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.
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.
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
Although it is not in its final state, this preview gives you the opportunity to have a look around the new site at your leisure and familiarise yourself with the new look and functionality before it replaces the current site.
Things are still being finalised and the preview site is not yet fully functional, so please do not use the new website for any important weather information. MetService’s official weather information continues to be posted at metservice.com.
In this project we reviewed the design of our site to make it easier to find information and to update its overall look and feel to a fresher, more modern interface. We did so after getting lots of feedback from our users and each stage of the process was extensively trialled with user testing groups.
All of the information available on the current site is also available on the new site – the range and type of such content has not been changed under this project, but the way it is presented to you has been. There are also a few new additions to the site, including 10-day forecasts for towns and cities and the addition of more rural regions. We have also optimised pages to ensure they load as fast as possible.
Metservice.com is an ever-evolving platform, on which we are planning to make continuous improvements. We’re working on other ideas for new or expanded content following this launch, so keep an eye out for those.
Metservice.com is well loved by New Zealanders and is consistently one of the most visited sites in New Zealand (as rated by Nielsen Online), so we’ve made any changes very carefully!
At this stage we are looking to turn the current website off and launch the completed website in early December.
We hope you like the new features and new design. We can be contacted at: firstname.lastname@example.org
As we approach summer in NZ, the Sun is getting higher in the sky and is increasingly warming the Earth and the air around us. In the early 1600s Galileo Galilei explained that the Earth goes around the Sun, but there’s no reason why we can’t discuss the apparent movement of the Sun across the sky, as you see it from a frame of reference fixed to the Earth. Let’s do that, and investigate the different ways that the Sun drives our seasons.
On about 22nd December in the Southern Hemisphere we hit the summer solstice. The movement of the Sun across the sky on that date is like this:
If your house has a south-facing wall you may be noticing that it’s getting some direct sunlight in the early morning or late evening these days.
You can compute how high the Sun gets in the sky at your place by first letting the point directly over your head be 90° angle above your horizon. I’ll explain what happens at an equinox, then I’ll go into the solstice computation.
• At the equinoxes
On about 22nd March and 22nd September the Sun is directly overhead the equator. At your place the Sun reaches a maximum angle of 90° minus your latitude.
For example, the latitude of North Island is approximately 40°S (you can be more precise for your location by looking at an atlas). So, at an equinox, the Sun reaches a maximum elevation angle of 50° above the horizon.
• At the solstices
At the December solstice the Sun has moved to its southernmost point, directly overhead latitude 23°S, called Tropic of Capricorn. E.g. at Rockhampton the Sun will reach 90° directly overhead around midday on 22 December.
Over North Island, the elevation of the Sun increases to a maximum of 50° plus the 23° (the tilt of the Earth’s axis), i.e. 73°, as shown in the animation at the start of this post. That’s pretty high in the sky. And now, in November, you can notice how high the Sun’s getting in the middle of a sunny day by seeing how short your shadow is.
In the depths of winter over North Island the Sun only reaches 50° – 23° = 27° above the horizon (see the animation below), which is not very high Roll on summer.
For interest, you may have noticed that in the tropics the duration of sunset is short as the Sun sinks almost straight down below the horizon. It’s a similar story for sunrise. The next diagram shows how the Sun moves on the equator.
At the South Pole, the Sun moves parallel with the horizon, always keeping the same elevation, as shown below. Nearer the Antarctic coast, e.g. at Scott Base, the Sun rises very gradually. Even on 22 Dec it doesn’t get very high in the sky there (about 35° max) and, in winter, the Sun doesn’t even make it above the horizon as it goes around.
To protect our eyes we never look directly at the Sun. But, as I alluded earlier, we can infer the Sun’s position from the shadows it casts. I remember Augie Auer quoted a rough rule of thumb, that if your shadow is shorter than your height you should be protecting yourself from the sun. In other words, if the Sun is higher than 45° elevation, you are more likely to get sunburn. I hope this post has given you a better understanding of why we get these higher Sun elevations in the summer months.
You can check out detailed data such as sunrise/sunset times at your place, Sun angle and lots more besides, at the US Naval Observatory website.
As I indicated at the end of the recent post about surface tension, I’ve started a new thread about the amazing properties of water. This time I’ll write about saturation, what it is and what it isn’t.
The reason I included the bit about “what it isn’t” is that a close friend once asked what saturation actually was – they thought that if the air were “saturated” it was like walking through a swimming pool. Not an unreasonable deduction based on our everyday meaning of “saturation”. But in meteorology the word has a precise meaning and, to understand it, we first need to look at the three forms that water takes.
States of water
1. Liquid: like the dew and rain drops mentioned in the surface tension post, this fluid form is probably our most familiar. It’s what we drink, clean with and swim in. Our lives depend on it, and we suffer whenever there’s a shortage.
2. Ice: as mentioned in the post on late frosts, occurs at temperatures below 0°C. It is solid and, in meteorology, most often occurs as hail or snow (see types of weather).
3. Water vapour: perhaps the form least familiar to us, is a gas that is:
abundant in many parts of the world, but also
very unevenly distributed.
All three forms of water can be found in your kitchen: either turn on your tap, look in your freezer, or feel the air around your face.
Characteristics of water vapour
Examples of water vapour are all around us. If you breathe out on a cold morning you can see the tiny droplets in the air – the droplets aren’t vapour, they’re actually tiny bits of liquid but, once they’ve disappeared into the air around you, they’ve evaporated (become vapour). Or perhaps you’ve had a hot shower and bathroom surfaces are damp – if you leave the door or window open the room will ventilate and the water will evaporate.
Sometimes, after a clear sunny morning, you may notice bubbly cumulus clouds forming towards lunchtime, seemingly from nothing. These clouds are made of many, many tiny droplets of liquid water that have condensed from water vapour in the air (perhaps originating from your shower!).
If you put a dish of (liquid) water on a sill, then the water will eventually disappear by changing to vapour. A real-life disappearing act! Provided the air isn’t saturated that is, and here lies the answer to my friend’s question.
When liquid water is in contact with air there is an exchange of the H2O water molecules between the two forms. Eventually an equilibrium is reached where the number of molecules going from liquid to vapour matches those going from vapour to liquid. Once equilibrium is reached the air is saturated. If you’ve heard of the term relative humidity (RH), it is at this point that RH is 100%. Nothing like a swimming pool although the air is moist, holding a lot of water in vapour form.
At this time of the year we often get strong wind events, as Bob McDavitt explained at the end of his post on the Spring Equinox. The weather maps for the 24 hours to midday are below, and show a deep low (or depression) passing just south of the country. The strongest winds were at about the time the low was closest to us.
Note how closely the isobars are bunched together in this map (I have chosen to display isobars at 2 hectoPascal intervals). When the isobars are squeezed together, that means a strong air pressure gradient and lots of wind (see here).
In addition to the strong pressure gradient, the topography of NZ modifies the flow (see the post on Bottlenecks) which is why Castlepoint also features on the short list above. But the very strongest winds were at the bottom of South Island in this event.