Post by: Petrochemicals on 16/04/2021 23:35:28
Post by: alancalverd on 17/04/2021 09:12:38
However all things are not equal. Cloudless nights are associated with high pressure systems. In winter these tend to be arctic, so the mean air temperature is low, and in summer, high pressure may build from the south, producing a persistent blanket of warm (often hazy) air.
The phenomena are particularly noticeable in the UK because winter temperatures hover around freezing, so a small change produces "ice or no ice" in winter but would not be noticed in summer. That said, dewy nights are equally common any time.
Post by: Colin2B on 17/04/2021 09:22:09
There are 2 factors that will affect this:
Ground heat capacity. During winter/spring the sun is lower in sky and doesn’t heat the ground as much as in summer when sun is higher in sky. This heat is radiated back out at night.
Water vapour. Summer has more water vapour in air. This absorbs heat during the day and not only acts as a thermal blanket at night, but also traps the reradiated heat from the ground. This is also why deserts get so cold at night.
Post by: Bored chemist on 17/04/2021 12:26:54
https://en.wikipedia.org/wiki/Diurnal_temperature_variation#/media/File:July_diurnalvariation_US.svg
Or maybe there are other effects to consider.
Post by: evan_au on 17/04/2021 12:57:28
Sydney_Max_min_temperatures.png (22.8 kB . 700x324 - viewed 4764 times)This shows that the difference between average daily maximum & minimum temperatures stays (roughly) constant all year round in Sydney. It has a slightly greater range in spring (10°C) than in autumn/fall (8°C).
Why don't you try for where you live?
Note: These are monthly averages. If you looked at a cloudless 24 hours, you would find a big difference in temperature between day and night
- While for a completely overcast 24 hours in the same month, the difference between maximum and minimum temperatures would be much less (because clouds reduce the intensity of sunlight during the day, and hold in the heat at night)
Post by: Petrochemicals on 17/04/2021 13:45:25
The maximum temperature is higher in summer because the sun is higher and the day is longer, but if all else is equal the difference between day max and night min won't change much - indeed it might even be bigger because of the T4 characteristic of radiant loss at night.
I would say the maximum temperature is because it starts from a higher threshold. I know that the higher in the sky the sun is the greater the concentration of solar radiation, but it seems that heat can build during the summer where as it doesn't seem to during winter.
However all things are not equal. Cloudless nights are associated with high pressure systems. In winter these tend to be arctic, so the mean air temperature is low, and in summer, high pressure may build from the south, producing a persistent blanket of warm (often hazy) air.I suppose that is the jet stream, the jet is beneath the uk at present where as during the summer it is above us I believe.
The phenomena are particularly noticeable in the UK because winter temperatures hover around freezing, so a small change produces "ice or no ice" in winter but would not be noticed in summer. That said, dewy nights are equally common any time.But in summer a temperature of 25 degrees does not yield a freezing night.
Post by: Petrochemicals on 17/04/2021 13:54:09
Just to add to what Alan is saying - same thing but different wordsIt cannot be as greater effect as that, if the sun can raise the air to a temperature of 25 degrees.
There are 2 factors that will affect this:
Ground heat capacity. During winter/spring the sun is lower in sky and doesn’t heat the ground as much as in summer when sun is higher in sky. This heat is radiated back out at night.
Water vapour. Summer has more water vapour in air. This absorbs heat during the day and not only acts as a thermal blanket at night, but also traps the reradiated heat from the ground. This is also why deserts get so cold at night.That foxes every theory about jet streams, ground temperature, sun angle and polar air. I suppose that is why the September October months can still be warm day and night.
Post by: Petrochemicals on 17/04/2021 13:55:36
If I Google "Monthly Temperatures Sydney" (where I live), and click on "Graphs", I get the following graph:Not average temperature. I was asking why heat doesn't build during hot periods during the cooler months.
This shows that the difference between average daily maximum & minimum temperatures stays (roughly) constant all year round in Sydney.
Why don't you try for where you live?
Note: These are monthly averages. If you looked at a cloudless 24 hours, you would find a big difference in temperature between day and night
- While for a completely overcast 24 hours in the same month, the difference between maximum and minimum temperatures would be much less (because clouds reduce the intensity of sunlight during the day, and hold in the heat at night)
Post by: alancalverd on 17/04/2021 14:15:56
But in summer a temperature of 25 degrees does not yield a freezing night.Nor does the temperature regularly drop by 25 deg in winter.
Post by: Colin2B on 26/04/2021 15:05:42
It cannot be as greater effect as that, if the sun can raise the air to a temperature of 25 degrees.Not sure what you mean.
Last night we went down to -2°C, but daytime only peaked at 11°C.
In a desert daytime can peak at 50°C and still go below 0 at night. The ground doesn’t hold much heat overnight if it is able to radiate the heat away and air is able to rise, as it will on a clear night with no cloud cover and a low level of water vapour.
That foxes every theory about jet streams, ground temperature, sun angle and polar air. I suppose that is why the September October months can still be warm day and night.If “That foxes” means is “consistent with” then yes.
