6.1.1 The Temperature Effect

The temperature effect is the positive correlation between local air temperature and local precipitation stable isotope δ values. Neither the temperature nor the stable isotopes are instantaneous values, but rather are the long-term (annual) means, to avoid sharp fluctuations that reflect only temporary factors related to a particular storm (cloud base elevation, precipitation intensity, and so on). The temperature effect results in water having more negative values in areas with lower temperatures, and is due to several factors. First, evaporation from colder oceans, in mid-latitude (and polar) regions, is subject to a greater fractionation factor, because of the colder temperature, and produces vapor with more negative values than those from warmer oceans. Second, condensation in colder clouds will produce more complete condensation of the vapor present, thereby incorporating more lighter isotopes. Third, the progressive rainout from low to high latitudes occurs in parallel to decreases in temperature, and so results in a higher correlation between T and .

Araguás-Araguás and others (2000) noted this T-δ correlation for Vienna’s precipitation, as shown in Figure 16. The correlation was improved by statistical smoothing and manipulation, mainly to remove the seasonal variation so that the interannual changes in T and δ values could be displayed, and to show the change in T or δ versus the average, rather than the absolute T or δ values.

Figure 16 – The relationship between surface air temperature and δ¹⁸O in Vienna from 1960-1996. Statistical removal of seasonality was accomplished by creating 12-month running means for T and δ¹⁸O, after which ΔT and Δδ¹⁸O were calculated by subtracting the monthly running averages from the long-term means, and finally the curves were smoothed by reapplying a 12-month running mean (after Araguás et al., 2000).

The temperature effect manifests relatively well in high latitudes, but is not as strong in tropical regions, where the amount effect dominates (Jasecko, 2019; Rozanski et al., 1993; Yang et al., 2011).

6.1.2 The Latitude Effect

Most evaporation occurs over the tropical oceans (an estimated 65 percent as reported by Peixoto and Oort, 1983) because the higher temperature of the sea allows greater evaporation than in middle or high latitude ocean areas. Consequently, atmospheric moisture evolves isotopically as it moves away from the tropics. Condensation and rainout favor the removal of the heavier isotopes and so the precipitation at higher latitudes has more negative δ values. Evaporation occurs from the mid-latitude oceans and because the temperatures are colder, the fractionation factors are greater, resulting in vapor relatively more depleted in the heavier isotopes than vapor forming above the tropical oceans.

The latitude effect is only easily perceptible over the continental to global scale, as the myriad local variations in moisture source, humidity, precipitation amount, altitude and so forth, create a large amount of “noise” when looking for the latitude-δ trend. As such, the latitude effect is mainly of interest in modeling global patterns of precipitation (Jasecko, 2019; Rozanski et al., 1993), but can be detected regionally in some cases (Laonamsai et al., 2020) as shown in Figure 17.

Figure 17 – Stable oxygen isotopes for precipitation in Thailand based on monthly cumulative samples from 2013 to 2015. Rainout causes depletion of heavy isotopes with distance, which: a) in the case of the SW monsoon is the typical negative correlation with latitude; while b) in the case of the NE monsoon, is a positive correlation. During the NE monsoon, the weather systems move south-westward, or right to left in b), creating a continental effect where isotopic rainout over Vietnam and Laos lowers the delta values of rainfall with decreasing latitude (after Laonamsai et al., 2020).

6.1.3 The Continental Effect

Progressive rainout is the main cause of increasingly negative values for precipitation that is further and further inland (Figure 18). The continental effect is seen as a decrease in delta values of precipitation with distance from the coast, as shown in Table 2. In some cases, where winter precipitation occurs, cooler air inland may also reduce the amount of evaporation and isotopic change that occurs as rain drops fall through unsaturated air below the cloud. These colder inland temperatures will also increase the equilibrium fractionation factor that applies during condensation, thus removing heavier isotopes more effectively from the vapor and resulting in precipitation further inland being even lighter isotopically.

Figure 18 – Schematic diagram illustrating both latitude and continental effects. Generalized values of δ¹⁸O are given, and these apply to both the evolution of moisture away from the tropics towards the poles (latitude effect) or evolution of moisture inland from the coast to the interior of a continent (continental effect). In both cases, progressive rainout (depletion of atmospheric moisture) is the key driver of the isotope composition.

In other cases, rainout can be so effective at removing the heavier isotopes that the temperature effect, due to seasonality, is overridden by the continental effect. For example, precipitation during summer (June to September) on the Tibetan Plateau is 6‰ (δ¹⁸O) lighter than in winter when temperatures are 10 C colder (Araguás-Araguás et al., 1998). The continental effect is best observed over continental scales, but does operate at smaller, regional scales (Jasecko, 2019; Laonamsai et al., 2020).

