Atmospheric Circulation Processes Leading to the Generation of Halcyon Days in Athens, Greece ^†

Abstract

Halcyon days are characterized by periods of one to three or more consecutive, typically sunny and mild days, occurring during winter (from 15 December to 15 February) in Attica, the region where Athens is located. We examined meteorological data from the station of the National Observatory of Athens in Thission, over a 54-year period, applying criteria that include daily maximum temperatures equal to or greater than 12 °C, minimum temperatures equal to or greater than 4 °C, wind speeds equal to or less than 6 knots, and mean total cloudiness (between 06:00 and 18:00 GMT) equal to or less than 3.2 oktas. This analysis identified all Halcyon days that occurred during this period. Our statistical study revealed the annual evolution of these days and any possible relationship with climate change. We focused on understanding the dynamics of the atmospheric circulation processes associated with the occurrence of Halcyon days. The primary atmospheric circulation feature responsible for the generation of Halcyon days appears to be the establishment of a subtropical anticyclone over Greek territory.

1. Introduction

In contrast to the Etesians, the well-known seasonal winds which blow during summer over the Greek Seas, and especially the Aegean, studied broadly and in depth by many authors, Halcyon days (HDs) lack such study since very few researchers have delt with them. Worth mentioning is the fact that no attempt to investigate the dynamic processes of the atmospheric circulation leading to the synoptic features associated with the occurrence of HDs in Athens had been made. The aim of this paper is to cover this gap, by utilizing similar criteria to those used in the previous studies, to identify and analyze all HDs. That is, we examined meteorological data from the Thission station of the National Observatory of Athens (NOA) in Attica (the oldest meteorological station in Greece) during a period spanning 1970 to 2024, focusing on the dates from December 15 to February 15. The criteria applied for sunny (mean total cloudiness ≤ 3.2 oktas), mild (max temperature ≥ 12 °C, min temperature ≥ 4 °C), and almost windless (wind speed ≤ 6 knots or 3.5 m/s) days. This statistical analysis resulted in spells of one, two, or more (up to ten) successive HDs, which occur in Attica during the heart of winter. These days were well known to the ancient Greeks, with the term Halkyon, or Halcyon Days, deriving from the mythical bird halcyon, which according to ancient lore, was believed to lay and hatch its eggs during these spells. With the exception of the brief reference by Dikaiakos [1] and Carapiperis [2] to the Mythology and History of Halcyon days, ancient documents and their authors mention them and the close connection with the bird Halcyon. The excellent study by Chronopoulou and Mavrakis [3] investigates largely and in detail the weather conditions and circumstances under which the audience of the open theater of Dionysus in Athens would have watched drama in the middle of winter, during the Attic month of Gamelion (approximately 15 January to 15 February) and more outdoor activities.

This paper will be restricted mostly to the new topics mentioned above, with its main aim being the statistical analysis of the data from the Thission meteorological station of NOA. Using a 54-year period (1970–2024), we shed abundant light on the variability of the occurrence frequency of HDs due to the climate crisis occurring already in Greece. In addition, interpreting well-known basic features of the atmospheric circulation in a completely different way than they used to in the past mentioned extensionally in Prezerakos and Dafis [4], we lead on understanding the dynamics of the atmospheric circulation processes associated with the occurrence of HDs, which is the primary atmospheric circulation feature responsible for the generation of the establishment of a subtropical anticyclone over Greek territory.

This article is structured as follows: Section 2 describes the data and the methodology utilized with Section 2.1 Data and Section 2.2 Methodology, Section 3 presents the results deriving from the processing data of this study discussed properly, and Section 4 contains the conclusions.

2. Data and Methodology

2.1. Data

The data utilized in this study are as follows:

(a)

NOA, the original meteorological observations from 15 December to 15 February of the 54-year period of 1970–2024, treated by Excel Microsoft program;

(b)

Hellenic National Meteorological Service (HNMS), the climatological bulletins of the winter months from 1970 to 2019;

(c)

The European Center of Medium-range Weather Forecasts (ECMWF) gridded data, visualized mostly by http://resources.eumetrain.org.

