Thermals generate turbulence as they push upwards and disrupt the normal airflow. This creates eddies along the thermal boundaries. The intensity of the turbulence will depend on the vertical velocity of the thermals, the volume of air rising, wind, humidity, altitude and atmospheric pressure. If wind is mixed with thermals, you have a mixture of horizontal and vertically moving air, which will be more turbulent. In strong wind, if the thermals are weak, the wind can blow them apart. If the thermals are strong, the wind will move around them, creating an area of rotor on the leeward side (see The Lee and Rotor). Generally dry, hot, high altitude areas (deserts) heat up rapidly and produce the strongest thermals and associated turbulence. Humid areas tend to moderate thermal activity, reducing the level of turbulence.
Atmospheric pressure can play a big role in thermal activity and thermal turbulence. With a low pressure, the atmosphere is generally rising, and it is easier for the thermals to rise with it, creating less friction and turbulence at the boundaries. However, if the pressure is too low, thermal activity can become extreme and lead to over-development or thunderstorms, with severe turbulence. An average pressure (29.92) is generally better for thermal conditions. With moderate high pressure, the thermals can build in volume and the friction turbulence could be moderate. If the pressure is very high, there is more atmosphere pressing down on the thermals, making it harder for them to rise, so they build in intensity. When the thermals do force their way upwards, it is in smaller areas with more distinct boundaries, which can feel ‘sharp-edged’. The exact relationship between pressure systems and thermal development is unclear but experienced pilots will attest to its relationship.
The presence of thermals is a consideration when on your landing approach. The wind directions can switch suddenly, if a thermal releases near you, drafting air into it. If you’re low, you will be landing downwind or cross-wind. You may also be lifted or dropped unexpectedly, if you encounter lift or sink. You may have experienced mild versions of this during your training, but it may be much more intense in strong thermic conditions. The greatest concern is encountering turbulence sufficient enough to cause a deflation, surging, or other issues at low altitudes. For these reasons, as a newer pilot, you will want to be very thoughtful about when you chose to fly and land. If you start flying the first thermals of the day and stay up for an extended period of time, you will likely be landing in stronger conditions, keep this in mind and maybe plan to land early. It may be better to fly late afternoon, when the conditions are declining and the landing is more likely to be smooth. If the landing area is green and/or near water, the chances of encountering turbulence is less than in a hot dry field, something to consider when making your decisions.
The USHPA recommends that P2 pilots not fly in thermic conditions exceeding 200 fpm until they understand the risks involved. You will not be required to practice stalls, spins, or in-air reserve deployments before flying in thermic conditions.