Atmospheric soundings
 
An atospheric sounding is the bread and butter for a forecasting meteorologist. It represents a vertical profile of the atmosphere at a single point in time and above a single position on Earth. Usually, a small instrument package called a radiosonde is tethered to a weather balloon, which is released from the surface and rises through the troposphere. The radiosonde is able to measure the vertical profile of temperature, dew point, wind speed and direction, as well as other properties, as it ascends. After the balloon bursts in the lower reaches of the stratosphere, (radiosondes are designed to be disposable), the data is radioed back to the receiver at the launch location and is plotted onto a chart. In an ideal case, this would represent a perfect vertical slice through the atmosphere - however, strong upper-level winds often blow the balloon about and so the path is rarely vertical. It should also be noted that it usually takes around 20 minutes for the balloon to ascend fully. The charts that the data is plotted onto are called tephigrams - used by the Met Office and in Canada and skew-T log-P diagrams - used primarily by the National Weather Service of the USA.
 
These charts only contain minor variations but however the data is plotted, they provide valuable information about the vertical structure of the atmosphere that is used to supplement satellite data, surface observations and model output to produce more accurate and reliable forecasts. 
A sounding from Norman, Oklahoma from the 19th January 2015. The red line represents the temperature, while the green line represents the dewpoint temperature. 
The above example is a skew-T log-P diagram, typically used by the US National Weather Service. Temperature is always plotted along the x-axis, with pressure plotted along the y-axis. The height can be inferred relatively easily by the pressure, which can be more easily measured by the radiosonde. Temperature contours are angled because the lower values which are often experienced high in the troposphere would not fit on the graph otherwise. 
 
The lifted condensation level (LCL) is the pressure (or height) at which saturation occurs (or the cloud base) due to forced ascent from the surface - ie following the dry adiabat from the surface to condensation level.
 
The convective condensation level (CCL) is the height at which saturation occurs when the air parcel ascends, following the dry adiabat through buoyancy from the surface due to heating here. 
 
Along with the LCL and CCL, other important levels include the LFC - level of free convection. This is the height where the environmental temperature decreases at a faster rate than the saturated adiabatic lapse rate of a parcel that has reached its dewpoint. 
 
The EL - equilibrium level, is the point where the temperature of an initially buoyant parcel of air once again becomes the same as the environment around it. Again, for thunderstorm development, this point is often in the stratosphere, although it can occur anywhere there is a temperature inversion. 
 
The MPL - maximum parcel level, is the maximum height that a parcel of air which has risen convectively, can reach in the atmosphere; the point where its vertical velocity is zero. The vertical velocity decreases when the parcel encounters a stable layer of air, usually in the stratosphere with deep convective clouds such as thunderstorms. 
 
The primary advantage of using an atmospheric sounding is that they provide the primary method fer assessing the instability of the atmosphere. They help to determine whether thunderstorms or convective clouds will initiate due to the cap strength, what temperature will need to be reached at the surface for storms to form and the character of any thunderstorms that do form. Globally, radiosondes are released at 00Z and 12Z, to ensure that measurements are taken at the same time.