Although this is mainly a boundary layer effect (up to 2km) high level jet stream and polar air can have an effect at our latitudes. As you know, hot air rising at the equator does not go directly to the poles in a one cell circulation, but is broken up by the Coriolis effect into 3 vertical circulatory cells. Our jet stream forms at the junction of 2 of those cells, where the depressions develop that result in most of our weather. As the depression moves over us stable air at the warm front develops under good cloud cover keeping the ground warm. After the warm sector has passed (behind the cold front) the air is unstable and rising air adds to the loss of radiative heat loss from the ground; typically these are clearer, cold nights with some Cu cloud.
Post by: evan_au on 26/04/2021 22:25:53
Quote from: Petrochemicals
jet streamI see that the jet stream is regularly mentioned on US weather forecasts, but I've never seen it mentioned in Australia.
- I assume that is because the jet stream is usually well south of Australia, so it doesn't often come near Australia.
The sinuous wanderings of the jet stream impose a randomness on the weather. But the average daily temperatures are measured over many years, and "average out" short term variations like the jet stream. This leaves you with the genuine seasonal variation.
Petrochemicals, why don't you post the average monthly temperatures for your location, to show us what you mean in the OP?
- So we can see if it is real, or just confirmation bias?
Post by: Colin2B on 26/04/2021 23:28:05
As far as I’m aware it generally goes straight through the centre of Australia. As you say, it does move N & S.Quote from: Petrochemicalsjet streamI see that the jet stream is regularly mentioned on US weather forecasts, but I've never seen it mentioned in Australia.
- I assume that is because the jet stream is usually well south of Australia, so it doesn't often come near Australia.
As here, in the N Hemisphere, it is on the boundary of cold and warm air masses, but Rossby waves form moving large masses of air either N or S resulting in a ‘meandering’ of the stream. When the air mass moves north it starts to spin clockwise (from above), forming a high-pressure system - usually settled weather. When air moves south it spins anti-clockwise, low pressure system - often wet weather. Assuming you get wet weather in Australia ;D
Post by: Petrochemicals on 27/04/2021 00:07:04
Yep the temperatures are becoming less extreme, Sunday was not as warm, the night not as cold, but it was still a substantial drop. In summer the temperature fluctuation does not seem as great. March 30 was 25 degrees up from 4 overnight in Cambridge, that is early.It cannot be as greater effect as that, if the sun can raise the air to a temperature of 25 degrees.Not sure what you mean.
Last night we went down to -2°C, but daytime only peaked at 11°C.
In a desert daytime can peak at 50°C and still go below 0 at night. The ground doesn’t hold much heat overnight if it is able to radiate the heat away and air is able to rise, as it will on a clear night with no cloud cover and a low level of water vapour.
Foxes meaning to outwit or out-think, cunning etc. The trouble is with the jetstrean etc is that they are present during the summer yet the temperature dues not seem to fluctuate as it does in spring.That foxes every theory about jet streams, ground temperature, sun angle and polar air. I suppose that is why the September October months can still be warm day and night.If “That foxes” means is “consistent with” then yes.
Although this is mainly a boundary layer effect (up to 2km) high level jet stream and polar air can have an effect at our latitudes. As you know, hot air rising at the equator does not go directly to the poles in a one cell circulation, but is broken up by the Coriolis effect into 3 vertical circulatory cells. Our jet stream forms at the junction of 2 of those cells, where the depressions develop that result in most of our weather. As the depression moves over us stable air at the warm front develops under good cloud cover keeping the ground warm. After the warm sector has passed (behind the cold front) the air is unstable and rising air adds to the loss of radiative heat loss from the ground; typically these are clearer, cold nights with some Cu cloud.
Post by: Petrochemicals on 27/04/2021 00:10:36
Again Evan not the average. We have had a period of warm days and cold nights, yet the same temperatures in summer are not accompanied by similar swings. This has been accompanied by drough and fires.Quote from: Petrochemicalsjet streamPetrochemicals, why don't you post the average monthly temperatures for your location, to show us what you mean in the OP?
- So we can see if it is real, or just confirmation bias?
https://www.theguardian.com/uk-news/2021/mar/30/uk-records-warmest-march-day-in-53-years-with-245c-temperatures
Post by: evan_au on 27/04/2021 03:36:45
Quote from: Petrochemicals
Again Evan not the average. We have had a period of warm days and cold nights, yet the same temperatures in summer are not accompanied by similar swingsSo you want to make a generic claim about spring and summer, but you don't want to use averages?
How do you do that?
- On date A in Spring (where I live), the temperature range was X
- On date B in Summer (where I live), the temperature range was Y
- X > Y
That is not a statement about Spring and Summer, that is a statement about dates A and B where you live.
The only way to make a generic statement about Spring and Summer (rather than about dates A & B) is to show the monthly averages.
- So why don't you want to do it? It's easy enough!
