Thunderstorms!

The last week in January 2010 will be remembered by many over the central North Island for the frequent thunderstorms that developed in the afternoon, often lasting well into the evening. Conditions changed little during the week with a slack pressure gradient over the North Island allowing afternoon sea breezes to combine with abundant low level moisture, triggering heavy showers and thunderstorms inland. Many of these storms were slow moving, prompting a number of Severe Thunderstorm Warnings as radar detected torrential rain and hail in some cells.

To illustrate the impressive number of lightning strikes that resulted, and just how widespread the thunderstorms were, here’s the North Island strike maps (courtesy MetService and Transpower) covering the afternoon and evening periods (all about 12 hours) from Monday 25th through to Thursday 28th January.

Monday 25th – more than 9,900 strikes:

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Tuesday 26th – more than 32,200 strikes:

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Wednesday 27th – more than 18,900 strikes:

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Thursday 28th – more than 17,100 strikes:

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The colours of the lightning strikes give an indication of when the strikes occurred, with each colour on these maps representing two hours worth of lightning. The times are pink and blue for early to mid-afternoon (about midday to 4pm), green and brown for late afternoon and early evening (about 4pm to 8pm) and orange and red for late evening and night (about 8pm to midnight). Armed with this knowledge you can now deduce where the lightning started in the afternoons and where the last strikes were in the evening. For example, on Wednesday some of the first strikes occurred on a line from South Taranaki to Gisborne with the last strikes of the day occurring in Waikato near Hamilton.

In addition to the colours for each lightning strike, you’ll also note different symbols on the maps. These tell us something about the nature of each strike including the charge. Cloud-to-ground strikes are plotted as a ‘plus’ or a ‘circle’ for positive and negative strikes respectively, while cloud-to-cloud strikes are plotted as a ‘square’.

While it’s not unusual to get afternoon and evening thunderstorms in summer, this week certainly stands out as a particularly active period with a large number of strikes on four consecutive days. And as I write this on Friday afternoon we are again expecting more thunderstorms inland over the North Island today (but perhaps not as many strikes as the above examples).

Lastly, I’ll leave you with a nice example of a large thunderstorm with anvil seen over the Wairarapa on Wednesday 27 January. I took this  panorama from the roof of the MetService building at about 7:25pm:

Click for full size image.

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.

Summer so far

One way of defining summer in NZ is calling it the three months December, January and February. By that definition we have just passed the half-way mark. What has the weather been like at your place so far this summer?  

In a recent blog post by Peter Kreft, he reflected on the 2009 winter. I thought I’d do a similar thing but looking at summer so far. In their climate summary, NIWA noted that Dec 2009 was “very sunny in the north”, with below normal soil moisture in the Far North, Central North Island and Eastern Bay of Plenty.  

Here is the average pressure pattern for December 2009:  

December 2009 sea level pressure; contours in hPa (same unit as millibar). Images courtesy U.S National Oceanic & Atmospheric Administration Earth System Research Laboratory

  

And here is the long term average for December, based on over 50 years of December data:  

December average sea level pressure

  

Note the higher than usual pressure values over the North Island in Dec 2009, together with a stronger anticyclonic flow (there is high pressure inside the 1016 hPa isobar in the top chart).  

In his post Peter referred to the importance of the upper air in driving our weather. So let’s also take a look at what the upper air was doing in December. The next chart shows the variation of upper level pressure heights, but you can think of it as showing the areas in the upper atmosphere where the flow was cyclonic and anticyclonic. For example, the yellow area over South Australia was anticyclonic, and the blue area east of the North Island was cyclonic. 

December 2009 upper flows: blue/purple = cyclonic, yellow/red = anticyclonic.

 The long term average in the upper air for December looks like this:  

December average upper flow, colours as previous chart

Not surprisingly, in December last year we saw stronger anticyclonic conditions than usual (shaded light green) across the north Tasman Sea towards the North Island. Also, interestingly, the usually very strong anticyclonic area south of Chatham Islands was much weaker last month. 

