Upper air data is taken from balloons which measure upper air conditions over a particular location. This includes plots of data on the standard mandatory and height levels, contours of various upper air parameters and a set of sounding plots for most US sites. These data update once every 12 hours at around 9:45 EST.
Sample SkewT plot
A Skew T plot is a standard plot used by meteorologists to analyze data from a balloon sounding. This is a plot of temperature with height as denoted by pressure.
The pressure lines are plotted horizontally in blue and are also on an inverse log scale. The concept of Skew T means that the temperature is not plotted vertically but angles off to the right at a 45 degree angle. The temperature lines of the Skew T are in blue. The green lines are called dry adiabats. The light blue dashed lines are saturation adiabats. The yellow dashed lines are lines of constant mixing ratio.
The sounding is plotted as two white lines. The right line is the temperature profile. The left line is the dewpoint profile.
The winds are plotted as wind barbs with height (see below) on the right edge of the plot.
The sounding also has height information plotted at the left, next to the pressure:
Specific sounding related parameters are plotted in the upper right. These values are:
TP: Tropopause level (mb)
MW: Max wind level (mb)
FRZ: Lowest freezing level (mb) or BG for below ground
WB0: Wet bulb zero (mb) or BG
PW: Precipitable water (in)
RH: Mean RH surface to 500 mb (%)
MAXT: Estimated max temperature (C) using a 150mb layer
TH: 1000-500mb thickness (m)
L57: 700-500mb lapse rate (C/km)
LCL: Lift condensation level (mb) from surface data
LI: Lifted index (C) using 100 mean layer above surface
SI: Showalter index (C)
TT: Total totals index
KI: K index
SW: Sweat index
EI: Energy index
-PARCEL- This is a parcel trajectory (the yellow line on the sounding) based on 100 mb mean layer.
CAPE: Convective available potential energy
CINH: Convective inhibition (open ended)
LCL: Lift condensation level (mb)
CAP: Cap strength (C)
LFC: Level free convection (mb)
EL: Equilibrium level (mb)
MPL: Maximum parcel level (mb)
-WIND- Wind parameters
STM: Estimated storm motion (knts) from 0-6000m AG mean layer, spd 75% of mean, dir 30 deg veer from mean.
HEL: Storm relative helicity 0-3000m AG (total value)
SHR+: Positive shear magnitude 0-3000m AG (sum of veering shear values)
SRDS: Storm relative directional shear 0-3000m AG (directional difference of storm relative winds)
EHI: Energy helicity index (prop to positive helicity * CAPE)
BRN: Bulk Richardson number 500-6000m AG (prop to CAPE/bulk shear)
BSHR: Bulk shear value (magnitude of shear over layer)
SkewT plot with parcel trajectory (solid yellow)
Sounding diagrams provide a important means for determining the stability of the atmosphere above a specific location. By using the concept of an air parcel, lifting it or lowering it and comparing the resulting parcel conditions to the conditions of the surrounding environment as defined by the balloon sounding.
Parcels of unsaturated air tend to follow the dry adiabat lines (green lines) as they ascend or descend. The saturation adiabats (light blue) lines show how parcels saturated with water vapor will ascend or descend. Descending parcels will tend to unsaturate immediately. The mixing ratio lines (yellow) relate to the amount of water vapor in a parcel in grams of water vapor per kilograms of dry air. Parcels of air attempt to maintain a constant mixing ratio as they ascend or descend. Generally, a parcel will rise, following the dry adiabat until it saturates. This occurs when dry adiabat crosses the initial mixing ratio line. This is considered the LCL. If lifting continues, the parcel cools following the saturation adiabat. If the parcel descends, it will always follow the dry adiabat as it will immediately unsaturate if saturated. By then comparing the parcel temperature to the environment, you can determine whether it is stable (parcel cooler) orunstable (parcel warmer). An unstable parcel will accelerate upwards and is the primary means for thunderstorm development. A stable parcel will decelerate and eventually descend. This is the typical atmospheric condition and it the primary condition in high pressure areas. The descending air desaturates the atmosphere and leads to clearing skies and calm conditions.