus (CS). As this conveyor belt of rising air affects the mid-troposphere (alto = middle), the cloud layers are called Altostratus (AS) and when dense enough to produce rain, they are Nimbostratus (NS). When this layer feels the undulating ground the resulting cloud is called Stratocumulus (SC), and when it touches the ground, Stratus (ST). These clouds produce drizzle and are often identifiable in satellite imagery as a band that is commonly called a warm front. The process which forms these organised clouds is called a warm conveyor belt.
Secondly, let’s look at the ‘buoyant air’ forcing method. This produces localised rushes of upward motion that make showers. Most of us have heard the adage ‘hot air rises’, but this should be rewritten that ‘less dense air rises’. We can see this when we take a bottle of soft drink and shake it, bubbles of carbon dioxide rise straight up. Even if we tilt the bottle, the bubbles go up, not along a slope, but directly in the opposite direction to gravity. This is the concept of buoyancy; whereby the air’s density acts as an energy source to work against gravity.
When the air is buoyant enough and gets triggered appropriately, it rises quickly – think of hot air balloons rising and drifting in the wind – perhaps as quickly as 80 kilometres per hour – in ‘chimneys’ rather than as a whole layer. The latent heat released as the water vapour condenses and forms clouds acts to warm the air and increase its buoyancy, which is why the upward motion in some shower clouds can be so large. Each chimney is surrounded by a counterbalancing moat of sinking air, something like a mushroom, but surrounding winds usually arrange it so that the falling zone is on the downwind side of the cloud. Each chimney may only last an hour or so and covers just a small area.
The resulting clouds are lumpy – Cumulus (CU), Towering Cumulus (TCU) and Cumulonimbus (CB).
Shower clouds can occur:
* In and around cold fronts, where incoming cold air displaces warmer air
* When buoyant moist air from the sea flows towards hills or mountains
* When air is heated by the Earth’s surface – perhaps cold air moving over warmer sea, or inland on a warm afternoon
Rain in the Ranges
New Zealand’s mountain ranges often have an air flow that hits them broadside from the sea. This produces upward forcing of both the ‘solid object’ and ‘buoyant air’ kind. Thus the resulting clouds can be a combination of the flat ‘stratus’ kind and the bubbly ‘cumulus’ kind, and the precipitation can be a mix of steady rain and showery bursts. A sheet of Altostratus cloud moving over orographically induced cumulus clouds can encourage dense rainfall in a process called ‘seeder-feeder’.
MetService forecasts concentrate on the timing of the heaviest rain likely from passing fronts. When conditions for both rain and showers coexist, the weather forecast, for sake of brevity, will usually refer to just ‘rain’ or just ‘showers’.
Thunderstorms = Serious Buoyancy
Each thunderstorm has an updraft and a downdraft. In the updraft the buoyancy drives the growth of a large amount of water and ice particles within the cloud. When this water reaches the downdraft, gravity will take effect and these particles, gathered from a large area, will rush to the ground and fall out (principate) into a small confined area. If there isn’t much wind to push the cloud around, they’ll all hit the ground in the same area. This is why thunderstorms are often associated with very heavy local rainfalls and flash flooding, which can cause rapid rises in creek levels that reach the sunny gullies miles away from the thunderstorm. And why MetService operates a ‘Severe Thunderstorm Warning Service’.
Buoyancy is the defining point between rain and showers, and can be read by observing the clouds. However in the mountains, conditions for both rain and showers can coexist.
MetService has cloud posters to help you read buoyancy and decide on rain or showers. These are downloadable from here.
Bob McDavitt retired from the MetService in 2012. FMC thanks Bob for his valued contribution as the Bulletin weather columnist over many years. This column was originally published in the August 2010 FMC Bulletin.