Introduction
This short article is in response to an emailed complaint about temperatures displayed on the Christchurch Towns & Cities page on Thursday evening, 24 May 2012.
Sent: Thursday, 24 May 2012 10:26 p.m. To: Enquiries Subject: Christchurch weather issues! Dear sir/madam, I am writing regarding the situation with the Christchurch weather forecasts and current temperatures since the introduction of the split map into suburbs. I live in the eastern suburbs and have yet to see a day when the current temperature for the eastern suburbs has been correct! It is complete and utter rubbish and I am very intrigued to know where you are getting the temperatures from? Attached is a screenshot image of your map this evening taken at 10.20pm …. I have just been outside and I can tell you for sure that the eastern suburbs are not currently at 12.7 degrees. More like around 6 degrees! Also I am certain that the port hills suburbs are not currently sitting at 17.9 degrees as I doubt anywhere in Canterbury reached that temperature today at all!
Please please get on top of this or at least put it back to just a temperature for Christchurch if you are unable to give us accurate readings and forecasts for the suburbs!
Many thanks
Situation
This is the analysis chart for midnight 24/25 May 2012. It shows a strong northwest airstream across the South Island ahead of a cold front advancing from the southwest. It shows two troughs east of the South Island which were producing rain on the West Coast.
Fohn wind
This is a very good example of a “Fohn wind” situation. A Fohn wind is a hot and dry wind in the lee of a mountain range. The air is warmed by the latent heat given up by the condensing water vapour on the upwind side of the mountains. The condensed water falls there as rain and is no longer available to re-evaporate and potentially cool the air again as it descends on the lee side. Sometimes in Canterbury, this warm descending northwesterly wind flow is unable to displace the cool low-level northeasterlies which often precede a cold front. Higher locations like Sugar Loaf and Le Bons Bay are the first to show these higher temperatures as they protrude up into that wind flow.
There are certain geographical situations that will direct these warmer wind flows down to altitudes lower than would otherwise occur. One such location is the Port Hills – the wind flow “sticks” to the lee side and warmer temperatures (and strong northwesterlies) are experienced there.
Observations evening and overnight 24 May 2012
The “Eastern Suburbs” observations are taken from the New Brighton Pier automatic weather station. This graph is a plot of wind direction, wind speed and air temperature from that station between 8pm on 24 May and 8am 25 May 2012.
In the evening of the 24th the wind speed is fairly light, less than 10 km/h, and the direction is vary variable – sometimes from the west or northwest but mostly from the southeast. At 10:20pm the temperature was 12.6° and the wind was blowing from the southeast at 6.1 km/h. The “Port Hills” observations are taken from the Lyttelton Pier automatic weather station located at the Port. This graph is a plot of the same elements over the same period.
It shows a period between about 9pm and 1:30am when there was moderate to fresh northwesterlies. During that period the temperature had warmed from about 9° to 16-18° then lowering to 12-15° when the direction changed to northeast. This is clearly a Fohn wind.
The Fohn wind can also be seen in the data from Sugar Loaf:
With the strong northwesterlies, the temperature is 14-15° with low dew point and Relative Humidity – a typical Fohn wind. Note how the temperature fell dramatically as the wind direction changed to southerly.
The Fohn wind is also observed at Le Bons Bay:
At Christchurch Airport, the Fohn wind lowered to ground level only briefly, between 4am and 6am.
Note the difference in air temperature in relation to the wind direction. Later in the afternoon the direction was from northeast, off the sea. Later in the evening, the direction was more from the north and not from the sea; consequently the temperature was lower.
It appears to me that, apart from the Fohn wind flowing well above Christchurch (to be detected by the higher AWS sensors of Sugar Loaf and Le Bons Bay, and by Lyttelton in the down-slope wind there), there was a further complication, in that air from the northeast and east was warmer by a few degrees than air flowing from the north. I suspect that our email correspondent’s place is located in that northerly flow and it was that colder air stream that he experienced, rather than the warmer northeasterly airstream that was affecting the New Brighton Pier AWS.
Conclusion
Considering the larger meteorological situation on the evening of 24 May 2012, and the common behaviour of the Fohn wind (Canterbury Nor’wester) it seems to me that the weather stations around Christchurch were illustrating correctly the wind flows that were present and where their effects were being experienced.
