Layers and streets of cloud

Clouds come in many different types and are characterised and named according to both their shape and height in the atmosphere.  While a single snapshot in time at a given location may only contain one type of cloud, there are many days when multiple cloud types can be observed in the sky at once. On Saturday, 3 July 2010, we had a nice example in satellite imagery of different cloud types sitting at different levels above Waikato.

Here’s what the satellite image looked like Saturday afternoon for southwestern parts of Waikato near Kawhia Harbour:

(Image courtesy of MODIS Rapid Response Project at NASA/GSFC.)

Looking at that image you can probably spot three different cloud types quite easily. The broad, flat-looking cloud near the top right of the image is an area of fog sitting on the ground. Above this, and casting shadows on the fog layer, are lines or “streets” of puffy looking cumulus cloud. Higher still are thin wisps of cirrus both over the sea and extending onto the land, some of which you can see through to the cumulus and coastline below.

To see how these clouds changed during the afternoon, here’s a short animation of hourly images covering the period from midday to 3pm:

You should be able to spot the area of fog slowly shrinking, while cumulus forms near the coast in warm updrafts over the land eventually spreading inland across Waikato as streets of cloud.  The lower quality of this animation makes the small wisps of cirrus hard to spot.

So how do these cloud streets form?
One might be tempted to compare each individual cloud street to a smoking chimney, and that is perhaps partially true. The cloud continuously forms at a stationary point, over a coastal hill for example (acting like a chimney), and is then blown away by the wind – a southwesterly in this case. However, this doesn’t completely explain the situation as you might expect the cloud to form all the way along the coast, and then move inland as a solid layer, not as lines of cloud with gaps between them. The answer is clearly a little more complicated and is best illustrated by the following image from Wikipedia:

(Image courtesy Daniel Tyndall, Department of Meteorology, University of Utah, via Wikipedia)

Clouds form when rising air cools and reaches its saturation point, causing the moisture in the air to condense into water drops. Here we see lines of rising air or updrafts between counter-rotating tubes or rolls of air that are aligned with the flow. In between the updrafts are lines of sinking air in which cloud can’t form. The result is narrow lines of cloud, separated by narrow lines of cloud-free air.

Note that this is quite a different process to lines of cloud that are often observed downstream and parallel to a mountain range and perpendicular to the wind flow, but that story can wait for another blog post …


You may have heard in the news recently about a large group of  icebergs in the ocean south of New Zealand. There is an impressive photo of one of the icebergs here. The story got me thinking again about the amazing properties of water, so I will continue the thread of a previous post by focusing on ice.

Ice in the water

Firstly, the icebergs. You may remember from school that approximately 9/10s of an iceberg is submerged in the sea, so that only a tenth is visible above. You can test this property of ice for yourself by putting an ice cube from your freezer into a glass of water. The ice will float like a mini-iceberg.

Extending this experiment a bit, if you were to take a glass of water at room temperature and warm it up, the water would expand, i.e. become less dense. Conversely, if you cooled the water it would shrink slightly, i.e. become denser. But shrinking stops at 4°C – any further cooling leads to expansion again. This is a very special property of water and is quite the opposite to what usually happens when objects cool. For example, when air cools it shrinks and it keeps shrinking the more you cool it.

Once cooling water reaches 0°C the change of phase to ice takes place and the lighter ice will float if placed in relatively heavy water.  In the real world there are some complications, for example the effect of sea-salt on the density of water, but the general principle applies.

Ice in the air

The image below is from the NASA/GSFC MODIS Rapid Response System and shows the lower South Island as viewed from the Aqua satellite in visible wavelengths of light. Invercargill is at the bottom of the image, right of centre.

MODIS visible satellite image, 4pm 24 November 2009

The image brings out some interesting characteristics of ice. Look at the wispy clouds just west of Fiordland. These are cirrus clouds and are made of ice crystals suspended high up in the troposphere. If you look closely you can see shadows from the clouds cast on the sea by the afternoon sun. It is the crystal composition of these clouds that gives them their fibrous or streaky appearance.

Cirriform clouds are common in New Zealand, and you can observe their texture for yourself by looking out for them high in the sky, especially when a front is approaching. There’s a nice example in the photo below. You can also see them when towering cumulus clouds develop into cumulonimbus, with ice crystals making the glaciated anvil top of the cloud appear fibrous.

Cirrus cloud, with thin filaments at the edges
Cirrus cloud, with thin filaments at the edges

Ice on the ground

Referring back to the satellite image, in the top right of the picture there’s snow on the tops of the mountains. This can be inferred from the texture of the white colouring which follows the contours of the land, a bit like branches on a tree.
But over Fiordland the white colouring is not snow but cloud, consisting of water droplets. I know it’s cloud from the form it takes – lines of cumulus clouds orientated towards the bottom right of the image. And there are shadows from the clouds too. At some point there is a transition between the snow and the clouds, and it is difficult to detect precisely where the change occurs. Can you?

Even though the ice (in the form of snow) and water (in the form of cumulus) look similar, I hope I’ve shown that the “snow form” of ice looks quite different from the “crystal form” as in the cirrus cloud. It seems that water has many fascinating aspects to its character, and I have only touched the surface :-)