Holiday fun

Schools are breaking up for the holidays, and if you’re a parent or caregiver you may be wondering how you’re going to get through the next several weeks. :-) How about setting your child(ren) a challenge – to keep a weather log for a continuous 14-day period. Depending on their age and sophistication, you could make the log shorter or longer, and easier or harder.

Here are some suggestions…

  • Get a nice big sheet of paper and draw a grid of 14 boxes (or however many days you choose). At the end of each day your child draws a picture in each box of what the weather was that day at your place. They could even do it on a computer if they have the skills. Here are some symbols from that they could use:montage_icons

            They could draw extra symbols as required.

  • If you have a thermometer that can be left outside, they could record the temperature. Make sure they put it in a shady well-ventilated spot though, otherwise it will give you a reading that isn’t representative of the air temperature. If they read the thermometer at the same time each day, they’ll get a good record of how weather systems are driving your local weather. If they’re keen they could read the thermometer at 2 or 3pm (the typical time of maximum temperature) and 7am (approximate minimum temperature).
  • They could come up with an estimate of the wind strength by using the Beaufort scale. Wind direction might be tricky, but you could try flying a kite to estimate where the wind is coming from! Remember that, for example, a southerly wind comes from the south, and a nor’wester comes from the northwest.
  • They could measure the rain very simply by putting a straight-sided container outside where it won’t get knocked or blown over. If there’s some decent rain they can measure the depth  approximately with a ruler. Note that if the container doesn’t have vertically straight sides the rainfall total would need a correction to be accurate. And a small amount of rain could evaporate if it’s left in the Sun.
  • You might even have a barometer in your home. This piece of equipment measures air pressure, and would be fun to record.
  • Another twist could be to save the weather report from your local newspaper, or perhaps print out the latest weather map each day from the MetService website. This would allow a comparison between the local weather and the larger weather systems that are driving it. There’s usually lots of climate information in the newspaper too.

Once the weather log is underway your child could start a graph that shows how the weather is changing from day to day. For example, a plot of the temperature or wind speed. They may notice some relationships between different features that they are recording, and might have learnt enough by the end of it to give you a tailored local weather forecast!

If this activity is successful for you, or if you come up with some neat variations, send me a comment. :-)

Jean Batten

I recently passed through Auckland International Airport, and paid special attention to the various displays about Jean Batten. You may have seen her Percival Gull aircraft beautifully on display high up in the international terminal heading, as below. There is also a statue of her just outside the international terminal building.

The Percival Gull at Auckland International Airport

A plaque near the aircraft reads: “The aircraft displayed above you is the Percival Gull monoplane in which Jean Batten, New Zealand’s most famous aviator, made her historic solo flight from London to Auckland in 1936. The elapsed time of the 14,224 mile flight was 11 days, 45 minutes…” The plaque goes on to describe a little of the life of Jean Batten and her achievements. I wasn’t previously aware that the international terminal is actually named in her honour.

I have worked in aeronautical meteorology for many years, and am aware of the challenges in providing good “met” advice to aircrews. The 3-D nature of our atmosphere can make the air motions and distributions of water in its various forms incredibly complex.

I can hardly imagine what it must have been like, over 70 years ago, to travel by air for such a long distance through inevitable periods of difficult weather conditions. Even in the 21st century, with great advances in meteorological observing and forecasting systems, the weather can be a real hazard for aviation. For example, there are thunderstorms with their hail, poor visibility, electrical activity and down-bursts. And there is severe turbulence and in-flight icing to avoid, as well as fog with its associated very poor visibility and, in some parts of the world, violent sand-storms and dust-storms.

Before her historic voyage in 1936, Jean Batten travelled through New Zealand using weather information obtained from our then Director of the New Zealand Meteorological Service, Dr Edward Kidson. In our archives we hold a precious letter from her, thanking the Director for his help with aviation weather forecasts.

An early letter from Jean Batten to Edward Kidson of the NZ Meteorological Service

The letter reads:

Hotel St. George,
14th August, 1934.

Dear Dr Kidson,

I am writing to thank you for your kindness in sending to me a weather forecast for each day’s flight during my tour of New Zealand.

The telegrams which I received with unfailing regularity were a great help, and enabled me to make my plans accordingly.

I take great pleasure in sending you an autographed photograph of myself which I hope you will accept.

