It is known that the proportion of ice-topped clouds depends on dust particles in the cloud that act as nuclei for ice crystal formation. However, it was not clearly demonstrated using large data set. In a study published on 31 July 2025, researchers have confirmed this relationship using 35 years of satellite data. They have showed that the proportion of ice-topped clouds (viz., cloud-top ice-to-total frequency or ITF) in the Northern Hemisphere between −15° and −30°C strongly correlates with the abundance of dust particles in the clouds. This is important for climate modelling because radiative forcing and precipitation of the clouds will be influenced by whether they are topped by a ice or water cloud layer.
The word “dust” triggers a feeling of inconvenience and discomfort, which is rightly so because dust from natural sources and human activities (such as construction, industrial processes, and vehicle movement) contribute particulate matters in the air leading to air pollution which has adverse health impact on respiratory and cardiovascular systems. In arid and semi-arid regions, sand and dust storms pump in large amounts of mineral dust particles in the air. The resulting air pollution impacts public health, environment and radiation budget.
The airborne mineral dust also plays an important role in climate system. It absorbs and scatters solar and thermal radiation hence directly affect the energy balance of the earth system. Any change in atmospheric mineral dust load changes the radiation balance of a region (i.e., net change in radiation flux due to dust or dust radiative forcing). The airborne particulate matters up to 0.2 μm size range also act as seeds for cloud droplet formation when water vapor condenses onto them. Called cloud condensation nuclei (CCN), these particles serve as the foundation for cloud droplets and are essential for the initiation of cloud droplet formation and the development of clouds and rain. It indirectly affects the Earth’s climate system, including radiative forcing. Changes in concentrations of airborne particulate matters acting as CCN have significant impacts on cloud properties, radiative forcing and climate.
Cloud types and Ice-to-total frequency (ITF)
Clouds can be of three types depending on whether they are primarily composed of ice crystals or liquid water droplets. Ice clouds are composed of ice crystals formed through nucleation around ice nucleating particles (INPs) like mineral dust. They usually form at high altitudes where freezing temperature prevails. Water clouds, on the other hand, are primarily composed of liquid water droplets and form when water vapor in the atmosphere cools and condenses into liquid water droplets around cloud condensation nuclei (CCN) like dust or salt particles. Mixed-phase clouds contain both ice crystals and supercooled water droplets. This process when supercooled water droplets freeze onto ice crystals or other ice particles, causing a significant increase in their mass and density is called riming. Riming is seen primarily in mixed-phase clouds at temperatures between -5°C and -25°C at places where supercooled water droplets freeze upon collision with ice crystals. Ice-to-total frequency (ITF) is the proportion of ice clouds compared to the total number of clouds observed at the cloud top level.
The processes involved in mineral dust’s effects on climate system is well understood, however there were at least two issues for researchers to address.
Firstly, there was uncertainty in estimation of direct and indirect climate effects of mineral dust on global scale. EMIT (Earth Surface Mineral Dust Source Investigation) mission of NASA installed onboard ISS addresses this by mapping the mineral dust composition of arid regions of Earth and providing global data set for climate modelling. It achieved a milestone on 27 July 2022 when it provided its first view of Earth. Last year in 2024, it transitioned into an extended mission phase at least until 2026.
Secondly, while it is known for long time that the proportion of ice-topped clouds depends on dust particles in the cloud that act as nuclei for ice crystal formation. However, it was not clearly demonstrated using large data set. In a study published on 31 July 2025, researchers have confirmed this relationship using 35 years of satellite data. They have showed that the proportion of ice-topped clouds (viz., cloud-top ice-to-total frequency or ITF) in the Northern Hemisphere between −15° and −30°C strongly correlates with the abundance of dust particles in the clouds. This is important for climate modelling because radiative forcing and precipitation of the clouds will be influenced by whether they are topped by a ice or water cloud layer.
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(Acknowledgement: Dr. Sachchidanand Singh, Chief Scientist, CSIR-NPL, India for his valuable inputs on the subject matter and editing)
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References:
- Villanueva D., et al 2025. Dust-driven droplet freezing explains cloud-top phase in the northern extratropics. SCIENCE. 31 July 2025. Vol 389, Issue 6759 pp. 521-525. DOI: https://doi.org/10.1126/science.adt5354
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