Diurnal Cycle Temperature

[Pavido Scalf]

Temperaturelag, also known as thermal inertia, is an important factor in diurnal temperature variation. Peak daily temperature generally occurs after noon, as air keeps absorbing net heat for a period of time from morning through noon and some time thereafter. Similarly, minimum daily temperature generally occurs substantially after midnight, indeed occurring during early morning in the hour around dawn, since heat is lost all night long. The analogous annual phenomenon is seasonal lag.

Diurnal temperature variations are greatest very near Earth's surface. The Tibetan and Andean Plateaus present one of the largest differences in daily temperature on the planet, as does the Western US and the western portion of southern Africa.

In the absence of such extreme air-mass changes, diurnal temperature variations typically range from 10 F (5.6 C) or smaller in humid, tropical areas, up to 40 to 50 F (22.2 to 27.8 C) in higher-elevation, arid to semi-arid areas, such as parts of the U.S. Western states' Intermountain Plateau areas, for example Elko, Nevada, Ashton, Idaho and Burns, Oregon. The higher the humidity is, the lower the diurnal temperature variation is.

In Europe, due to its more northern latitude and close proximity to large warm water bodies (such as the Mediterranean), differences in daily temperature are not as pronounced as in other continents. However, places in Southern Europe significantly far from the Mediterranean tend to have high differences in daily temperatures, some around 14 C (25 F). These include Southwestern Iberia (e.g. Alvega or Badajoz) or the high-altitude plateaus of Turkey (if considered part of Europe) (e.g. Kayseri).

Diurnal temperature variation is of particular importance in viticulture. Wine regions situated in areas of high altitude experience the most dramatic swing in temperature variation during the course of a day. In grapes, this variation has the effect of producing high acid and high sugar content as the grapes' exposure to sunlight increases the ripening qualities while the sudden drop in temperature at night preserves the balance of natural acids in the grape.[4]

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Feature papers represent the most advanced research with significant potential for high impact in the field. A Feature Paper should be a substantial original Article that involves several techniques or approaches, provides an outlook for future research directions and describes possible research applications.

Abstract: The diurnal cycle of land surface temperature (LST) is an important element of the climate system. Geostationary satellites can provide the diurnal cycle of LST with low spatial resolution and incomplete global coverage, which limits its applications in some studies. In this study, we propose a method to estimate the diurnal cycle of LST at high temporal and spatial resolution from clear-sky MODIS data. This method was evaluated using the MSG-SEVIRI-derived LSTs. The results indicate that this method fits the diurnal cycle of LST well, with root mean square error (RMSE) values less than 1 K for most pixels. Because MODIS provides at most four observations per day at a given location, this method was further evaluated using only four MSG-SEVIRI-derived LSTs corresponding to the MODIS overpass times (10:30, 13:30, 22:30, and 01:30 local solar time). The results show that the RMSE values using only four MSG-SEVIRI-derived LSTs are approximately two times larger than those using all LSTs. The spatial distribution of the modeled LSTs at the MODIS pixel scale is presented from 07:00 to 05:00 local solar time of the next day with an increment of 2 hours. The diurnal cycle of the modeled LSTs describes the temporal evolution of the LSTs at the MODIS pixel scale. Keywords: land surface temperature (LST); diurnal temperature cycle (DTC); MODIS; MSG-SEVIRI

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Extreme heat is increasingly being acknowledged as a serious hazard to human health, through a combination of physiological responses to heat, expressed as dry and wet bulb temperatures, and personal factors. Here we present an analysis of the diurnal variability of dry and wet bulb temperatures using station data in South Asia during both regular and heatwave days. We find that diurnal cycles differ, with the daily maximum wet bulb temperature occurring several hours after the daily maximum dry bulb temperature. Using radiosonde profiles, we show that the timing and amplitude of the diurnal variability of wet bulb temperature can be explained by changes in boundary layer depths and water content. Physiological thresholds for uncompensable heat stress were exceeded even in the evenings, many hours after dry bulb temperature peaks. Cumulative exceedances occurred in 105 instances, corresponding to at least 300 hours of exposure to uncompensable heat stress in South Asia between 1995 and 2020. We conclude that physiologically relevant thresholds provide a more robust way to estimate health impacts, and that wet bulb temperature alone is insufficient as an indicator of hazardous heat.