2.2. Methodology

The methodology applied by this paper is the usual approach suited for diagnostic synoptic studies. The careful study of the real time available data, sourced from climatological bulletins or clusters outputs of gridded data, reveals a clear picture qualifiedly and quantitively of the physical processes leading directly to the achievement of the main goal of this paper, as mentioned just above. We focused mainly on revealing the linking physical mechanisms, having been determined accurately. Then, we are ready to use statistical-climatological methods if needed. The use of simplified—idealized numerical models isolating and simulating only two physical meteorological elements—features for discovering their physical linkage, without the inseparable impact of the other remaining meteorological parameters, we think, leads mostly to misleading or incorrect results.

3. Results and Discussion

3.1. Data Analysis and Identification of the Halcyon Days in Athens

The previously defined criteria for identifying HDs were applied to the original meteorological observations from NOA’s Thission station, covering the 54-year study period (1970–2024). The analysis focused on the period between 15 December and 15 February. This statistical process resulted in the selection of a list of dates corresponding to spells of one, two, or more (up to ten) successive sunny, mild, and nearly windless days, which occur in Attica and almost all of Greece during the heart of winter.

3.2. Results

Figure 1 shows the summary of the statistical analysis of the days that meet the criteria that characterize the Halcyon days (HDs). “NX” denotes the frequency of occurrence of each spell of successive days, spanning from individual days (N1) to sequences of up to 10 consecutive ones (N10). However, for presentation reasons—and in order to calculate the sum of HDs for each year—each NX value is multiplied by its corresponding number of days (X).

From Figure 1, we can see that the mean number of Halcyon days per year is 12.75 days (the thin black dotted line) and that the maximum number of successive HDs (10) occurs once, precisely in 2024. The spell of nine consecutive days occurs three times, in 2023, 1993, and 1984, and the value 8 occurs solely in 2022. The maximum total number of HDs in a single year (36 days) is again recorded in 2024, while the next highest total (28 days) is in 2022; 2023 also has a significant number of 22 HDs. That is, in recent years climate crisis has been here, and its effects have become more evident. Also, apart from 2012 and 2017, the annual number of HD’s is kept above the mean value 12.75 from 2011 onwards, increasing until 2024, where the trend peaks. These results are in perfect agreement with those of Dikaiakos and Perry [1], who examined daily data from 1931 to 1977 (46 years) from the National Observatory of Athens–Thissio revealing 630 HD’s (702 in this study but for 8 years more) during the period. They reported an average yearly frequency of 13.5 days, with individual years ranging from 3 to 22 days. Longer spells of HD’s are rare, with only four occurrences of seven or more successive days over the entire period, with the longest of 11 days occurring in February 1958. The close to 1-day difference in the mean value between the two studies, may be due to the fact that Dikaiakos and Perry [1] examined February until the 20th, covering five more days than this study. We do not try to compare the results of this investigation with that of Carapiperis [2] since the criteria applied for identification the HDs were different, aiming to identify spells of three or more consecutive days. The statistical analysis per month (

Table 1. Average HD numbers per month and frequency of occurrence per spell of successive days.

Month	Avg HDs	N1	N2	N3	N4	N5	N6	N7	N8	N9	N10
January	6.04	2.05	0.82	0.31	0.13	0.04	0.04	0.00	0.02	0.05	0.00
February	3.07	0.91	0.35	0.29	0.11	0.02	0.02	0.00	0.00	0.00	0.02
December	3.64	1.15	0.45	0.11	0.07	0.04	0.07	0.00	0.00	0.00	0.00

), regarding the 12.75 days per year, shows that January is the month with the most days that can be characterized as Halcyon Days. However, focusing on the meteorological parameters (

Table 2. Mean monthly values of major climate parameters.

Month	Tmax (°C)	Tmin (°C)	RH (%)	MSLP 1 (hPa)	W Speed (m/s)	Clouds (oktas)
January	16.1	7.8	70	1020.8	1.9	2.1
February	17.1	8.1	66	1019.5	2.1	2.1
December	16.4	8.3	70	1022.0	1.7	2.0

¹ Station Atm. Pressure adjusted to sea level.

) we don’t see a notable difference between the 3 months, or at least a result that was not expected.