You may now want to ask how January is shaping up. Here is the average pressure for the first twelve days:  

1 to 12 January 2010 sea level pressure (contours in Pa rather than hPa, so 101200 Pa = 1012 hPa)

And the long term average for January is:  

January average sea level pressure

So the anticyclonic anomaly over the northern North Island has continued from December into the first part of January at least. The tendency so far this summer towards more anticyclonic conditions, both at the surface of the Earth and in the upper air, has driven the drier than usual weather in the north.

Feels Like

MetService adds a “Feels like” temperature in the Towns and Cities places page at www.metservice.com. This gives, rounded off to the nearest degree, an idea of what sort of temperature the air feels like to a person in the shade. It is an estimate of a sensation, and not an actual temperature.

FeelsLike

Humans make hopeless thermometers; we are internally heated by our metabolism, and our skin cools in the wind and rain or by perspiring, or it warms in the direct sunshine.  When we are outdoors, how warm or cold the air really feels to us is often different from the measured air temperature.

To estimate this “Feels like temperature”, in colder weather MetService calculates a “wind-chill” that accounts for  the coolness of the wind as it removes heat from us.  In warmer weather we instead calculate an “apparent temperature” that accounts for the sultriness we feel when our perspiration is unable to cool us down efficiently on a warm muggy day. There is a roll-over, between 10ºC and 14ºC, from the wind chill formula to the apparent temperature. These calculations take into account air temperature, wind speed, and humidity:  three of the major weather-related variables for chilling and mugginess.

Two factors not taken into account are wetness or sunshine, so the “feels like temperature” only applies to dry people in the shade.  It’s about how the air feels to you.

Wetness can matter, because if you get caught in the rain and your skin gets wet, or if you have been swimming on a warm, dry day, cooling of your skin as the water evaporates will increase the cooling power of the wind. But we assume your skin is dry, aside from natural perspiration.

Similarly direct sunshine (solar radiation) will make you feel hotter, but this will vary a lot depending on factors such as how sunny it is, the angle of the sun and your skin type.  The air temperature is effectively the same in the sunshine and the shade; we assume you are not in direct sunshine.

Things such as our physical make-up (age, fitness, height and weight), activity, acclimatisation, mood, alertness, or even when we last slept and ate, all have an impact on how warm or cold we really feel, but are virtually impossible to take into account.

The rest of this post is for those wanting the mathematics of the MetService “Feels Like” formulae.

WIND CHILL

1The wind chill formula used by MetService is the one that was adopted internationally after being agreed to by the  Northern American Joint Action Group for Thermal indices in 2001. It is based on results of measurements of the loss of heat from the face of human subjects in various air temperatures and wind speeds in a refrigerated wind tunnel, dressed in winter clothing under various types of activity and with both wet and dry faces.  For more details on the formula see NOAA news release 2001 or Environment Canada’s Wind chill pages here and here.

W = 13.12 + 0.6215 x T – 11.37 x K0.16 + 0.3965 x T x K0.16

Where W is the wind chill in degrees Celsius, T is the air temperature in degrees Celsius, and K is the average wind speed in km/h at a standard height of 10 metres above ground.  Note that wind chill is only defined for K with a minimum speed of 5 km/h, and is only designed for air temperature of 10ºC or less.

Wind chill relates how cool your skin feels in the wind by giving an equivalent temperature out of the wind.  For example, using the table above ….when it is 9ºC and you bike to work doing 20 km/hr (down-hill) into a 20 to 30km/hr head-wind (total 50 km/hr), you’ll lose as much heat as if you were sitting in a fridge (4 ºC, circled)). Since the wind chill index represents the sensation of cold on your skin it is not actually a real temperature, so it is useful to always precede it with the words “feels like”.

Wind chill relates how cool your skin feels in the wind by giving an equivalent temperature out of the wind. For example ….when it is 9ºC and you bike to work doing 20 km/h (down-hill) into a 20 to 30km/h head-wind (total 50 km/h), you’ll lose as much heat as if you were sitting in a fridge (4ºC, circled).

