Regardless of what's being plotted, there are some common elements that are useful to understand. Take either the route forecast plot, or the point forecast plot across time. Each of thse plots show height on the left axis. Time or distance is shown on the bottom axis. There are several plots offered showing different background values. They include thermal strength, thermal index, temperature, dewpoint and relative humidity analysis.
All plots will typically show the same set of values plotted as lines across the base scenes. These are:
The red line depicts the calculated top of usable lift. This level is determined by lifting the parcel of air using the dry adiabatic lapse rate and finding the level where the vertical velocity reaches roughly 1.2 meters per second (200 fpm).
The Black line shows the convective boundary layer tops. There will often be a large difference between the boundary layer tops and the top of usable lift. There are a variety of reasons as to why this can occur.
The pink line traces the lifted condensation level (LCL) or dewpoint of the airmass.
The grey area (when applicable) shows where dewpoint is reached within the boundary layer curve. When this intersects, a grey shaded area will show indicating cloudbase. Calculating cloud tops is not applied in these plots to keep them simple and easy to read. Where the top of usable lift is actually higher than dewpoint, the top of usable lift curve is adjusted to be where dewpoint was found. This is because "usable" lift implies not using lift within cloud for most soaring aviation.
See AlsoXC Skies - XC Map Basics
The minimum surface elevation is used as the botom value for the left axis, and the top is the maximum height of lift plus a 2000 feet.
All plots are zoomable. On a mobile device with touch screen, a pinch-zoom gesture will allow detailed inspection of point along the plot. For desktop, the mouse wheel or "zoom" gesture on a track pad will do the same.
All units are consistently shown based on your selected units preference on your account.
All plots show the winds aloft across the route or across time for a point. Wind barbs are used and are represented in the units you have chosen. Reading windbarbs can feel tricky for the first time you see them, but once you warm up to these symbols, you'll appreciate the amount of information they convey without needing to clutter plots with too many numbers.
You can read more about wind barbs here.
Parcel Sensitivity to Surface Temperature Variances
Parcel Sensitivity to Surface Temperature Variances
There is another button called "Toggle Sensitivity" on all plots. When clicked, two more lines will show on the plot... and it will start getting harder to read. This is turned off by default. Here is an example zoomed in to one day on a point forecast.
The idea of sensitivity is to show how senstitive the airmass is if more surface heat were added or removed from the current forecast. There can be large variation across even small areas and this attempts to provide a quick look into how the soaring potential might respond to more or less engery added to lifting parcels of air.
The yellow line shows the convective boundary layer tops if a full degree of temperature C was removed from the surface heating. If the yellow line is nearly identical to the black line (boundary layer top) there is little sensitivity to less surface heating. If there is a large difference, there is likely high sensitivity. In this example, we can see the latter half of the day is very sensitive to less surface heating. In fact, a full hour becomes unsoarable as the day ends.
The white line shows the convective boundary layer tops if a full degree of temperature C was added to the surface heating. This shows the upper bounds of sensitivity if more energy is added to a rising parcel. When the white line is above the black line, the boundary layer top is higher and might imply the top of usable lift could be higher as well.
Investigating the sensitivity of the airmass is useful in identify if otherwise good looking soaring potential days are actually borderline days. Exploring a skew-t chart for a single location at a given time will provide a large amount of information about the internal sturcture of the air parcel's characteristics.