Table 2 – Some examples of the continental effect from around the world. The effect is given as a gradient, i.e., the change in mean values of precipitation per 1000 km of distance.

Species	Gradient     ∆‰ 1000km	Location	Reference
D	-13	Europe: Belgium to Poland – summer	Rozanski et al., 1982
D	-33	Europe: Belgium to Poland – winter	Rozanski et al., 1982
δ18O	-1.6	Europe: Poland to Russia	Rozanski et al., 1993
δ18O	-3.8	Europe: Poland to Russia	Rozanski et al., 1993
δ18O	-3 to -4	North America: Atlantic to Rockies	Clark and Fritz, 1997
δ18O	-10	Canada: Pacific to Prairies	Yonge et al., 1989
δ18O	-0.75	Amazon: Atlantic to Andes	Salati et al., 1979

6.1.4 The Altitude Effect

As with the continental effect, the altitude effect is caused mainly by rainout, in this case triggered by orographic uplift, as well as a decrease in temperature, resulting in greater fractionation factors, which will drive rainout of heavier isotopes and cause a faster shift to lighter isotopes with altitude. Also, rain falling at higher elevations will have less distance to travel to the ground and less chance for evaporative enrichment, in which the lighter isotopes preferentially evaporate. The altitude effect is shown as a decrease in δ values per 100 m elevation gain in Table 3.

Table 3 – Some examples of the altitude effect from around the world. The effect is given here as a gradient: the change in δ values of precipitation per 100 m increase in elevation. The range of elevation over which the precipitation was sampled is also given.

Location	Country	δ18O Gradient   ∆‰100 m	Altitude masl	Reference
Mount Cameroon	Cameroon	-0.16	0-4000	Gonfiantini et al., 2001
Eastern Andes	Bolivia	-0.24	200-5200	Gonfiantini et al., 2001
Hérault	France	-0.27	500-1800	Ladouche et al., 2009
Whole Island	Taiwan	-0.20	0-2500	Peng et al., 2010
Fuego Volcano	Guatemala	-0.67	800-1200	Mulligan et al., 2011
Table Mountain	South Africa	-0.075	100-1100	Diamond & Harris, 2019
Mount Shasta	California, USA	-0.21	1000-3100	Peters et al., 2018

The altitude effect for an area is derived by collecting precipitation in at least two locations close to each other, but at different elevations. If using only two locations, then several precipitation seasons (several years) of data should be collected so as to average out the differences between years or unusual years caused by climatic variability. It is important to collect all rain that falls, and to weight the data for each sample (be they daily or monthly precipitation samples) by precipitation amount, to get a representative average isotope composition. If more precipitation stations are used, then sampling can be undertaken over a shorter period, but ideally there should be several precipitation stations sampled over several rainy seasons, including climatically different years, for example El Niño and La Niña fluctuations.

The altitude effect creates large enough variations in stable isotope compositions to allow stable isotopes to be used as tracers over fairly small distances (kilometers to tens of kilometers). This is commonly used to delineate recharge locations for groundwater discharging at springs or boreholes at lower elevations (Blarasin et al., 2020; Diamond and Harris, 2019; Jasecko, 2019).

6.1.5 The Amount Effect

The amount effect also has a close relationship with rainout. The amount effect manifests as a shift to lighter isotope compositions for heavier precipitation events (Figure 19). First, heavy individual rainstorms will tend to remove more of the vapor and cloud droplets in the air, and so with increasing precipitation in one location, the isotopic signature should become lighter. Second, the air below the cloud base will gradually become more saturated and colder, as rain and air from higher in the cloud descends (the downdraft), both of which will reduce evaporative enrichment of the later rain drops. The amount effect is known to be more pronounced in low latitudes (Dogramaci et al., 2012; Rozanski et al., 1993; Yang et al., 2011).

Figure 19 – Oxygen isotopes of precipitation in winter (triangles) and summer (circles) for a) Guangzhou and b) Changsha from June 2006 to May 2009. The negative correlation with precipitation amount is apparent and has been quantified in the equations displayed on the graphs (from Yang et al., 2011).

Of the 5 isotope effects, temperature and rainout are the main underlying processes that drive the various ‘effects’. It is important to note that all these effects and their underlying causes occur in a highly complex natural system where many variables contribute to the final isotopic composition of a rainwater sample. Other than temperature and rainout, factors such as humidity, storm track, preceding atmospheric conditions and source region also modify the isotopic composition. Isotope content of rainwater varies by the minute in a rainstorm (Muller et al., 2015) and between rain events, as is typical of most meteorological phenomena (e.g., temperature, cloud formations, precipitation amount, storm duration). Averaging the isotope composition of precipitation over longer periods, such as a month, has been found to be the most useful way of understanding the variation in isotopic signatures in an area (Yurtsever and Gat, 1981).