Straight winds, or wind shears, which can blast straight down at speeds of 100 mph, are caused from cool, dry air from aloft blowing into the storm, maybe 10,000 feet above the ground. As rain falls into the dry air, some of it evaporates. This cools the air and makes it heavier. The force can destroy light construction and twist off the tops of trees, making people think they've suffered a twister. Although funnel clouds may also form, these straight winds are more common along the leading edge of large squall line.
Thunderstorms require instability, moisture and lifting air.
Here in south Texas, the heat and sea breeze are common culprits for their propagation. So much for the nuts and bolts of these storms - what are more fascinating to me are the lights and sounds provided by these moving disco shows. Lightning strikes the ground somewhere around the world between 1 to 100 times per second. One in every 200 houses is struck every year. That's a 1 in 200 chance that a house will be struck.