A thermal is a rising column of warm air, a natural phenomenon driven by solar heating of the Earth’s surface. This atmospheric convection powers everything from soaring birds to gliders and is a fundamental part of the planet’s weather system. Understanding when these air currents begin to form requires looking closely at atmospheric physics and how the sun interacts with different types of ground.
The Physics of Thermal Generation
The fundamental process that creates a thermal is differential heating, where the ground absorbs solar radiation and warms faster than the air above it. As the surface temperature rises, it transfers heat to the lowest layer of air through conduction, causing that air to become less dense. This lighter, warmer air then forms a discrete parcel that detaches from the ground and begins to rise due to buoyancy.
As the air parcel ascends, the surrounding atmospheric pressure decreases, causing the rising air to expand. This expansion leads to a cooling of the air parcel without exchanging heat with the environment, known as adiabatic cooling. For an unsaturated air parcel, this cooling occurs at a predictable rate of approximately 9.8 degrees Celsius per kilometer, called the Dry Adiabatic Lapse Rate.
The persistence of a thermal depends on its temperature compared to the surrounding air, governed by the Environmental Lapse Rate (ELR). The ELR is the actual rate at which the ambient air temperature decreases with altitude. If the rising parcel cools more slowly than the surrounding air, it remains warmer and less dense, maintaining buoyancy and continuing to ascend. When the ELR is steep, the atmosphere is considered unstable, and strong thermals are likely to form.
When Morning Thermal Activity Begins
Thermal activity cannot begin immediately at sunrise because a stable atmospheric condition, known as the nocturnal inversion layer, typically forms overnight. This layer consists of cool, dense air trapped near the ground with warmer air sitting above it, suppressing vertical air movement. The sun must first warm the ground sufficiently to erode and eventually break through this inversion layer.
The time it takes for thermals to start is highly variable, but generally falls within one to three hours after sunrise. On a clear, dry day, thermal generation may begin around 9:00 a.m. to 10:00 a.m. local time, once the surface temperature overcomes the overnight stability. In mountainous terrain, initial activity can start earlier on east-facing slopes that catch the direct morning sun.
Atmospheric moisture also influences this timing by diverting solar energy away from surface heating. If the ground is saturated from overnight rain or heavy dew, a portion of the incoming solar radiation is consumed by evaporation. This consumption of latent heat reduces the energy available to warm the air layer, delaying the moment the air becomes buoyant enough to initiate the first thermal columns.
Ground Conditions That Influence Initial Lift
The first thermals tend to appear over specific surfaces that act as localized “hot spots” or thermal triggers. Dark-colored surfaces, such as paved asphalt roads, freshly plowed fields, or dark rock outcrops, absorb a higher percentage of solar radiation. This greater absorption leads to faster and more intense heating of the air immediately above them, causing the first buoyant air parcels to detach earlier than over surrounding areas.
Conversely, light-colored surfaces like snow cover, green grassy fields, or large bodies of water reflect more solar energy, causing them to warm more slowly. Air parcels over these areas will be slower to reach the necessary temperature threshold for lift, delaying the onset of convection. This differential heating is why a glider pilot may circle over a dark field while the air over a nearby lake remains stable and cool.
Topography also plays a role in concentrating heat and initiating lift. Slopes and ridges that face the morning sun receive direct, perpendicular solar radiation, accelerating the warm-up process. Furthermore, sheltered areas, such as shallow bowls or ravines, can allow warm air to pool and accumulate before finally bursting free as a substantial thermal column. These topographical features focus the heating effect, ensuring that initial lift is localized and predictable.