Why are Day/Night temperature changes larger in summer than in winter?

Submitted by photenth t3_zf2ubk in askscience

On my weather app on my phone I always see the temperature line across the week and I just noticed that during winter the peaks and valleys are a lot shallower than in summer. So I checked historical data from my location and indeed, on clear summer days, the temperature can reach 35 degrees celsius during the day and drop to around 15 at night, that's a temperature change of 20 degrees. In winter, the biggest change I found for my location is 10 degrees. But overall, it seems to me that the changes are more dramatic in summer than in winter.

Is it overall cloud coverage? Is it absolute humidity? Is it the energy cost of water freezing and melting? Or is it just my location that is like this?

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CrustalTrudger t1_iz9x3gi wrote

Some terminology first, what you're describing is usually referred to the 'diurnal temperature range' or DTR (so if you want to poke around more, searching for this along with 'seasonality' will get you much of the relevant literature).

The pattern you're noticing, i.e., a maximum in DTR during the summer and a minimum during the winter, is characteristic of some (but importantly not all) locations. For example, Ruschy et al., 1991 found similar patterns in portions of Minnesota and attributed these to tradeoffs between cloud cover, solar radiation intensity, and albedo (from snow) throughout the year. Schwartz, 1996 found a similar pattern over much of the northeastern US, but added a variety of other possible controls (e.g., seasonal humidity changes, changes in vegetation behavior, etc).

However, the above is not a global pattern, but rather regional. For example, broader consideration of just the contiguous US highlights three general behaviors (1) the northern and western US have a maximum DTR in the summer and a minimum in the winter, (2) the central and southeastern US have a maximum DTR in the winter and a minimum in the summer, and (3) the mid-latitudes of the US have two maxima in DTR in the spring and fall and two minima in the summer and winter (e.g., Leathers et al., 1998, Robinson et al., 1995). As with the geographically more limited view from above, a variety of factors (that vary be region) are proposed as causing these different patters in DTR (and subsequent work has proposed even more, e.g., Durre & Wallace, 2001a, Dai et al., 1999, Portmann et al., 2009). If you browse many of these, you'll find that common suggested controls are cloudiness, various aspects of the hydrologic cycle (e.g., precipitation, soil moisture, etc), and vegetation changes (which are linked to the hydrologic cycle).

Also of note is a wide body of work that highlights that patterns and magnitudes of seasonal DTR are changing as a result of climate change (e.g., Durre & Wallace, 2001b, Balling & Cerveny, 2003, etc.) so longer term historical ranges for your (and other) location(s) may differ from more recent seasonal patterns in DTR.

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photenth OP t1_iz9xijz wrote

What a great answer. So essentially highly complex issue with multiple factors at play and I just happen to live where it's the way I noticed it.

Thanks!

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Busterwasmycat t1_iza38x7 wrote

yes, you understand correctly. Proximity to water bodies, dryness of the air (cloud cover or lack of it) and other issues definitely matter.

A major factor, however, is the fact that daytime heating is a lot less intense when the sun is a lot lower to the horizon, so the heating is less than it would be if the sun were higher (it isn't only that there is less time of sun exposure, but also that the sun is not even close to as high in the sky).

The local temperature "wants" to be at the median temperature for the location (where it would stay if input and output was the same across the entire 24 hour period). In winter, the amount of heating is a lot less (much less sunlight per unit area), so the increase ABOVE median is smaller, and thus the cooling is equally less severe to get back to median, and the magnitude of variation is smaller.

One cannot ignore the role of atmospheric moisture though. This is a major factor in why cloudless winter nights tend to be so dang cold but cloudy weather isn't generally cold. The clouds and the greenhouse effect of water limit the amount of heat which can make it out into space, so the lower atmosphere stays warmer (loses a lot less heat to space). The flip side, of course, is that cloudiness also reduces the amount of sunlight making it down to the ground and heating things up in daytime. It is definitely a complicated process, many factors matter.

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DrunkenGolfer t1_izco443 wrote

One factor to consider is humidity. The air can only cool to the dew point and at that point there is an equilibrium. Instead of getting colder, things just get wet. Humid air has a higher specific heat capacity than dry air, so the same amount of solar energy will heat an equivalent volume of air to different temperatures.

I lived in Bermuda where the humidity is always high, but the diurnal range is greater in winter when the humidity is lower.

Here is light reading and formulas: https://www.engineeringtoolbox.com/amp/enthalpy-moist-air-d_683.html

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raiu86 t1_izbbf9b wrote

This is so cool...especially because my first thought reading OPs title was "you mean the other way around, right?" I live in the SE US.

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Brosambique t1_izctxm7 wrote

Wow amazing and interesting answer.

Can I ask why you know all this and what you do for work?

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Sagacity89 t1_izbymgn wrote

Wow. I never knew meteorology could be so sexy and interesting.

Thank you.

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ggpopart t1_izc57vt wrote

This is fascinating! Thank you so much for sharing.

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brettscharff t1_izd8ch0 wrote

Wow. Impeccable answer. Sited and all! Wowzers, I just made a mess in my trousers 🤓

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funnyman95 t1_izbmsdy wrote

Could it also just be a % change as well? Like in the summer if it’s 90F during the day but 65F at night, vs 40 during the day and 30 at night, both are around a 25% temp drop.