Yours sincerely,

(signed) Jean Batten

P.S. I have not yet had time to read the two booklets which you gave me, but intend to read and digest them during my holiday in Rotorua.”

I would love to see what these weather forecasts contained as they must have been based on very limited information but, to the best of my knowledge, they sadly no longer exist.

If you’re passing through the international terminal at Auckland Airport, do have a closer look at the Percival Gull towards the departure lounges. And have a look at “The Jean Batten Story” display of memorabilia on the 2nd floor, but you’ll have to see it before you go “airside” through the security checks. It gives a fascinating insight into our “most famous aviator”.

High humidity

The current weather pattern is feeding subtropical air onto the North Island.

Pohutukawa blosom , 3 Dec 2009
Pohutukawa blosom , 3 Dec 2009

Pohutukawa trees are revelling in the warmth and high humidity.  This one, near Auckland Harbour Bridge, is already near full bloom and it will not be long before its colleagues follow, all around the coastal North Island.

Weather pattern , 1pm Wed 2 Dec 2009
Weather pattern , 1pm Wed 2 Dec 2009

On the weather map shown here, for 1pm on Wednesday in the middle of the first week of December, some typical signs of summer can be seen:  The incoming high pressure system is, for a while, at near 45°S. The low pressure system in the Tasman Sea has been transporting some warm air from the subtropics on to northern New Zealand, raising our humidity.  (Next week, the  high is expected to return to its typical El Nino summer latitude of 35°S, so let’s enjoy some subtropical warmth when we can.)

The relative humidity has been “flat-lining” at 100% in many places all around the country this week.  The bottom part of the graph shown here gives the relative humidity at Whangarei, Auckland, Wellington, Christchurch and Dunedin for comparative purposes.  Out of all these locations, Christchurch was the place hugging 100% the most, so did it feel the most humid?

Dew point (degrees C) above and Relative Humidity % below for WRA Whangarei, AAA Auckland, WNA Wellington, CHA Christchurch and DNA Dunedin  . Timesteps are in UTC so 010000=1pm Tues 1 Dec 2009, 020000=1pm Wed 2 Dec, etc
Dew point (degrees C) above and Relative Humidity % below for WRA Whangarei, AAA Auckland, WNA Wellington, CHA Christchurch and DNA Dunedin . Timesteps are in UTC so 010000=1pm Tues 1 Dec 2009, 020000=1pm Wed 2 Dec, etc

The answer is NO.

Relative humidity measures how close the air is to saturation (see here for more) but not necessarily how muggy it feels to people.

In cool air, saturation is reached at relatively low amounts of water vapour – when the air temperature is below around 8°C your breath

can often contain sufficient moisture  to produce saturated air or misty cloud – at standard pressure that’s 6 g of water vapour per kg of air,

and this has been the amount of water vapour in Christchurch at times this week.    Places such as Auckland or Whangarei have this week been having around 12 g of water vapour per kg of air – twice the ‘humidity’ of Christchurch,  even when the relative humidity is 100%!

A better index for measuring mugginess is the dew point … the temperature at which, if cooled, water vapour from the air will condense on to a surface such as grass.  This condensation requires 100% relative humidity and occurs when the air temperature equals the dew point.

The air feels muggy when we get hot and sticky – and that’s when 1) the air is warm enough to make us perspire, and 2) the air contains sufficient water vapour to interfere with the evaporation of our perspiration and not allow us to cool down much from that.  So mugginess depends on both heat and humidity.

In the graph shown above the dew point is plotted on the upper table.  Notice how Whangarei and Auckland have both been on top this week.  They have often been having nearly the same dew point and so have been nearly equally muggy. However, only Auckland has, occasionally at night, been flat-lining at 100% relative humidity.  Even though both places are equally muggy, Whangarei has slightly warmer air than Auckland and thus slightly lower relative humidity.

The dew point temperature is a great mugginess parameter, for it combines heat and humidity into one number. Wikipedia has more information about the human perception of mugginess depending on the dew point, ranging from extreme discomfort above around 24°C, to noticeably humid from 16°C-18°C, to noticeably dry below 10°C

I hope this has posting has helped to clear up any misconceptions between relative humidity and perception of mugginess.  However, leave me a comment if you have any queries.

Jon Tunster of Wellington comments:

Here’s a photo of the same tree at Oriental Bay, Wellington taken on 6 Dec 2008, and a year later on 7 Dec 2009.

