Posted 2 September 2008

What is Wind Shear?

According to Webster’s definition, wind shear is a difference in wind speed and direction over a relatively short distance in the atmosphere. Wind shear can be broken down into vertical and horizontal components. Horizontal wind shear is seen across weather fronts and near the coast. Vertical shear is experienced typically near the surface, though also at higher levels in the atmosphere near upper level jets and frontal zones aloft.

According to the National Weather Service (NWS) wind shear is "shear" and is categorized as speed shear and directional shear.

Speed shear is the component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.

Posted 3 August 2008

Avoiding Thunderstorms is the Best Policy

One beautiful, hot summer day, I provided a standard briefing to a VFR-only student pilot. The radar was indicating a broken line of thunderstorms about 30 miles away and moving toward the station. Convective SIGMETs (WST) and other advisories were associated with the line.

The National Weather Service reports that an average thunderstorm cell is 15 miles in diameter and lasts an average of 30 minutes

The departure forecast called for scattered clouds with an occasional ceiling of four thousand overcast, visibility three to five miles with thunderstorms and moderate rain showers. The pilot's route of flight penetrated the line. His destination on he other side of the line was reporting scattered clouds and haze. I issued the VFR flight not recommended statement and the pilot asked "why."