What Are Ice Clouds?

Ice clouds are clouds composed wholly or predominantly of ice crystals suspended in air rather than of liquid water droplets. They include cirrus, cirrostratus, and cirrocumulus in the upper troposphere, the anvil tops of deep convective systems, polar stratospheric clouds, and the contrail cirrus generated by aircraft exhaust. Because ice crystals are non-spherical and span sizes from a few micrometers to several millimeters, ice clouds scatter and absorb radiation very differently from liquid clouds, and that difference makes them one of the harder components of the atmosphere to observe and to represent in climate models.

Physically, an ice cloud is a dispersed system: solid crystals held in a gaseous medium, with optical behavior governed by particle shape, size distribution, and orientation. Satellite lidar shows thin cirrus over the tropics almost continuously, so ice clouds are a persistent feature of the upper troposphere rather than an occasional one.

Ice Nucleation and Crystal Habit

Two nucleation routes produce atmospheric ice. Homogeneous freezing occurs when a solution droplet cools below roughly -38 degrees Celsius and freezes without any solid substrate, which requires high ice supersaturation and yields large numbers of small crystals. Heterogeneous nucleation occurs at warmer temperatures on ice-nucleating particles such as mineral dust, volcanic ash, or certain biological aerosols, and it produces fewer but larger crystals. Which route dominates sets the cloud's microphysical and radiative properties. The crystal habit that grows afterward depends on temperature and supersaturation: plates near -15 degrees Celsius, hollow columns and bullet rosettes at colder temperatures, and irregular aggregates once crystals collide and stick.

Radiative Effects

Ice clouds influence Earth's energy budget through two competing mechanisms. They reflect incoming solar radiation back to space, which cools the surface, and they absorb and re-emit outgoing thermal radiation at their cold cloud-top temperature, which warms it. The balance depends on optical thickness and altitude. Thin, high cirrus generally produces a net warming, while thick anvil cloud can produce net cooling. Combined radar and lidar analyses, including assessments of global ice cloud radiative effects using CloudSat and CALIPSO, quantify these terms at the top of the atmosphere and at the surface, and the residual uncertainty in ice cloud feedback remains a leading contributor to spread among climate projections.

Remote Sensing and Retrieval

Ice clouds are retrieved from space using instruments chosen for their sensitivity to small, non-spherical particles. Polarization lidar separates ice from liquid by the depolarization that randomly oriented crystals impose on backscattered light, millimeter-wave cloud radar at 94 gigahertz penetrates thick cloud to profile ice water content, and split-window thermal infrared channels near 10.6 and 12.05 micrometers constrain effective particle diameter. Passive submillimeter and microwave radiometry adds ice water path over a broad swath. NASA distributes the resulting products, described in its cloud data resources for Earth science, and the retrievals feed directly into model evaluation and numerical weather prediction.

Contrails and Contrail Cirrus

Aircraft engine exhaust forms line-shaped ice clouds when the mixing plume becomes supersaturated with respect to ice, and in ice-supersaturated air those contrails persist and spread into diffuse contrail cirrus indistinguishable from natural cirrus in appearance. A global assessment of contrails and contrail cirrus in climate change places their radiative forcing on the same order as that of aviation's cumulative carbon dioxide emissions, which has made flight-path rerouting around ice-supersaturated regions an active area of operational research.

Applications

The study of ice clouds has applications in a range of fields, including:

• Climate modeling and radiative forcing assessment
• Numerical weather prediction and precipitation forecasting
• Satellite instrument design and retrieval algorithm development
• Aviation planning, including contrail avoidance and icing avoidance
• Free-space optical and satellite communication link budgets
• Atmospheric chemistry, where crystal surfaces host heterogeneous reactions