Winds

Horizontal or so called u- and v- winds are represented by wind velocity vectors. The arrows point downwind and the length of the symbol is indicative of the wind speed. Winds are given for all heights. At the higher levels the winds tend to blow geostrophically or along the pressure contours, closer to the surface the winds are affected by topographic and frictional effects. Therefor local surface winds are likely to be a result of small-scale influences and are likely to be poorly represented by model wind-fields. The mean 1000-850 mb wind is calculated for the 850 mb plot and can be used in conjunction with 1000-850 mb thickness to diagnose temperature advection. The 750 mb level winds are near 8000'(2500m) and roughly represent ridge-top winds. The 300 mb winds are contoured and shaded to indicate the location of jet maximums and allow for easy recognition of entrance and exit regions.

Temperatures

Temperatures are given in degrees Celsius.

Celsius	Fahrenheit
-40	-40
-20	-4
0	32
20	68

Surface temperatures are for the model surface, which does include topography and so are terrain following within the resolution of the model surface data.

Thickness

The 1000-850mb thickness is calculated from forecast pressure heights. Large thickness values are indicative of warmer average temperature for the layer, cooler air is indicated by the thinner or small thickness value. Warm or cold air advection is indicated when the wind vectors point across thickness contours.

Geopotential height

Contours of all plots indicate the height in meters above sea-level of the indicated pressure surface. Some common pressure surfaces and their approximate heights in meters and feet above sea-level are noted below.

Remember that in cold conditions thickness levels decrease and the true elevation of a given pressure surface is lower than in the standard atmosphere.

Vertical velocity

The vertical velocity at 700 mb is given in units of microbars/sec or 10^-3 mb/sec. A negative vertical velocity is associated with rising motion. This is because increasing elevation (going up) implies moving to a level of lower atmospheric pressure (higher to lower, a negative direction)

Relative humidity

Humidity is shaded for levels of 50, 70 and 90 percent with the highest contour level indicating greater than 90% relative humidity. Upward vertical velocities (negative values of omega) and relative humidities of greater than 70% at 700mb indicate conditions where precipitation is likely and the presence of clouds at 10,000' can be expected.

Vorticity

Absolute vorticity is indicated in units of 10^-4 sec^-1. Vorticity is a measure of the strength and sense (cyclonic or anti-cyclonic) of fluid circulations. Positive vorticity at 500 mb level indicates good upper-level support for storm systems. The green colors indicate higher values of absolute vorticity, and the orange shading indicates smaller values of absolute vorticity. Regions where the absolute vorticity is increasing with time are sometimes referred to as undergoing positive vorticity advection (PVA). This is recognized as being associated with upper-level divergence and the enhancement of upward vertical motions. This is often marked by an increase in the prevalence of clouds. In the same manner negative vorticity advection (NVA) is associated with downward vertical velocities and tendency towards dissipation of clouds.