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Background: Raynaud's phenomenon is the earliest and most common systemic sclerosis manifestation. Episodes can be triggered by cold exposure and ambient temperature changes. Small studies have found that Raynaud's phenomenon outcomes were associated with season. We aimed to map the degree that differences in ambient temperature are associated with Raynaud's phenomenon outcomes across the temperature spectrum., Methods: People with Raynaud's phenomenon secondary to systemic sclerosis in the Scleroderma Patient-centered Intervention Network Cohort completed past-week Raynaud's phenomenon severity assessments (0-10 numerical rating scale) at enrolment and longitudinally at 3-month intervals. Mean daily temperature and feels like temperature, which incorporates wind chill and humidity, for the week before each assessment were extracted for each participant from a weather site close to the participant's recruiting centre via the Iowa Environmental Mesonet. We used linear mixed models with basis splines to flexibly model non-linear changes in Raynaud's phenomenon severity across the temperature spectrum. People with lived experience of systemic sclerosis contributed to the study design and interpretation., Findings: Between April 15, 2014 and Aug 1, 2023, we included data on 20 233 Raynaud's phenomenon severity assessments from 2243 participants. 1964 (88%) of 2243 participants were women, 279 (12%) were men, and 1813 (82%) were White. Mean age was 54·8 (SD 12·7) years. The maximum predicted Raynaud's phenomenon severity score was 6·8 points (95% CI 5·6-8·1), which occurred at -25°C. Severity scores decreased minimally from -15°C to 5°C (0·05-0·21 points per 5°C difference), then decreased in larger steps between 5°C and 25°C (0·37-0·54 points per 5°C difference). The minimum predicted score was at 25°C (2·6 points [95% CI 2·5-2·7]). Scores increased at temperatures above 25°C to 3·5 points (3·0-4·1) at 35°C and 5·6 points (4·5-6·8) at 40°C. Results were similar for feels like temperature., Interpretation: Raynaud's phenomenon severity is worst at very cold temperatures but also increases with very warm temperatures, presumably due to air conditioning. Clinical management and Raynaud's phenomenon intervention trial designs should consider temperature patterns., Funding: Scleroderma Society of Ontario, Scleroderma Canada, Sclérodermie Québec, Scleroderma Manitoba, Scleroderma Atlantic, Scleroderma Association of BC, Scleroderma SASK, Scleroderma Australia, Scleroderma New South Wales, Scleroderma Victoria, the Canadian Institutes of Health Research, the Arthritis Society, the Lady Davis Institute for Medical Research of the Jewish General Hospital, the Jewish General Hospital Foundation, and McGill University., Competing Interests: Declaration of interests JDP has received speaker honoraria or consultancy fees from Astra Zeneca, Boehringer Ingelheim, IsoMab, Janssen, Permeatus, and Sojournix Pharma. MDM has received research grants or contracts from Prometheus Biosciences, Mitsubishi Tanabe, Boehringer Ingelheim, EICOS, Corbus, and Horizon Pharma; consulting fees from Cabaletta Pharma; an honorarium from GSK Pharma; and was a member of a data safety monitoring board or advisory board with Mitsubishi Tanabe, Boehringer Ingelheim, and EICOS. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)

Competing Interests: E-LN, M-EC, LK, RSH, SH, SP, SJB, LM, SBP, JV, AB, and BDT conceptualised and designed the study. E-LN, M-EC, LK, RSH, and BDT acquired data. E-LN, GV-G, and MG directly accessed and verified the data and did the statistical analysis. E-LN and BDT drafted the manuscript. All authors revised the manuscript, approved the final version, had full access to all the data in the study, and had final responsibility for the decision to submit for publication. Scleroderma Patient-centered Intervention Network (SPIN) COVID-19 patient advisors were Catherine Fortuné, Amy Gietzen, Geneviève Guillot, Nancy Lewis, Michelle Richard, Maureen Sauvé, and Joep Welling. De-identified individual participant data with a data dictionary and analysis codes will be made available upon request to the corresponding author and presentation of a methodologically sound proposal that is approved by the SPIN Data Access and Publications Committee. Data will be available after publication. Data requesters will need to sign a data transfer agreement. Additional, related documents (eg, study protocol or informed consent form) can be made available on request to the corresponding author. This study was funded by the Canadian Institutes of Health Research (CIHR; VR4–172745, GA4–177764), the McGill Interdisciplinary Initiative in Infection and Immunity Emergency COVID-19 Research Fund, Scleroderma Canada via an educational grant from Boehringer Ingelheim, the Scleroderma Society of Ontario, Scleroderma Manitoba, Scleroderma Atlantic, Scleroderma Australia, Scleroderma New South Wales, Scleroderma Victoria, Scleroderma Queensland, the Scleroderma Association of Saskatchewan (Scleroderma SASK), the Scleroderma Association of BC, and Sclérodermie Québec. E-LN was supported by a Fonds de recherche du Québec—Santé (FRQS) Doctoral Research Award, a Canadian Behavioural Interventions Trials Network Platform, and a CIHR Doctoral Studentship. RSH was supported by a CIHR Postdoctoral Fellowship. SP was supported by a FRQS Masters Research Award. BDT was supported by a tier 1 Canada Research Chair. All other authors declare no competing interests.