How does one balance wind, sun and air temps when forecasting warm or cool days?

hatrickpatrick · 28-04-2021 7:04pm #1

I've always wondered a bit about this, and this could well be partly down to living in Glasthule which is literally right on the eastern seafront (beside Dun Laoghaire), so when it's windy here it tends to be cold wind regardless of other factors.

Essentially what I'm wondering is, how does one balance the air temperature and expected wind speed when determining whether a day is going to feel warm or cool? I know there are other factors which play in to the "absolute temperature" vs "feels like" temperature, but this one seems to be the biggest one if you live in a coastal area. And how does sunshine or cloud cover impact this?

To take one example: This coming weekend's Dun Laoghaire forecast suggests that Saturday afternoon (4PM) will have a temperature of 8ºC with a wind speed of 8 km / h, under a partly sunny sky. Sunday afternoon, however (also 4pm) will have a temperature of 10ºC with a wind speed of 20 km/h, again under a partly sunny sky.

Going by temperature alone, Sunday will be the warmer of the two days. However, the difference in wind speed is substantial, and at least to my mind, suggests that Saturday will feel warmer - even though it's two degrees lower in air temperature, the amount of wind expected on Sunday would in my experience make the day feel a lot less warm, particularly if walking on the seafront or sitting in the park beside the sea. That, however, is pure instinct and could turn out to be entirely incorrect.

Furthermore, sometimes (especially at this time of year and earlier in the Spring / late Winter) you can have cold high pressure systems in which a cloudless day with slack winds and plenty of sunshine will oddly feel cooler than a day with a lot of cloud around and low pressure. If you have two days, both forecasting a temperature of 10 degrees, but one with clear skies and another with a lot of cloud cover, which day will actually be warmer out in terms of which clothes you would wear, etc?

What's the generally accepted way to balance these factors in deciding, for instance, which day would be nicer to go for a walk or have a picnic?

In the era of outdoor, socially distant coffee dates and catchups, these are the important questions dammit

Oneiric 3 · 28-04-2021 9:25pm

Not sure how it is calculated 'officially' but on my weather station there is a 'THW index' (Temp/Heat/Wind) that calculates a presumed 'feel' of the current temperature.

https://www.davisinstruments.com/blog/meteorology-101-understanding-indexes/

I think such calculations can give a general idea, but whether you feel warm or cool or just neutral in any given condition is a highly subjective experience.

Gaoth Laidir · 28-04-2021 10:38pm

Wet-bulb Globe Temperature (WBGT) is generally accepted to be a good indicator of heat stress as it also takes into account solar radiation, etc. I've yet to come across one of these instruments, though. It's not really applicable to a temperature of 10 degrees, though, and not really easy to calculate without the black globe.

https://ksi.uconn.edu/prevention/wet-bulb-globe-temperature-monitoring/

riffmongous · 30-04-2021 8:33pm

There are a number of different indexes that can be used, the most sophisticated ones use a full energy balance model for a human. An example..

The Perceived Temperature (PT) is an equivalent temperature based on a complete heat budget model of the human body. It has proved its suitability for numerous applications across a wide variety of scales from micro to global and is successfully used both in daily forecasts and climatological studies. PT is designed for staying outdoors and is defined as the air temperature of a reference environment in which the thermal perception would be the same as in the actual environment. The calculation is performed for a reference subject with an internal heat production of 135 W m(-2) (who is walking at 4 km h(-1) on flat ground). In the reference environment, the mean radiant temperature equals the air temperature and wind velocity is reduced to a slight draught. The water vapour pressure remains unchanged. Under warm/humid conditions, however, it is implicitly related to a relative humidity of 50%. Clothing is adapted in order to achieve thermal comfort. If this is impossible, cold or heat stress will occur, respectively. The assessment of thermal perception by means of PT is based on Fanger's Predicted Mean Vote (PMV) together with additional model extensions taking account of stronger deviations from thermal neutrality. This is performed using a parameterisation based on a two-node model. In the cold, it allows the mean skin temperature to drop below the comfort value. In the heat, it assesses additionally the enthalpy of sweat-moistened skin and of wet clothes. PT has the advantages of being self-explanatory due to its deviation from air temperature and being--via PMV--directly linked to a thermo-physiologically-based scale of thermal perception that is widely used and has stood the test of time. This paper explains in detail the basic equations of the human heat budget and the coefficients of the parameterisations.

https://pubmed.ncbi.nlm.nih.gov/21336880/#:~:text=The%20Perceived%20Temperature%20(PT)%20is,daily%20forecasts%20and%20climatological%20studies.

And if you can read German
https://www.dwd.de/DE/service/lexikon/begriffe/G/Gefuehlte_Temperatur_pdf.pdf?__blob=publicationFile&v=4

Danno · 30-04-2021 10:39pm

Any wind over 10mph (16kph for the funny bunnies) and it starts to take the edge off what would be an otherwise glorious day. For me anything -2c and above in calm conditions with unbroken sunshine is perfect weather.

During the cold spell in January there wasn't a puff of wind, and daytime temperatures barely got above freezing. Leisure walking in our locality involved going through woodland and once the sun was shaded the "nip" was notable in the air, but out in the open it was grand.
Last weekend here was fantastic, clear sky for the most part and temperatures nudged 17c to 18c, again with little wind it felt perfect. Much of April has been superb owing to the lack of wind allowing the sun to work on the skin.

The only time I'd appreciate a breeze of any kind is when it hits 24c, 25c and upwards. For that reason, I'd detest a coastal location to live in. Outdoor labouring is horrible in a wind. For me, stuck in the bottom of a valley at the south end of Laois is about as good as it can get in Ireland.

01-05-2021 11:31am

Temperatures in April don’t seem to reflect what people feel. A lot of people were sunning themselves in 8-10° a week or two ago. The breeze was slight, the sun was warm. It felt warm.

Oneiric 3 · 01-05-2021 12:29pm

Danno wrote: »

Any wind over 10mph (16kph for the funny bunnies) and it starts to take the edge off what would be an otherwise glorious day. For me anything -2c and above in calm conditions with unbroken sunshine is perfect weather.

During the cold spell in January there wasn't a puff of wind, and daytime temperatures barely got above freezing. Leisure walking in our locality involved going through woodland and once the sun was shaded the "nip" was notable in the air, but out in the open it was grand.
Last weekend here was fantastic, clear sky for the most part and temperatures nudged 17c to 18c, again with little wind it felt perfect. Much of April has been superb owing to the lack of wind allowing the sun to work on the skin.

The only time I'd appreciate a breeze of any kind is when it hits 24c, 25c and upwards. For that reason, I'd detest a coastal location to live in. Outdoor labouring is horrible in a wind. For me, stuck in the bottom of a valley at the south end of Laois is about as good as it can get in Ireland.

I think I'm a curious case because the colder it gets, the less I feel the cold. With temps of -5c or well below, I am quite comfortable wearing a teeshirt outside! The more usual temps of around +5.0c and I'm perished.

How does one balance wind, sun and air temps when forecasting warm or cool days?

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