Galactic cosmic rays (GCRs) in the Earth's atmosphere can ionise the gas molecules they pass close to. The total rate of atmospheric ion production due to GCRs is modulated primarily by three factors: heliospheric modulation, atmospheric depth and Earth's magnetic field.

The heliospheric modulation potential ф defines the energy spectrum of GCRs reaching the top of the atmosphere at a given time. When the GCRs collide with air molecules, they trigger a cascade reaction which results in the creation of ions.

Because these reactions occur probabilistically, the atmospheric depth h can be used as a measure of the likelihood that a reaction has occured by the time a particular cosmic ray reaches a given altitude. Atmospheric depth is the mass of a column of air above a surface, with dimensions mass / area.

Since GCRs are charged particles, they are affected by the Earth's magnetic field. The geomagnetic cutoff rigidity is the minimum energy per unit charge that a cosmic ray must have to reach sea level without being blocked by the Earth's magnetic field.

From time to time the sun's magnetic field experiences brief periods of intense activity, when large quantities of highly charged plasma are thrown out into the solar system. This plasma blocks cosmic rays from reaching the Earth's atmosphere, resulting in a temporary reduction in the ionisation rate. These events are known as Forbush decreases.

Several recent papers have linked observations of changes in cloud characteristics with Forbush decreases.