First Documented Observation and Meteorological Analysis of Cirrostratus undulatus homomutatus

Abstract

On the morning of 4 April 2025, a rare formation of Cirrostratus undulatus homomutatus was observed over Barcelona. This variety of the homomutatus form of the Cirrostratus cloud genus—originating from the transformation of persistent aircraft contrails—has not previously been documented in the International Cloud Atlas or in any scientific publication, making this observation unique within the current literature. The event was visually recorded and meteorologically analyzed using upper-air data from the Barcelona radiosonde and the ECMWF ERA5 reanalysis at 300 and 500 hPa geopotential heights. Synoptic and thermodynamic analyses revealed a localized region of enhanced wind shear activity coinciding with a thin, moist layer near the tropopause. These conditions likely facilitated the transformation of persistent contrails into cirriform layers exhibiting undulated patterns characteristic of the undulatus variety. This case provides new insight into the microphysical and dynamic mechanisms underlying the evolution of anthropogenic cirriform clouds, contributing to the growing body of knowledge on homomutatus phenomena and their interaction with upper-tropospheric processes. It thus represents the first formal documentation and meteorological interpretation of Cirrostratus undulatus homomutatus, offering a valuable reference for future observational and classification efforts within the WMO framework.

1. Introduction

Homomutatus clouds constitute a relatively recent category within the international cloud classification, formally recognized by the World Meteorological Organization (WMO) in the 2017 edition of the International Cloud Atlas. These clouds originate from the transformation of persistent aircraft contrails (most commonly Cirrocumulus homogenitus) which, under the influence of upper-tropospheric winds and internal microphysical processes, can evolve into cloud structures resembling natural cirriform formations such as Cirrus or Cirrostratus. When atmospheric conditions are favourable, they may also exhibit species, varieties, and features typically associated with natural cloud types; however, they remain classified as homomutatus due to their explicitly anthropogenic origin.

The study of homogenitus and homomutatus clouds is of particular relevance, as these cloud types illustrate the interaction between natural atmospheric processes and anthropogenic activity. Homomutatus high clouds are explicitly classified as anthropogenic, originating directly from aviation, and may exert measurable impacts on the Earth-s radiative balance and, consequently, on climate change (e.g., [1,2]). Their formal recognition within international cloud nomenclature provides a standardized framework for their description and supports systematic research into the influence of air traffic on cloud formation and its broader climatic implications [3,4,5].

The study of high-level cloud formations has gained increasing importance in recent decades due to their influence on radiative forcing and their susceptibility to anthropogenic modification [6]. Among these formations, Cirrostratus undulatus homomutatus represents a particularly rare phenomenon, arising from the transformation (mutatus) of anthropogenic contrails into cirriform layers that exhibit undulating patterns. Such observations provide valuable insight into the interaction between aviation-induced ice clouds and the upper-tropospheric environment, linking traditional synoptic-scale meteorology with emerging perspectives in atmospheric microphysics [7,8]. Previous studies have also highlighted the significance of systematic ground-based and citizen observations for documenting the evolution of anthropogenic cirrus and related optical phenomena [7].

In high-level homomutatus clouds, species such as fibratus (cirrus fibratus homomutatus) or nebulosus (Cirrostratus nebulosus homomutatus) are relatively common, as atmospheric conditions frequently favour the development of translucent veils or fine bands [3]. By contrast, the occurrence of the undulatus variety in these clouds (Cirrostratus undulatus homomutatus) is exceptionally rare, since undulations require the presence of atmospheric gravity waves or pronounced wind shear, abrupt changes in wind direction and/or speed at the level where water vapour sublimates into ice crystals, producing wave-like or banded patterns under conditions of stable stratification. For such a combination of processes to occur, contrails must not only persist and evolve into Cirrostratus layers but must also coincide spatially and temporally with the regions of wind shear or gravity-waves [9]. To the best of the authors’ knowledge, no previous scientific study has documented the formation or observation of this high-level homomutatus variety.

Thus, this study has a twofold objective: first, to formally document this singular observation, and second, to describe the meteorological conditions present at the time and location of its occurrence.

