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 …

It must have been an insightful man…

It must have been an insightful man who decided to build the new meteorological office on the end of the ridge above the Botanical Gardens. If you are lucky you can observe some beautiful meteorology from the roof of the building. The Director at the time, Dr John Gabities, probably had a big say in the matter. Being a meteorologist, he would have appreciated the value of that siting. 

At lunch time on Friday (22 January 2010), I was on the roof and noticed a little drama unfolding on the harbour and took a photo with my cell phone. The photo was taken a few minutes after the scene was at its best, and the framing is not what I had hoped. However, it shows enough to share. 

Fog to cloud on Wellington Harbour
Looking northeast from the roof of the Meteorological Office, Wellington. Thorndon and the container teminal are in the middle foreground. SH2 runs north along the left (west) side of the harbour. My small bright cloud is the one partially obscuring the hills in the middle-left of the photo. Petone and Lower Hutt are obscured by light rain in the middle-centre of the photo. Matiu/Somes Island is the dark line above the container ship just right of middle-centre. Eastbourne is on the right (east) side of the harbour.

There was a light drift of wind over most of the harbour – the Oriental Bay fountain indicated that it was northerly there; the anemometer vane at Kelburn here showed a light westerly; but aircraft were landing at the airport towards the south, so it was southerly there. 

It was certainly southerly in the harbour entrance because there was a finger of very low stratus or fog extending from between Pt Halswell and Eastbourne towards Matiu/Somes Island in the developing southerly change. This can be seen in the photo, although Point Halswell is just out of frame to the right. The finger of cloud and fog had not extended much past Matiu/Somes, but look on the escarpment above SH2 near Newlands! The narrow finger of moist southerly air was being forced upslope and was condensing to form a bright, dense little cloud with a base of no more than about 60 metres. When I took the photo, a small cloud had also formed over Matiu/Somes Island by the same mechanism. 

Not long later, only a few minutes, the stream of cloud/fog emerging from the harbour entrance was dissipating. The cloud over Newlands was growing bigger, and the top was being caught in the northerlies above it, and pushed southwards out over the harbour again. 

Ten minutes later it was over – the finger of fog and low cloud, and the Newlands cloud had evaporated; gone. The northern and eastern parts of the harbour were being overtaken by light rain and lowering stratus from the body of the southerly air stream. 

The whole thing lasted no more than 15 minutes. I feel privileged and humbled to have seen such a wonderful little meteorological drama.

Types of weather

The World Meteorological Organization (WMO) sets recommended practices for coding and reporting weather observations and forecasts. For aviation reports, these codes are set in consultation with the International Civil Aviation Organization (ICAO).

One of the bits of information that is coded up and distributed to the aviation community is what is called present weather. There are many types, including precipitation that falls from the sky, and obscuration due to particles suspended in the air. You can check out the details here (section 4.4  – note this is a large pdf file). When one of these present weather types occurs it is reported in the interests of safe flying.

The recognised types include the following: DZ, RA, SN, GS, GR, TS, HZ, BR, FG, FU, DU, SQ, FC. See if you can guess what the codes stand for -  I have written the answers further down your screen. In a few cases a knowledge of European languages will help.  :-)

Last week’s weather in New Zealand was full of variety. We had a taste of many different types of weather in a short period of time.

Precipitation

The most common types of weather last week were rain and drizzle resulting from fronts crossing and/or moist onshore flows. These types were experienced in many places with varying intensity.

On Tuesday a very unstable atmosphere generated active thunderstorms with hail stones over south and mid Canterbury. There were also occurrences of smaller hail – by small I mean less than the size of a pea.

CH_Metro_0030Z_22Sept2009_smaller

Christchurch radar, 12:30pm Tues 22 Sep 2009. Showers are yellow, heavy showers blue.

CH_Metro_0115Z_22Sept2009_smaller

As previous image, but 45 minutes later. A hail shower is shown as red over the western side of the city.

Another less common type of precipitation (in populated areas at least) was snow, which fell to low levels in southern Hawkes Bay, Wairarapa, and the eastern hills of Wellington on Thursday. This came about from a moist flow at upper levels of the troposphere (the part of the atmosphere that contains our weather) sliding over an unusually cold layer of air near the ground.

Obscuration

A common way that the air is obscured in NZ is with fog and mist, especially at night and in the morning due to night-time cooling in anticyclonic conditions (see my blog post on late frosts for a description of the typical conditions for fog as well as frost).

Haze is also common when the wind blows in from the sea. Tiny particles of sea salt get caught up in the air, dry out, then get blown onto the land. The effect is most noticeable in anticyclones when a temperature inversion concentrates the haziness in the air near the ground. You can read more about the effect of inversions in Erick Brenstrum’s blog post about ridge-top winds. Smoke is a variation, occasionally occurring in our big cities due to vehicle emissions and open fires.

Much less common here is widespread dust, as occurred late in the week over upper parts of North Island, in particular, when fine red dust from Australia was blown across the Tasman Sea.

Other types

There were several periods of showers and thunderstorms over western parts of North and South Island through the week. Some of these were accompanied by squalls as the heavy showers induced downdrafts that hit the ground and spread out as strong wind gusts.

The satellite image below shows the extent of the cloud covering NZ as the week neared its end – a rain-band over central and southern regions extends into a swirl marking a depression in the middle-west of the picture. Shower cloud affects most northern regions, and there were lightning strikes over the northwest of North Island at about this time. Interestingly, do you notice the shadow effects due to the low morning sun shining from the east?

Visible satellite image, 8am 25 Sept 2009

Satellite image based on visible light, 8am 25 Sep 2009.

Last week our trainee meteorologists completed a practical test when we simulated the work of our forecast room based on real-time weather as it unfolded. What a week it was, with such a variety of challenging weather types to diagnose and predict.

Of the types I listed above, we had them all somewhere around the country. All except a funnel cloud, that is. But even that could have happened given that funnel clouds often go unreported or aren’t close enough to our current weather radar network to be detected.

Here are the present weather codes referred to above:

DZ= DRIZZLE
RA= RAIN
SN= SNOW
GS= SMALL HAIL (from the French gresil)
GR= HAIL (from the French grêle)
TS= THUNDERSTORM
HZ= HAZE
BR= MIST (from the French brume)
FG= FOG
FU= SMOKE (from the French fumée, and also perhaps Spanish fumar and Italian fumare)
DU= DUST
SQ= SQUALL
FC= FUNNEL CLOUD (Tornado or Waterspout if it reaches the surface)

Did you get most of them?