Ross, I see from driving around the country that MetService has installed a number of new sophisticated AWS and that there is one located at Little River on SH75 at the base of the hill. Are these stations reserved for commercial or other activities ? Wouldn’t the Little River station be better to use for the Banks Peninsula readings than the Le Bons Bay station. That’s great that you may be able to provide more station data in the future, would be good to get a higher resolution.
Thank you. On your prompting we have checked the applicability of the Cooptown AWS (that is what we called the weather station) to represent Banks Peninsular. It would be better than Le Bons Bay for several reasons. The change is scheduled to happen on Tuesday. We would have liked to do it sooner, but there were other changes in the queue, and these things, even the simple ones, need to be carefully tested. Thanks again, and I think there will be a noticable difference.
Hello. I would like to add to Ross Marsden’s comments. I live in South Brighton at the start of the spit. Our home is 200 meters in a straight line from the beach. Four of my friends have homes situated from North Brighton to the foot of the spit. These homes are between 100 & 800 meters from the beach. We all have garden thermometers and have been informally comparing the temperatures we see at our homes with what the Met service displays for the Eastern suburbs.
Invariably we find that our temperatures are different from yours. Mostly lower by up to 4-6 degrees over the winter period.
Is there no cheap and simple method to move the met service measurement equipment on shore?
As someone who cycles from South Brighton to Sockburn and back every weekday I have become intimately aware of how the weather varies across Christchurch. Having reliable information on metrological conditions for my ride is actually quite important to me. Thanks, Andre`
The temperature readings at New Brighton Pier can be a poor representation for the Eastern Suburbs. On calm clear nights the warming affect of the sea will be confined to the coastal fringe.
Yes, of course, the Eastern Christchurch thermometer is out on the pier over water. On a frosty morning there will be a big difference between the temperature on land and the air temperature several metres up over coastal water. In frosty situations the ground temperature (where the frost is) can be 4°C cooler (and often more) than the ambient air temperature at 1.25 metres above ground which is the temperature we are trying to forecast.
In calm and frosty conditions, the ambient temperature in Eastern Christchurch is going to be much like that of Christchurch City (Christchurch Airport). As funding permits, we would like to install weather stations in more representative locations for the purpose of these metropolitan sub-region pages. In the mean time we hope that people will recognise the situations where the stations we have available now will be unrepresentative, and make the appropriate adjustments in the assessment of what that weather situation means for them.
Living in North Beach, we’ve always known the area felt warmer and drier than the city centre. We can also look ‘down’ towards the city and see the pollution etc which the easterly keeps away from us. It’s often been overlooked just how much warmer the seaside suburbs really are and feeling sheltered from much of the directly cold southerlies by being in the extended lee of the Port Hills. Glad to see these more accurate readings from New Brighton which some people out west may find hard to accept! The easterly may have a reputation, but it’s easily tamed with fencing and shelter trees so we get nice micro-climates with sub-tropical fruits common.
This seems to illustrate the fact that these days that many members of the general public generally are willing to pronounce themselves expert enough on weather/meteorology issues to make comments on validity of readings or meteorology in general, when in reality the huge majorityof them know very little about it. I blame the climate change deniers for a lot of this, plus the proliferation of amateur observing networks.
wow awsome response, just shows how complex weather forcasting is. I never really thought of chch having little micro climates, its so flat (apart from the port hills)
The Banks Peninsular observations are taken from our Le Bons Bay automatic weather station. Banks Peninsular is a large and meteorologically complex area and it would be hard to pick one place that was representative of the region. Le Bons Bay AWS is the only one we have in that large area, and it is probably representative for it’s vicinity.
That explanation applies to the particular situation of a Fohn wind regime. We hope that for most of the time and for most people in the greater Chrischurch area, the stations we are using, and the modelling will give useful information. If, for a partucular location, people find the reported observations (or forecasts) consistently biased one way or the other in certain regimes, then we hope that they take this into account when using this information.
Thanks.
Thanks for the explanation of differing temperatures in the Christchurch area.
Can you tell me where the Banks Peninsula readings are taken, as we are in Okuti Valley, close to Little River and if the only one is in Le Bons Bay our climate is quite different. We have been discussing the readings and are not sure how accurate they are, Banks Peninsula being a large area. Thanks for your response….