Viewing a single comment thread. View all comments

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.

353

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!

112

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.

41

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

0

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.

17

Brosambique t1_izctxm7 wrote

Wow amazing and interesting answer.

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

2

Sagacity89 t1_izbymgn wrote

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

Thank you.

1

ggpopart t1_izc57vt wrote

This is fascinating! Thank you so much for sharing.

1

brettscharff t1_izd8ch0 wrote

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

1

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

−4

CrustalTrudger t1_izc604f wrote

This is not supported by any of the cited literature.

4

funnyman95 t1_izfllkz wrote

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

1

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.

1