APPARENT TEMPERATURE

There are several indices that take into account the sultriness or mugginess we feel on a hot, humid day.  The heat index 2,3 is used most commonly in the USA and is based on work done by Robert Steadman in the 1970s, humidex is used in Canada, and the apparent temperature 4, based on later work done by Steadman as published in 1994,  is used in Australia.  The non-radiation version of the apparent temperature that MetService uses takes into account air temperature, wind speed and humidity:

AT = T + 0.33 e – 0.70  M – 4.00

Where AT is the apparent temperature in degrees Celsius, T is the air temperature in degrees Celsius, M is the average wind speed in m/s at standard height of 10 metres above ground, and e is the water vapour pressure (humidity) and can be linked to rh or percent relative humidity using e= rh/100 x 6.105 x exp( 17.27 x T/ (237.7+T))  where exp(x) is short for the exponential function, ex .

Sometimes the apparent temperature comes out less than T.  In those cases we take T as the “feels like“  temperature.

ROLL-OVER

The wind-chill formula is designed to be used in cool temperatures of 10ºC or less, but still give some idea of the chilling effect of the wind for temperatures above 10ºC. The wind chill at 12ºC for a wind speed of 5 km/h is exactly the air temperature, so it is logical to take this as the midpoint of a pragmatic roll-over between pure wind chill (up to 10ºC) and apparent temperature (from 14ºC upwards).

In the roll-over zone  for T from 10ºC to 14ºC, MetService uses a pragmatic linear roll-off of the wind chill, so the “Feels like temperature” is T – (T-W) x (14-T)/4

If  T is less than 10 then W is the “Feels like” temperature, and if T is more than 14 then either AT or T (whichever is the higher) is the “Feels like” temperature.

LAYERS OF CLOTHING

In 1971 Robert Steadman5 designed a wind chill index that measured the thickness of clothing required to maintain thermal equilibrium.  Click here for a copy.

Steadman’s Clothing index (1971) gives the thickness of clothing required to maintain thermal equilibrium in a cool wind.  It assumes standard wool/cotton clothing covering 85% if the body including a hat and gloves/mittens.

Steadman’s Clothing index (1971) gives the thickness of clothing required to maintain thermal equilibrium in a cool wind. It assumes standard wool/cotton clothing covering 85% of the body including a hat and gloves/mittens.

The Clothing index can be a useful reference for comparative purposes.   Assuming each layer of clothing is a couple of millimetres thick, the colour-coding in the accompanying table shows how many layers of clothing are required (orange =1, etc).  As can be seen in comparison with the wind chill table, they both slope in much the same way.  However the clothing requirement index changes very slowly for above-zero temperatures (left side of table) and then changes very quickly for below zero temperatures (right side of table).  This is correct: we need thicker clothing more rapidly as the temperature drops below zero.

And note that in the interests of modesty we assume you always need at least one layer of clothing even when it is very hot.  :-)

References:

1. Osczevski, Randall and Maurice Bluestein 2005: The New Wind Chill Equivalent Temperature Chart. Bulletin of the American Meteorological Society, Oct. 2005, p. 1453–1458.

2. The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, R. G. Steadman, Journal of Applied Meteorology, July 1979, Vol 18 No7, pp861-873

3.  The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature Journal of Applied Meteorology, R. G. Steadman, July 1979, Vol 18 No7, pp874-885

4.  Robert G. Steadman. 1994:  Norms of apparent temperature in Australia.
Aust. Met. Mag., Vol 43, 1-16.

5. Steadman, R.G., 1971:  Indices of Wind chill of Clothed Persons’, Journal of Applied Meteorology, Aug 1971, Vol 10, p674-683

Big Day Out – A Day of Two Parts

On Friday, Mount Smart stadium in Auckland’s Onehunga becomes once again the site for the Big Day Out – a whole day devoted to music and dancing.

MetService hopes that all involved in this year’s bash have fun, and as I look at the MetService data for the day I can see that the weather is likely to come in two parts.

Part One should be sunny and dry — as seen in the rain map below taken from the MetService site here showing the scene at 10 am as proceedings start.   A cold front is expected to wander over the Auckland area overnight and you can see the remains of it over Northland.  Since the winds following the front are forecast to come in from the southeast, this is likely to be a sunny dry clearance.  So during the early afternoon full sunshine can be expected and the air temperature is likely to rise to 24C.  If you are in the full sun it will feel 6 degrees warmer than that, so just add water to keep your cool.

