The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →In a 2004 study of natural cirrus and aircraft contrails, researchers reported unusually high relative humidity with respect to ice (RHi) in very cold clouds. They proposed that nitric acid on ice particles could help explain the finding—but the measurements and the proposed mechanism are distinct claims, and the study alone does not establish whether the explanation shapes atmospheric science today.
What the researchers measured
Gao and colleagues reported in situ measurements of RHi and nitric acid (HNO3) in upper-tropospheric natural cirrus and aircraft contrails. RHi means relative humidity referenced to ice, not liquid water; keeping that distinction matters when interpreting values above 100%.
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The paper’s abstract states: “At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types.” This is the study’s reported average for the conditions it examined, not a claim that every cirrus cloud or contrail reaches that humidity.
How nitric acid might affect ice
The authors attributed the elevated RHi to a proposed class of nitric-acid-containing ice particles they called “Delta-ice.” Their suggested mechanism was that HNO3 at the particle surface could impede equilibration between ice and surrounding water vapor. If that process occurs as proposed, the vapor and ice would not respond as a simple equilibrium picture predicts.
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That explanation is an interpretation of the observations, not the same thing as directly measuring the causal process. The paper’s terminology should likewise be understood as the authors’ proposed particle class, rather than assumed to be a broadly established classification.
Why the finding mattered for climate models
Gao and colleagues argued that adding Delta-ice to climate models would change simulated cirrus properties and the distribution of water vapor in the upper troposphere. The implication was that a particle-level process might affect how models represent high-altitude cloud and moisture—not that the study supplied a settled estimate of its climate impact.
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The 2004 paper and its bibliographic records do not establish whether later studies confirmed, revised, or rejected the mechanism, or whether current climate models include this proposed particle class. The finding is therefore best read as a historical research result and a modeling question, not as evidence of present-day scientific consensus.
Sources and scope
- Gao et al., “Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds,” Science, January 23, 2004 — the primary paper record and abstract, including the reported humidity finding and proposed mechanism.
- NASA Technical Reports Server record — bibliographic information and stated implications of the same paper.
Chemistry World published Emma Davies’s report “Nitrates blaze a trail” on March 1, 2004. Its account provides contemporary context, but the primary paper is the basis for the numerical finding and the proposed explanation.
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