2. Materials and Methods

On 4 April 2025 at 07:47 LT (05:47 UTC), a Cirrostratus undulatus homomutatus was observed over Barcelona, a very uncommon observation. Figure 1 shows photographs documenting this singular cloud observation. The upper panel offers a general view of the sky sector where the cloud developed, showing several primary contrails that progressively spread into cirrocumulus and subsequently evolved into Cirrostratus. The lower panel provides a closer view taken with a zoom lens, highlighting the undulated structure characteristic of the undulatus variety derived from the transformed contrail.

Based on these images documenting the cloud’s evolution, its formation mechanisms were analyzed. First, the synoptic situation under which the cloud developed were examined. Second, using data from the Barcelona radiosonde, the wind shear likely responsible for generation the undulatus structure was quantified.

Bulk Wind Shear (BWS) is one of the primary parameters used to quantify wind shear between different atmospheric layers, arising from changes and direction [10]. BWS is computed from the differences in the zonal (u) and meridional (v) wind components between two atmospheric levels, z₁ and z₂, and is expressed as follows:

$$BWS={\displaystyle \frac{\sqrt{({u_2-u_1)}^2+({v_2-v_1)}^2}}{z_2-z_1}}$$

(1)

where u and v are the zonal and meridional wind components at levels z₂ and z₁. Values between 0.01 and 0.1 s⁻¹ are considered moderate shear, while values above 0.1 s⁻¹ are considered high [10].

BWS values were computed from radiosonde data using fixed altitude intervals rather than standard pressure levels, ensuring consistent vertical spacing across all profiles and avoiding the variability associated with pressure-height conversions in the upper troposphere.

3. Results

3.1. Synoptic Analysis

ECMWF ERA5 reanalysis data (0.25° spatial resolution and 3 h temporal resolution) at 300 and 500 hPa for 4 April 2025 at 12 UTC indicate the presence of a cold-air trough west of the Iberian Peninsula, extending toward its central regions, with an associated surface low over southwestern Portugal. This synoptic configuration generated a southerly flow over north-eastern Iberia, influencing conditions in Barcelona. To the east, a ridge extends from North Africa to the central Mediterranean. The north-eastern Mediterranean coast of Iberia was positioned within a transitional zone between the stability imposed by the Mediterranean ridge and the instability associated with the western trough. Such ridge–trough boundary conditions favour the development of more than two octas of homogeneous high clouds (homomutatus) [11,12].

3.2. Radio Sounding Analysis

Using radiosonde data from Barcelona (41°23′64″ N, 2°9′32″ E) at 12 UTC on 4 April 2025, Bulk Wind Shear (BWS) was calculated from the measured horizontal wind components (u, v) at different pressure levels.

The upper panel of Figure 2 presents the vertical profile of BWS calculated using Equation (1), while the lower panel shows the corresponding relative humidity profile. Most atmospheric levels exhibit BWS values below 0.1 s⁻¹; however, between approximately 6000 and 6400 m, substantially higher values are observed, indicating strong to very strong wind shear. At these altitudes, relative humidity exceeds 80%, approaching 100% near 6000 m. BWS reaches 0.9 s⁻¹ at 6000 m and 2.5 s⁻¹ at 6446 m, reflecting intense shear coinciding with highly humid layers. Under such conditions, the additional moisture from aircraft exhaust may be sufficient to saturate the air, favouring the sublimation of water vapour into ice crystals.

4. Discussion

The 2017 edition of the International Cloud Atlas does not explicitly refer to the combined classification term Cirrostratus undulatus homomutatus, although it does list Cirrostratus homomutatus and Cirrostratus undulatus separately as valid components of the genus Cirrostratus. The Cloud Classification Summary of the International Cloud Atlas (https://cloudatlas.wmo.int/en/cloud-classification-summary.html (accessed 15 November 2025)) shows Homo under the Mutatus column for Cirrostratus and also include undulatus among its possible varieties. However, the Atlas does not provide explicit documentation or reference imagery for the combination of these three terms. The situation is not unique; other legitimated classification combinations are recognized even though they are not illustrated with reference image. For example, Stratocumulus cumulogenitus (https://cloudatlas.wmo.int/en/explanatory-remarks-and-special-clouds-cumulus.html (accessed on 15 November 2025)) is listed as a Stratocumulus formation derived from the spreading of Cumulus cloud, yet the Atlas does not depict the many possible species or varieties such a cloud may exhibit. The illustrated Stratocumulus cumulogenitus could, for instance, also be classified as the opaque variety, although this is not specified. Likewise, the International Cloud Atlas, does not include an explicit reference image for Stratocumulus cumulogenitus stratiformis, despite it being a fairly common combination of terms.

