### Bubble Model A horizontally and vertically isolated rising "bubble" or vortex ring. Bubbles are thought to originate from smaller reservoirs of warm air, and conditions of intermittent surface heating. ![[GFH bubble.png]] ### Plume Model A vertically continuous updraft forms a "plume" or "chimney". Plumes are thought to originate from larger reservoirs of warm air, and conditions of continuous surface heating. In reality, the two models are not mutually exclusive; individual bubbles may exist within a single plume. ![[GFH plume.png]] ### Vertical Structure Warm air at the surface begins to rise from a **trigger point**, and organizes as an ascending plume or bubble. As altitude increases, the thermal becomes wider, as a result of decreasing pressure and mixing with the surrounding air. As a very general rule, thermal height is approximately 4 times the diameter, with the thermal expanding 50% in diameter from base to top. The maximum updraft velocity of the thermal occurs at the point of greatest temperature difference with the surrounding air; this can vary considerably with the associated conditions, but is often closer to the middle or the top of the thermal, rather than at low altitude. ![[thermal vertical structure.png]] ### Horizontal Structure The strongest updraft velocity occurs in the center, or **core** of a thermal, with diminishing lift towards the edges. The thermal is surrounded by sinking air, with zones of shear or turbulence along the boundaries. Thermals may have multiple cores (ie bubbles within a plume), and are not always concentric. Horizontal distribution of thermals varies with their maximum height; as low as 1.5 times the height for low (<4000ft) topping thermals, and as high as 10 times the height for high (>10,000ft) topping thermals. ![[thermal horizontal structure.png]] ### Triggers With enough buoyancy, a thermal may spontaneously rise on its own; this is known as **autoconvection**. More commonly, a **trigger** causes localized initial rising, that causes a release of all of the stored warm air at the surface. Triggers often take the form of **discontinuities** at the surface; individual hotspots, changes in surface or terrain, etc. Light winds or [[Convergence]] often help to trigger thermals. Longer storage of warm air at the surface typically equates to longer release times, and longer recycle times. The individual lifespan of a thermal, from formation to dissipation, is generally 10-20 minutes. ### Wind Effects A thermal moves vertically through an air mass; if the air mass is moving (ie wind), the thermal will "tilt" downwind with respect to the surface. Light winds may help to trigger thermals, while higher wind speeds can encourage the mixing of air and inhibit thermal formation: 20kts can make thermals rough and inconsistent, while 30kts may break up thermals entirely. **Vertical shear** (wind speed and/or direction change with altitude) can increase the downwind movement of thermals, or break up thermals as they rise. Strong, well organized thermals tend to hold together better in shear, but strong shear (>10kts/1000ft) can make things challenging or impossible. ![[thermal vs winds.png]]