Oriental Parade Pohutukawa 6 Dec 2008
Oriental Parade Pohutukawa 6 Dec 2008

Oriental Parade Pohutukawa 7 Dec 2009

Oriental Parade Pohutukawa 7 Dec 2009

Flowering is considerably delayed this year, maybe due to the cold October?

Bob McDavitt answers:

The name for the study of plant (and animal) life cycle events in relation to the changing seasons is Phenology.

These photos dramatically illustrate the difference between the start of  a La Nina summer (Dec 2008) and an El Nino summer (Dec 2009). The chilly air and soil in October is indeed a likely source of the delayed flowering of the pohutukawa.

Continue reading High humidity

A few reflections on winter

In New Zealand, most people would not consider May and October to be winter months. This year, however, they were noticeably colder than usual. NIWA remarked upon this in their Climate Summaries for May and October 2009. To quote very selectively from these summaries:

  • May: “Temperature: Well below average over most of the country; many areas experiencing lowest recorded May temperatures.”
  • October: “Temperature: The coldest October in 64 years, with all-time record low October temperatures in many areas. Exceptionally late snowfalls. Record low October temperatures were recorded on the 4th/5th in most North Island locations, and on the 9th at many South Island sites.”

In contrast with this, here’s the equivalent quote from NIWA’s Climate Summary for August 2009:

  • August: “Temperature: The warmest August on record, with record temperatures at many locations.”

All of NIWA’s Climate Summaries do, of course, contain much more information than the simple quotes above.

I thought it might be interesting to look at the average pressure pattern for each of the months from May to October of 2009. As you can see below, these monthly patterns are different. In particular:

  • The patterns for May and October 2009 are remarkably similar, with a trough over New Zealand between anticyclones over southern Australia and in the mid South Pacific
  • The pattern for August 2009 shows a northwest flow over New Zealand. There’s a large anticyclone over the South Pacific too, but it looks rather different from those in May and October and it’s in a different place.


Images courtesy U.S National Oceanic & Atmospheric Administration Earth System Research Laboratory

Remember that these patterns are averaged over the whole month and consequently don’t show the day-to-day progression of weather features (anticyclones, depressions, fronts, etc). But especially in May, August and October 2009, what they do show is the persistence of weather features in particular places.

So why is it that, sometimes, weather features become stuck in one place?

The answer lies in what meteorologists call the upper air. The part of the atmosphere that the weather occurs in is called the troposphere and is about 10 kilometres thick. The surface pressure pattern – that is, the lines you see drawn on a weather map – are simply one way of representing everything that’s going on in that 10 kilometres or so of air above.

The atmospheric flow above the Earth’s surface – in the upper air – is what really matters. It’s the waves in this flow that drive the day-to-day weather. Most of the time, the winds in the upper air blow more or less from west to east, in a wave-like fashion, as explained in this blog post by Chris Webster. But sometimes the wave-like pattern, instead of progressing from west to east, comes to halt. When this happens, there’s invariably a strong anticyclonic pattern right through the depth of the troposphere over some part of the hemisphere. Think of it like a big boulder in a river: the water flows around it, and the presence of the obstacle is felt some way upstream. And like in a river, the obstacle can be pushed away if the flow against it is strong enough. In weather forecasting, this behaviour of the atmosphere is described as blocking.

In May, August and October 2009 the anticyclones over the South Pacific are blocking the upstream flow. Interesting, isn’t it, how weather features so far away can have such a big effect on New Zealand’s weather.

Thank you for your feedback

Wow, in less than 24 hours we have received over 400 emails! Thank you to everyone who has provided feedback on the new look website .

Based on your comments we are addressing the following issues:

  • 7 day and 3 day rain forecast maps – you have told us you would like to see the side scroll bar back and we are working to reintegrate this as soon as possible.
    Tip: If you click on the small thumbnails you’ll be able to view larger images and the dates and times will be easily viewable. Also, if you hover your mouse over the top right and left corners, you’ll see Next and Prev to scroll through the images easily
  • The current image size of the 7 day rain forecast and isobar maps.
  • Display of the New Zealand wind direction, rainfall and temperature maps .
  • Browser issues – correction of fonts overlapping and minor text display issues
  • We are also looking to bring back the 24 hour Observations and Month to Date Observations.

We appreciate your patience over the next few weeks while we address your concerns.

Please note we are looking at introducing a mobile version of the website early next year.