One contribution to instability is how warm the boundary layer can warm during the day. The amount of warming at any one place is going to depend on a variety of factors. The warm air tells part of the story but the other part of the story is the moisture content in the air. A location that is slightly cooler can be more unstable than a warmer location if it has a significantly higher moisture content. This writing will focus on factors at the surface that determine the heating and moisture content of the air near the surface. One factor that influences the amount of heating is clouds. Clear regions will generally warm more than cloudy regions. The sun coming out and staying out can be a critical ingredient for thunderstorm development. Another factor is the heat capacity of the surface that is being warmed by the sun. Land will warm faster than water. Dry soil will warm faster than wet soil. Bare soil will warm faster than densely vegetated soil. There is a trade off in that increasing moisture also helps make the air more unstable. Thus, regions undergoing a combination of both significant warming while retaining high dewpoints will be favored regions for the burst point for convection. Wet soils can have an overall positive impact on instability even though it may prevent the temperature from warming as high as it could have been. Solar heating depends on several factors such as the amount of unobstructed sunlight, time of year, air mass in place, cloud cover, particulate matter in the air and ground cover. Solar heating is cumulative during the day and tends to reach of maximum in the afternoon. This is a common time of the day storms will first form. During the late spring and early summer the amount of daylight is at a maximum. This produces more cumulative heating throughout the day and makes it more likely that storms will form when solar heating is needed to break a capping inversion. Severe thunderstorms are most common this time of the year also, from April to June. A solar heating max that initiates vertical motions can be the location of the burst point that starts a thunderstorm or severe storm outbreak. |