Thermal tops are determined by lifting a reasonable representation of the ground surface air to a neutral buoyancy. Taking just the 2 meter surface temperature and lifting it to where the thermal index equals 0 is not always the most accurate way to determine the absolute thermal tops. It is also important to note that the thermal tops are not always the top of the boundary layer.
The boundary layer, which is a physics fluid dynamics term, is the layer of fluid that resides near and is influenced by the bounding surface. In terms of the atmosphere, the usage of boundary layer refers to the following, quoted from Wikipedia:
"The planetary boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface."
The depth of the boundary layer is the area from the planet’s surface to the height where the surface no longer mixes with the air mass aloft. Although most paraglider and hang glider pilots can only exploit the upward movement in the boundary layer from thermals rising through it, the boundary layer can also take the shape of mixing air influenced by the surface that is not usable by pilots. You can read more about this here http://en.wikipedia.org/wiki/Planetary_boundary_layer
Although most pilots consider the micro elements when it comes to the depth of thermals, there are very large dynamic factors that influence the boundary layer. Sailplane pilots understand these influences better than most other types of soaring pilots. Top of Usable Lift
Determining the top of usable lift requires the consideration of many variables. As a general rule the following are used:
- Surface temperature
- Temperature profile directly above a given location
- Relative humidity profile directly above a given location
- Surface heat flux
- Ground heat flux
- Latent heat flux
- Sun angle (directly effects heat flux variables)
- Vegetation type and its affiliated heat capacity characteristics
- Soil type and its affiliated heat capacity characteristics
- Boundary layer depth
- Strength of wind profile within the boundary layer
- Influence of large bodies of water
It is no wonder that just looking at soaring tools like the thermal index is not enough to truly determine what the day can offer for thermal-based soaring. Skew-T plots likely provide the largest picture of understanding the day, but the coarse resolution and lack of ground based flux values can make it tricky to forecast how high you’ll get.
Determining Clouds and Dew Point
Clouds are perhaps the most challenging variables to forecast for any model. As pilots we are mostly concerned with convective cloud levels (CCL). This is the level where clouds will form as influenced by thermals rising. Other cloud determination concepts exist, such as the LCL, which is known as the lifted condensation level. This helps to characterize where upper level clouds will form. These are typically large scale forces at work, such as a very large mass of air slowly rising enabling a large area of cloud to form. Generally speaking, vertical velocities in these scenarios are not exploitable by gliders.
If you think of cloud determination backwards, you’ll quickly understand that if their location, depth, width, and existence are not accurate to what the day provides, then you know that the assumptions going into parameterizing them are wrong. The variables that determine clouds closely share the same variables that determine thermal tops and the top of usable lift. Hence, if clouds are not forecasted well for any given area, you can expect the other thermal forecasts to vary as well.
Cumulus clouds are creatures unto themselves. Anyone who has spent time at cumulus cloud base, in it, or even above it, can attest to the true magic that our atmosphere seems to provide. Like thermals, cumulus clouds are transitory by nature and are never exactly alike. On average cu clouds exist from 5 to 30 minutes in fair weather regimes.
Even if cloud base is determined accurately, finding cloud tops is extremely complex. The main reason for this is due to the specific heat capacity of the air and the latent heat of vaporization. Temperature greatly effects these values in the known schemes of determining the moist adiabatic lapse rate (MALR). XC Skies attempts to resolve cloud tops based on these characterizations. Essentially, a suite of equations are computed for each vertical level of rising air, adjustments are made to constants, and then computed again.
Loosely, three classifications exist within a cumulus cloud profile depending on temperature: below freezing (water), at or near freezing (glaciation), and ice above that zone. The equations to determine how much heat is given back to the rising parcel of air changes throughout these phases. At some point, temperatures become so low that clouds can no longer vertically build. At this point, in towering cumulus clouds, anvils are often seen as ice crystals which are blown downwind from their cloud tops.
Lift to Wind Ratios
Similar to the buoyancy to shear ratio and Richardson turbulence schemes, XC Skies uses another concept to determine where lift becomes eroded due to wind negating thermal velocity. For example, consider this scenario: as a thermal rises, its buoyancy increases and then starts to taper off near the top of lift (where the thermal index for that given vertical profile reaches zero in dry adiabatic conditions, ie no clouds). It is common for winds to gradually increase with height, so at some point in the thermal profile, the winds aloft start to effect the shape of the diminishing velocity of the thermal. This critical level does not have an official name but all soaring pilots recognize this phenomenon. At this critical level, winds start to overpower thermals and they become broken and often unusable. Gliders that can "sample" large areas with minimal sink, such as sailplanes or top performing hang gliders, can continue to exploit the lift of this mixed layer, although it is often quite turbulent. Eventually, however, the ride upwards stops and it's time to find more lift elsewhere.