This alone seems like it could mostly explain away OPs findings

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CrustalTrudger t1_izc604f wrote

This is not supported by any of the cited literature.

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funnyman95 t1_izfllkz wrote

but like, just do the math.... How is it wrong?

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CrustalTrudger t1_izfslal wrote

It's wrong because much of the data is not consistent with your idea if you do the math. The data presented in Leathers et al 1998 is useful here as they consider how Tmax varies with DTR. Even for those with a linear Tmax to DTR relationship, these don't have consistent percentage drops (and so your explanation doesn't work). Additionally, as described in the original answer, many sites have very complicated relationships between Tmax and DTR and with variable timing of seasonal maxes of DTR. For example, one site where at days with a Tmax of nearly 30 C and those with a Tmax of -5 C have the same DTR of ~12 C (with a minimum in DTR with a Tmax of 5 C). I.e., looking at actual data highlights that your explanation is not useful as a broad explanation.

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somewhat_random t1_izccrw0 wrote

weather is complex but a couple items that should be mentioned:

heat is added all day and removed at night. The net flux of energy is that solar radiation adds energy during the day and black body cooling sends energy away from earth at night.

In summer, the solar radiation is higher (that is why it is summer) so there is more gain during the day. At night, a warmer black body can emit more energy so the dark planet can lose more energy at night.

Cloud cover dampens both effects so cloudy winters make this more pronounced.

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nvrhsot t1_izc515q wrote

The simple explanation is in two parts In non tropical or subtropical climate regions where moisture content is moderate to low, in the warmer months , temps tend to rise rapidly . Warm air holds more moisture than cooler air . So let's say the high temp is 90% and the peak dew point is 55° Thats relatively dry air.. Once the sun begins to approach the horizon and ultimately sets, the air begins to cool rapidly. And while the relative humidity increases, the actual moisture content stays relatively the same. In this case the dew point of 55° Under certain conditions, the ambient temperature will cool to the dew point.. So the low temp will be in the mid 50s In winter, there tends to be more cloud cover, and if in a region where precipitation is at its highest, moisture levels in the lower atmosphere tend to remain high.. So let's say on a given day the high temp is 35° And let's say there is either a snow pack or the ground is moist. Moisture must evaporate. And does so into the atmosohere closest to the ground.. This actually drives the dew point higher.. And because the air temp can not fall below the dew point, this limits the range between today's high and low air temp.

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life_like_weeds t1_izd9v6c wrote

The answer is massively dependent on the location. High deserts have extreme temperatures swings in the winter. So do most temperate regions with average true winters (Midwest United States, New England).

It’s not uncommon to see 40-50°F swings from day to night in the winter. This does not happen in the summer in those regions.

A proper answer to this particular question would require specific geographic context.

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[deleted] t1_izb9648 wrote

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photenth OP t1_izba26l wrote

AFAIK Planets without atmosphere bleed heat into space at the same rate no matter how much sunlight they got during the day, so their temperature ranges are pretty much the same across the year, just at a different starting points

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Olclops t1_izcl5o8 wrote

A simplified but still true answer:

For most locations on the planet, temperature change is largely driven by the heat differential between the closest pole and the tropics/equator. The tropics always gets a lot of sunlight. In the summer (in the northern hemisphere) the closest pole gets a lot of sunlight too. So there's not a huge differential to trigger wind/shifting temperature in the areas in between.

In the winter, the north pole gets almost no sunlight and the equator still gets lots. So the differential is much higher. Which means there's lots of wind activity and disequilibrium, leading to more temperature change in between.

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brettscharff t1_izd99i2 wrote

Actually, the tropics are a typically cloudy area. If you look at a world weather map, you will almost always find super cloudy areas there around the equator. Take a city like Lima, Peru for example. Barely any rain, but always cloudy and on the equator. So while the solar radiation is high, and humidity is high, sunshine is low and so is precipitation. This leads to low diurnal temperature variation.

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financial2k t1_izsnb8t wrote

It's not that complicated as some make it out to be. You mostly have to just follow the net energy and understand that earth is a globe and rotating around a tilted axis in relation to the sun, which gives rise to a number of seasons on the northern and southern hemisphere the further you are away from the equator.

One more thing. What is temperature?

>At lower temperatures, the molecules have less energy. Therefore, the speeds of the molecules are lower and the distribution has a smaller range. As the temperature of the molecules increases, the distribution flattens out. Because the molecules have greater energy at higher temperature, the molecules are moving faster.https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/03%3A_Rate_Laws/3.01%3A_Gas_Phase_Kinetics/3.1.02%3A_Maxwell-Boltzmann_Distributions

Okay so temperature has to do with energy and virtuall all comes from the sun.

Then the main determinant is:How much energy is reaching the surface of which material for how long and how much is radiated back into space. Let's assume the surface is asphalt since > 90% of us are living in cities.

>Water has a very high specific heat. That means it needs to absorb a lot of energy before its temperature changes. Sand and asphalt, on the other hand, have lower specific heats. This means that their temperatures change more quickly.

So asphalt and concrete heat up very fast and release a lot of that heat very fast. Similarly to a desert. In Winter there is just much less heat to begin with and that heat is quickly irradiated away.

Look at this:Google images: https://www.google.com/search?q=energy+net+flow+earth+albedo

This is not enough to create a weather model, but it will make you understand the temperature range of winter vs summer.

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