Not a precise science, part 5

(… continued from Not a precise science, part 4, posted on 17 Oct 2015).

One further meteorological “known unknown” that is identified in the HS2 Phase 1 Environmental Statement (ES) is atmospheric temperature inversion leading to what the ES terms “positive temperature gradients” (see footnote 1). This is an anomalous, but not uncommon, atmospheric mechanism that, when the conditions are right, leads to an “inversion” of the normal decrease of air temperature with height, allowing the air layers closest to the ground to be cooler, and thus more dense, than those above them (see footnote 2). In still conditions, this disrupts the normal air circulation within the atmosphere, which can lead to the most visibly-obvious symptom of temperature inversion – trapped pollutants and even smog.

Because the speed of sound in air varies with its temperature, temperature inversion can affect the normal tendency of sound to travel in straight lines, and lead to sound being refracted, or bent, down towards the surface of the earth. This sound ray bending acts like a speaking tube, concentrating sound on its journey from its source to an observer, and leading to amplification of the sound level observed.

We are all familiar, I am sure, with the enhanced clarity with which distantly-generated sounds can be heard around dawn following a still and crisp night – this is the effect of temperature inversion in the atmosphere above the cold ground. One observer who is certainly aware of this phenomenon in his surroundings is Dr Roger Harrison, as he was at pains to apprise the HS2 Select Committee of the effects that positive temperature gradients can have on sound levels experienced by residents of the Chilterns (see footnote 3).

He introduced this topic by relating his own experience of the phenomenon during a summer’s night when he traced the source of music that was keeping him awake to a rock concert in Great Missenden “4km away”. But Dr Harrison’s evidence was not merely anecdotal, and it became obvious that he had thoroughly researched the topic – he told the Committee that he had a chemistry doctorate and that, with his professional experience, considered himself “very capable of assessing technical data, and the pros and cons of new technology” (see footnote 4).

Dr Harrison told the Committee that he considered that, in his neighbourhood, “the inversion effect is like to be common, judging from [his] personal experience and technical literature reports” and that the horizontal layering that resulted from inversion was likely to be “stable and long-lasting” in the Chilterns valleys. He added (see footnote 5):

“What turns this phenomenon into a problem, what will cause widespread noise nuisance is the introduction into the temperature inversion environment of an intense and near continuous noise source: HS2 travelling at up to 400km/h.”

In one respect Dr Harrison was wrong. He told the Select Committee that HS2 Ltd “have not even considered” the effects of temperate inversion on the noise impacts of HS2. As Counsel for the Promoter, James Strachan QC, was keen to point out, there are the two paragraphs on the subject in the ES that I have identified in footnote 1 to this blog. Mr Strachan echoed the view expressed therein that positive temperature gradients (see footnote 6):

“… only occur on still days and result in similar levels of sound increase at distance root as downwind conditions, and therefore the HS2 prediction method also holds for average propagation during clear, calm nights. So that, in effect, the worst-case scenario for the downwind conditions that are used for the modelling, result in the same noise spread, albeit in different atmospheric conditions, as where there is ground temperature inversion …”

Whilst it is undoubtedly true that sound propagation enhancement due to temperature inversion and positive wind are mutually exclusive, the plausibility of the claim in the ES that the impact of the former is already accounted for in the HS2 propagation model by providing for the latter hinges on the assertion that the two cause “similar levels of sound increase at distance” and no evidence to support that proposition is offered in the ES. So let’s test that claim as best as we are able.

Dr Harrison’s evidence can assist in this, as he quotes the claim in “a 2003 paper, in Applied Acoustics” (footnote 7) that “Inversions can result in sound pressure levels along the ground that are 20-30dB higher than those typically observed during neutral atmospheric conditions”. Reference back to the set of data relating to received sound levels at 200 metres from the track that I constructed for part 3 instructs us that the mean measured upwind noise level is 68.9dB LpAF,Max and the downwind mean is 79.4dB LpAF,Max. I suggest, therefore, that a typical value for still conditions may be derived from the mean of these two means, which is 74.1dB LpAF,Max. So the tentative range for received levels during inversion conditions is 94. 1dB LpAF,Max to 104.1dB LpAF,Max. This is clearly well above the 82dB, or thereabouts, that the HS2 model predicts. So it would appear that the ES is again being over-optimistic in its claims, which is surely no surprise based upon what we have uncovered so far.

Dr Harrison makes one further point that serves to underline the relevance of his concerns. He quotes from “an authoritative review of sound barrier design” (see footnote 8):

“… it is generally recognised that downward-curving sound paths, as in … during the temperature inversions that are common at night, do reduce the insertion loss of a barrier.”

And, of course, this double whammy applies not just to fence noise barriers, but to screening earthworks, cuttings and some natural screening features also.

It is just as well perhaps that the Chairman of the Select Committee, Robert Syms MP, indicated that he intended to ask the Promoter’s acoustics witness, Rupert Thornely-Taylor, “some questions about temperature inversion” when he was before them subsequently (see footnote 9).

(To be concluded …)


  1. The ES devotes just two paragraphs to this mechanism: paragraphs 1.3.29 and 1.3.30 in Annex D2 to Appendix SV-001-000 of the ES.
  2. A fairly straightforward explanation of temperature inversion and its effects on sound propagation may be found here.
  3. Dr Harrison’s petition was heard by the HS2 Select Committee during the morning session that was held on Thursday 24thSeptember 2015. The relevant part of his evidence is recorded from paragraph 100 in the transcript and at 10:11 hrs in the video.
  4. See paragraph 57 in the transcript of the morning session of the HS2 Select Committee that was held on Thursday 24thSeptember 2015.
  5. The quotes are taken from paragraphs 103 and 111 of the transcript of the morning session of the HS2 Select Committee that was held on Thursday 24thSeptember 2015.
  6. See paragraph 126 in the transcript of the morning session of the HS2 Select Committee that was held on Thursday 24th September 2015.
  7. I understand the paper to be Hole, L R and Hauge, G, Simulation of a morning air temperature inversion break-up in complex terrain and the influence on sound propagation on a local scale, Applied Acoustics 64 (2003) 401–414.
  8. Technical Assessment of the Effectiveness of Noise Walls, Final Report of a working party of the International Institute of Noise Control Engineering (I‑INCE), September 1999.
  9. See paragraph 123 in the transcript of the morning session of the HS2 Select Committee that was held on Thursday 24th September 2015. At the time of the posting of this blog, it is my understanding that these questions remain to be asked.

Important Note: The record of the proceedings of the HS2 Select Committee from which the quotes reproduced in this blog have been taken is an uncorrected transcript of evidence, which is not yet an approved formal record. Neither witnesses nor Members have had the opportunity to correct the record in such instances, and it may therefore be subject to changes being made in the light of any such corrections being requested.



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