10am Friday 15 January 2010

Twelve hours later, by 10 pm, weather conditions will have noticeably changed, as shown in the map below.   That cold front mentioned before is expected to stall in the area east of northern New Zealand and that will allow a family of small weak low pressure centres to form on the weather map.  Winds go clockwise around lows,  so the wind direction over Auckland is likely to switch and come from the southwest late in the day. The onshore wind and falling pressure will help to make the atmosphere more unstable, and set the scene for showers to form, especially around western parts of Auckland.   Temperatures are likely to drop to around 18C and the humidity is likely to noticeably increase.  The mosh pit may become moshier.

10pm Friday 15 January 2010

These weather maps get updated often so click here for the latest.

Bob McDavitt

Contrail over Tauranga

I received an inquiry by way of the Editor of the Bay of Plenty Times from Laurie Sanders of Tauranga with some photographs of an interesting cloud formation. Laurie saw it as he was going to work southeast-bound on 15th Avenue at the Cameron Street traffic lights at 7:40 am on Wednesday, 9 December 2009. He took several photos, and I have reproduced two of them here with his permission.

I think the cloud formation is a contrail from the Aerolineas Argentinas flight AR1182 from Buenos Aires to Auckland, which landed at 8:23 am on Wednesday 9 December 2009. The aircraft used on this route is the 4-engine Airbus A340. The great circle route from Buenos Aires to Auckland dips well south, and the approach to Auckland is from the southeast. The view direction of the photo is towards the southeast along 15th Avenue, and the continuation of the trail would cross the southwestern part of the sky towards the northwest and Auckland. Both the timing and the orientation of the contrail are consistent with its being from that flight.

For contrails to form, the ambient air temperature needs to be lower than a certain temperature. This critical temperature mostly depends on the altitude and the relative humidity. It depends on the jet engine type, and engine efficiency to a lesser degree. Usually contrails dissipate a few plane-lengths behind the aircraft, but if the air is moist enough to be ice-saturated they will remain in the sky and behave like any other cirrus cloud. Persistent contrails eventually dissipate or merge with other cirrus.

Atmospheric measurements by balloon-borne instruments released from Chatham Islands, Paraparaumu and Whenuapai indicate that conditions at jet cruising altitude were suitable for the formation of contrails. Also, the humidity was high enough that the air there was ice-supersaturated, meaning that contrails would not dissipate as they do in drier conditions. The aircraft probably commenced its descent into Auckland somewhere over Rotorua and out of the persistent-contrail layer. This is why the trail in the photo stops abruptly. It is possible the plane could have been observed to be making a short contrail for a few more kilometres after the point where the persistent trail stopped, but Laurie says he did not see it.


In the first photo, further back along the track of the aircraft, where the contrail is older, the wind is distorting the trail and spreading it a little. In the second photo, taken only 30 seconds later, the youngest part of the trail is also being visibly distorted compared to its appearance in the first photo.

Aerolineas Argentinas has recently increased the frequency of its services. There are now five flights each week, scheduled to arrive in Auckland at 7:55 am on Tuesday to Friday, and on Sunday. The return flight is on the same days and departs from Auckland at 7:30 pm, having been to Sydney and back in the meantime. Interested observers could watch for these in the future. Be aware that, due to the long duration of the flight and the variability of winds en route, the arrival time over New Zealand may be ahead or behind schedule by some time. This particular flight was about 30 minutes behind schedule; on other days it could be that far ahead.

Lan Chile and QANTAS also have flights to and from Santiago on six days each week (all except Wednesday), and they operate the same aircraft type. The scheduled arrival time at Auckland is 4:20 am, so observers will need to be up early to see anything of it. Any contrails will be visible in the night sky if the moon has enough of its disk illuminated. They return to Santiago the same days at 4:40 pm and, as with the Aerolineas Argentinas service, have been to Sydney during the day.

Because Buenos Aires is further east than Santiago, the Aerolineas Argentinas flights will usually be further south in the sky over the Bay of Plenty and Gisborne. The initial great circle path heading from Auckland to Buenos Aires is 138°, and to Santiago it is 130°; however, the route flown on any given day will vary depending on forecast wind conditions.