Extreme heat is increasingly being acknowledged as a serious hazard to human health1,2,3. The health impacts of exposure to extreme heat occurs due to a combination of physiological responses to heat and other factors such as age, co-morbidities and behavioural traits, making it difficult to quantify the impacts of extreme heat4. Furthermore, human thermal environments are determined by the ambient temperature, humidity, shortwave/longwave radiation and wind speed, not all of which are routinely and widely measured. These challenges in quantifying both heat exposure and heat stress have resulted in the development of a wide variety of metrics, some of which are simply indicators of the ambient environment whereas others are more physiologically based5,6,7,8,9,10,11,12. Temperature and humidity have been widely and reliably measured over many decades all around the globe, and since direct heat transfer and evaporation of sweat are both pathways for removal of heat from the human body, metrics that incorporate both ambient temperature and humidity have come to be widely used to quantify heat stress.

Few studies exist which actually look at health impacts of extreme heat in South Asia, primarily due to a lack of high quality health data24,25,26,27,28. Given this lack, using physiological data to contextualise TD and TW variability in South Asia provides an alternative methodology to identify regions and periods that are hazardous to human health. Furthermore, these studies represent each day using a single value of TD (maximum, minimum or mean)27. While this choice is made since the response variable (usually all-cause mortality) is also at daily resolution, ignoring the relationship between the diurnal variability of TD and TW makes it difficult to quantify the actual duration for which hazardous environmental conditions exist during each day.

In this study, we document the seasonal variability of extreme TD and TW to contextualise subsequent statistical analyses. We document the timing of daily maximum and minimum of TD and TW and how this timing changes during heat waves. We then use radiosonde data to understand change in timing and amplitude in terms of boundary layer heights and water content. To understand the likely implications on human health, we contextualise TD and TW variability using recent thermal physiological experiments which establish TD/TW combinations that correspond to uncompensable heat stress.

Most documented heatwaves in South Asia have occurred between the months of March-June (see Supplementary table S1 and the EM-DAT data). In fact, The top ten heatwaves in terms of mortality in the EM-DAT database have predominantly occurred in the months of May and June.

While the monthly distribution of extreme TD and TW may suggest a diminished role for extreme TW, this may not be true for individual high-impact heatwaves. For instance, in the analysis of the high impact heatwave in Karachi in June 2015, it was suggested that elevated temperature and humidity were responsible for the unprecedented mortality on the basis of the observed heat index30. However, station data for that period (supplementary figs. S3 and S4) shows that there was a considerable drop in daily minimum humidity (and TW) during the event, and the daily maximum values of humidity and TW were otherwise close to their seasonal mean values.

The mean hour in local time at which daily maximum (a) and minimum (b) TW is reached in South Asian stations. The lighter colours represent hours close to noon (hot times of the day) whereas the darker colours represent cooler times of the day.

Given the experimental evidence for the dependence of critical TW on TD in the context of uncompensable heat stress23, the significance of extremely high TW (which occurs in the coolest times of the day) in the context of human health is unclear; we will return to this issue in the subsequent section.

The change in amplitude of the diurnal cycle can occur either due to an increase in the daily maximum TW or reduction in the daily minimum TW or both. Figure 3 shows the relationship between the monthly mean and standard deviation of daily maximum and minimum TW for all stations in South Asia. Both quantities show an inverse relationship to the mean value, with stations that are more humid on average exhibiting lower variability in TW. This inverse relationship implies that stations with a higher mean humidity (such as those on the coasts) have smaller variability in the amplitude of the diurnal cycle of TW.

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