It is therefore logical and reasonable that the International Cloud Atlas acknowledges the theoretical possibility of various classification combinations without necessarily providing an associated image, as in the case with the cloud presented here. Nonetheless, it remains essential scientific community and meteorological observers worldwide to document unique observations with the most complete classification possible. Doing so will help build a record of real-world cloud combinations and improve our understanding of the physical processes that produce them. Distinguishing which combinations are theoretically possible and which are not is fundamental to the scientific method: systematic observation is key to validating classification criteria.

Although the International Cloud Atlas does not explicitly define the combined classification of Cirrostratus undulatus homomutatus, the present observation reduces potential ambiguity through a multi-step classification approach grounded in established diagnostic criteria and observational consistency. First, the identification of Cirrostratus relies on its characteristic optical and structural properties, which are well documented in the Atlas. Second, the undulatus variety is assigned only when wave-like oscillations are clearly discernible in multiple consecutive images, ensuring that the pattern is not an artefact of perspective or transient turbulence. Third, the homomutatus attribute is justified though analysis of the cloud’s temporal evolution, conforming its origin from persistent aircraft condensation remnants that subsequently transformed into cirriform layer, consistent with recent literature describing the transition from condensation trails to cirrus.

In the case of the homogenitus and homomutatus classifications, an additional benefit is their utility for assessing the influence of anthropogenic activities on the troposphere and their subsequent evolution. Documenting observations of infrequent clouds that exhibit multiple valid combinations of the classification elements proposed by the International Cloud Atlas is a scientific effort that should be encouraged, as it enhances our understanding of natural atmospheric behaviour and the imprint of human activity, much like systematic cataloguing in disciplines such as botany or entomology.

Homomutatus clouds produce a net positive radiative forcing, primarily by enhancing longwave capture, making them one of the largest non-CO₂ climate impacts from aviation [13]. Recent satellite-based analyses confirm that condensation-remnant cirrus outbreaks significantly perturb regional radiation balances and can rival natural cirrus in optical depth and lifetime [14]. Current studies identify homomutatus as a key factor in aviation-induced warming [2].

Observation constitutes the foundational pillar of the natural sciences, providing the empirical basis upon which hypotheses are formulated and theories validated. In atmospheric science, as in all natural disciplines, observation serves as the essential bridge between physical processes and conceptual understanding [3]. Systematic cloud observation, both visual and instrumental, remains indispensable for identifying morphological features, classifying cloud types, and documenting rare or transitional formations that may escape automated detection ([6,15]). Although the International Cloud Atlas formally recognizes numerous combinations of cloud genera, species, and varieties, the observation of a cloud simultaneously exhibiting three defining characteristics, such as Cirrostratus undulatus homomutatus, is of exceptional scientific importance. Such occurrences reveal complex interactions between dynamic and microphysical atmospheric processes, including contrail evolution, upper-tropospheric wind shear, and gravity-wave modulation. Documenting these phenomena enriches the empirical foundation of cloud climatology, and provides insights into anthropogenic influences on high-level cloud development. Observation, therefore, is not a passive act of recording but an active epistemological process that unites perception, measurement, and interpretation, ensuring the integrity, continuity, and advancement of atmospheric and natural sciences as a whole.

5. Conclusions

An unusual cloud observation has been reported, representing a first in academic literature and the latest edition of the International Cloud Atlas: a Cirrostratus undulatus homomutatus. The coincidence of strong wind shear and elevated humidity at 6–6.4 km provides a plausible mechanism for the formation of undulated structures in persistent homogenitus and homomutatus clouds. This explanation is supported by the high values of the BWS at this altitude, substantially exceeding 0.1 s⁻¹. Although the radiosonde measurements were recorded a few hours after the observation, the vertical profiles are consistent with the hypothesis that shear-driven perturbations within a moist layer contributed to the undulatus morphology. Atmospheric stability indicators (CAPE, 0 J/kg; Lifted Index, 4; Vertical Totals, 29) suggests that convection was negligible, reinforcing the conclusion that mechanical shear, rather than thermal instability, was the dominant factor in shaping the cloud pattern. Therefore, despite the time gap between the observation and the radiosonde measurement, it is plausible that the observed undulatus features resulted from shear-induced wave structures, as indicated by the elevated BWS values.