3.3. Dynamic and Physical Processes Leading to Occurrence of the Halcyon Days

The synoptic and large-scale atmospheric circulation features that typically dominate during winter and influence the weather in the Balkans are as follows: (a) The Polar Jet Stream, which is generally strong and meandering during winter, is typically positioned around 55°N latitude. (b) The Azores High (Subtropical High), which tends to be weaker or displaced northward, reducing its influence on the Balkan Peninsula. (c) The Icelandic Low (Azores Low), representing the surface response to upper tropospheric polar or arctic air intrusions. It often deepens, intensifying and shifting southward, triggering Rossby’s atmospheric circulation index from high to low, contributing to low-pressure systems and storm activity over Europe and the Balkans and the building up of subtropical highs of fair-mild weather. (d) The polar and arctic air masses of upper troposphere, which frequently extend over Eastern Europe and the Balkans. These are associated with cold, moving anticyclones behind cold fronts, resulting in cold and dry conditions. (e) The position and the meridian space balance of the Subtropical Jet Stream, which, being ordinarily south of Crete during winter, plays a significant role in the configuration of the subtropical highs. To analyze the dynamics of the circulation mechanisms associated with Halcyon days, we utilized climatological bulletins from the HNMS for the winter months spanning 1970 to 2019. For the remaining periods, we used the ECMWF gridded data (500 hPa clusters over the European region), primarily visualized via http://resources.eumetrain.org. A careful examination of these data reveals that almost all 12:00 UTC synoptic situations correspond to the presence of a subtropical high, often characterized by meridional blocking. Further analysis of the 3–4 days preceding the occurrence of HDs indicates the following mechanism: The Subtropical Jet Stream (SJS) is positioned just south of Crete, while the Polar Jet Stream (PJS) maintains an average position near 55°N. Between these two jet streams, subtropical atmospheric air is established, with the tropopause around 300 hPa and a temperature of approximately −63 °C. The Azores High, located at the surface and responding primarily to the mid-tropospheric planetary Rossby wave ridge (number 3), is mainly generated over the eastern North Atlantic due to abrupt sea–land transitions. Operational meteorologists often interpret this high-pressure system as steering unstable perturbations along its eastern flanks.

This atmospheric circulation pattern, predominantly governed by the PJS in a zonal direction, persists for most of the time, resulting in a high circulation index. The zonal region bounded by parallels 35°N and 55°N and meridians 30°W and 25°E acts as an atmospheric reservoir where potential energy gradually accumulates. When a critical threshold is reached, this energy shifts, transforming into eddy potential and prompting a circulation shift from high to low. The ridges of the Rossby wave train correspond to surface subtropical anticyclones, primarily of a meridional blocking nature, which are responsible for the sunny, mild, and nearly windless conditions during HDs in Attica. An important observation from the detailed study of the evolution of long HD spells is that, despite a temporary succession of a large-scale ridge over Greece by a minor trough, the weather conditions in Attica remain unchanged, and HDs persist. These are cyclonic-type HDs, characterized by the mean sea-level pressure (MSLP) remaining around 1000 hPa during their occurrence in Attica.

To validate this understanding, we examine the longest sequence of ten successive HDs from 1 February to 10 February 2024. Figure 2 presents a chart of 300 hPa geopotential height overlays, with MSLP isobars at 12:00 UTC on 5 February, showing high-pressure fields extending from the Iberian Peninsula up to the Balkans, except for a trough over the Adriatic Sea, indicating orographic influences. The pressure over the Iberian Peninsula, at 1033 hPa, indicates the presence of a blocking system, clearly depicted at 300 hPa. This is associated with perturbations triggered by polar air intrusions, which cause the shift from a high to a low circulation regime over the eastern North Atlantic.

The evolution of this blocking system over preceding days can be observed in Figure 3 (which includes four small-scale maps from 1 to 4 February 2024). On 7 February at 12:00, when the meridional axis of the blocking crosses Attica (not shown), the 300 hPa height reaches 9320 gpm, and the 1.5 PVU dynamical tropopause at 200 hPa, with a temperature of −67 °C, confirms the high and cold tropical-tropospheric conditions associated with warm subtropical anticyclones. The EUMETSAT SGM airmass RGB image (not shown) on 7 February at 12:00 UTC interpreted the air masses over Greece to be influenced by a mix of maritime tropical (mT) air masses bringing warm, moist air from the Mediterranean region, and possibly some continental influences from northern Europe.