Post by: Colin2B on 27/04/2021 08:33:10
Foxes meaning to outwit or out-think, cunning etc. The trouble is with the jetstrean etc is that they are present during the summer yet the temperature dues not seem to fluctuate as it does in spring.I don’t see how anything I’ve said foxes jetstream.
As I’ve pointed out, the jetstream is not the sole determinant of our weather and in particular temperatures. However, it does give a good indicator of the lines the depressions will move along which is why it is so useful in forecasting.
Like @evan_au I’m becoming confused about what you are really trying to say.
Look at the chart below, which is for a UK weather station, and reread what Alan and I have written. Can you explain what the problem is that you see?
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Post by: Petrochemicals on 27/04/2021 23:28:37
[
The only way to make a generic statement about Spring and Summer (rather than about dates A & B) is to show the monthly averages.
- So why don't you want to do it? It's easy enough!
The temperature drop between night and day in summer is far less than the temperature drop in winter. The post about the desert seemed to understand the theme.
Like @evan_au I’m becoming confused about what you are really trying to say.
Averages are no good because they are a sum total of various highs and lows, these will average out, days with a lower temperature will reduce the average temperature. I understand the theory of averages but as Colin says overcast days will be less extreme, averages are about as useful as max highs and lows, which for march are 25 and - 10, but they are not overly helpful, Wales has a temperature difference of max/min of over 40c for March. The average of 5+5+5 is the same as 0+5+10.
What I am trying to say is the day/night temperature difference is greater in spring than summer. I do not recall a temperature of 25 c in summer being followed by a 4c night time temperature.
The graph Colin I cannot see as useful as I can only see 5 to 10 drops per month, not the standard 30 is nights we have in most months.
https://www.dictionary.cambridge.org/amp/english/average-out
where as the example already given of temperature difference in march this year illustrates an extreme drop, a drop I do not remember in the summer over such a short period. The latter half of April whilst not as extreme still exhibited great temperature differences where the heat was not sustained, it dropped every night and did not build over the period.
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These illustrate max and min temperatures in June 202 and April 2021 in cambridge
Post by: alancalverd on 28/04/2021 10:31:28
Post by: evan_au on 28/04/2021 10:50:57
Quote from: petrochemicals
the example already given of temperature difference in march this year illustrates an extreme dropSo now, we are not talking about the temperature on a particular day, but you are looking at a particular month in a particular year.
- For example, in the week around 20th April, there is a sustained period with a large temperature range, indicating clear skies, while the 2nd April has a very small range, suggesting overcast conditions that day
- So this is still not a generic statement about Spring and Summer, that could be applied in the same place in different years, let alone different places in different years
- If it has no predictive power, it is an anecdote, rather than a hypothesis
Let's look at the multi-year average temperatures for Cambridge UK:
Cambridge High_Low_temps.png (23.15 kB . 693x320 - viewed 4593 times)The temperature range is less in winter than in summer
- But the temperature range is fairly uniform during spring and summer, at about 10C
We can see why the temperature range may be less in winter, because there are more overcast days in winter.
Cambridge_Cloud_Cover.png (69.31 kB . 751x503 - viewed 4563 times)See: https://weatherspark.com/y/45545/Average-Weather-in-Cambridge-United-Kingdom-Year-Round#Sections-Clouds
It is also possible that the moderating effect of the sea and it's significant latent heat when freezing contributes to fewer days that drop below 0C.
Quote from: petrochemicals
I do not recall a temperature of 25 c in summer being followed by a 4c night time temperature.That is the advantage of the weather bureau/Met Office - they do remember!
- In great detail!
- For many years!
Oops - overlap with alancalverd...
Post by: Petrochemicals on 28/04/2021 23:16:06
April 2021 has been an exceptional month with a persistent anticyclone - virtually no wind, whatever wind there has been has been from the east (cold, dry) and negligible cloud. Hence larger temperature swings than you get with a predominantly warm wet west wind with lots of cloud cover. The max for June 2020 didn't exceed 25 deg until the 31st, which indicates lots of cloud - we normally get several days around 30 - 35 degrees in June if there's an anticyclone.Perhaps you are correct Evan, maybe it is my memory playing tricks as both spring 2020 and 2021 have for some reason been sunny and dry! April 2021 is the frostiest on record.
Post by: Petrochemicals on 28/04/2021 23:21:35
We are looking at why heat is dissipated so much more quickly in spring than summer. As posted as reply to Alan, we ar having some exceptional weather in the 2 most recent springs. The heat does not seem to build in the way that it does during the summer months. The example of the desert seems to be the best fitQuote from: petrochemicalsthe example already given of temperature difference in march this year illustrates an extreme dropSo now, we are not talking about the temperature on a particular day, but you are looking at a particular month in a particular year.
- For example, in the week around 20th April, there is a sustained period with a large temperature range, indicating clear skies, while the 2nd April has a very small range, suggesting overcast conditions that day
- So this is still not a generic statement about Spring and Summer, that could be applied in the same place in different years, let alone different places in different years
- If it has no predictive power, it is an anecdote, rather than a hypothesis
Post by: Petrochemicals on 23/06/2021 02:15:20