Hatzaki et al. [5] emphasize the seasonality in system density and maxima of anticyclogenesis, which are influenced by the seasonal variations in broader atmospheric circulation affecting the Mediterranean region. This paper is the first to objectively investigate anticyclogenesis and anticyclone tracks using gridded data and the Laplacian of the central pressure—an effective measure of system intensity. Many findings from such data analyses align with previous research; however, they often lack a deep exploration of the underlying atmospheric circulation dynamics. System density peaks during winter over the Iberian Peninsula and northern Africa, following the extensions of the subtropical ridge (Azores High) [5] (p. 9281). These are not true extensions of permanent systems but rather apparent ones, resulting from the surface response of planetary Rossby waves (number 3), which tend to balance around their usual central position, as previously described. These apparent extensions, combined with changes in the circulation index (RCI) and the creation of Rossby wave trains, lead to the development of new, separate moving systems [6]. Furthermore, a notable winter maximum is observed over the Balkans, likely due to the extension of cold, persistent anticyclones originating from central Europe and Siberia—conditions conducive to anticyclogenesis [5] (p. 9281). This Siberian anticyclone, a large and stable high-pressure system over continental Siberia, tends to be stagnant, largely restricted by the Ural Mountains to the west and the Caucasus to the south. The very dry, cold air masses associated with this system often extend into other regions, mainly via upper-level patterns such as the polar and arctic air masses in the upper troposphere and stratosphere. These upper-level air masses tend to veer westward or southwestward around the periphery of the polar vortex, especially during winter, allowing cold air to be advected into eastern and southeastern Europe, including the Balkans. Such upper-level meandering of polar and arctic air is a common feature in winter circulation patterns. Thus, the term “extension” should be not understood as a persistent surface feature stretching far westward [7].

Figure 4 shows the low at surface and 300 hPa on 8 February at 12:00 UTC over Greece (left) and the high again on 10 February at 12:00 UTC (right). In the last chart, a developing low over the Genoa gulf moving eastwards will contribute to the end of HDs.

Giariki [8] investigated the halcyon days (HDs) of January 1995, focusing on the period from the 17th to the 30th. Aside from January 17th and 18th—both of which experienced very cold weather and did not meet the criteria for HDs set in this paper—these days were sunny. This is attributed to the presence of a cold anticyclone over Greece, resembling a Siberian extension. The remaining HDs were caused by subtropical anticyclones, considered extensions of the Azores high. The occurrences of HDs in January [(1, 2), 5, 8, (14, 15, 16, 17), (19, 20), (23, 24, 25, 26, 27)] and December [15, (17, 18, 19, 20), 22, 24, (30, 31)] are primarily the result of the regular shifting of the RCI high- to low-pressure systems. These shifts are mainly due to meridional subtropical ridges and weak troughs, which rarely develop into deep lows similar to those studied in detail in February.

4. Conclusions

This study provides a comprehensive analysis of the atmospheric circulation processes leading to the occurrence of Halcyon Days in Athens, Greece, over a 54-year period. The findings confirm that the primary atmospheric feature responsible for these mild, sunny winter spells is the establishment of a subtropical anticyclone, predominantly associated with the Azores High and modulated by the planetary Rossby wave train. The analysis of meteorological data and synoptic patterns reveals that the persistence of high-pressure systems, especially meridional blocking ridges and the positioning of the Subtropical Jet Stream (SJS) and Polar Jet Stream (PJS), play crucial roles in generating conditions conducive to Halcyon Days. Furthermore, the variations in the atmospheric circulation index and the development of Rossby wave trains influence the frequency and duration of these beneficial spells. The results indicate an increasing trend in the number of Halcyon Days in recent years, which could be linked to climate change impacts on atmospheric circulation patterns. The detailed meteorological and dynamical insights highlighted in this study contribute valuable knowledge to understanding the regional climate variability and its implications for local weather phenomena. In conclusion, understanding the atmospheric mechanisms behind Halcyon Days not only enhances regional meteorological knowledge but also has practical implications.