Moisture Convergence


This series of Haby Hints looks at 10 mechanisms that can create a burst point for convection. The burst point is the location where a thunderstorm first forms and climbs vertically throughout the troposphere. Convection is air rising due to positive buoyancy. The topic in this writing is moisture convergence.

Moisture is one of the critical ingredients for thunderstorm development. A region with more moisture will be less dense than an adjacent region that has less moisture when both have the same temperature. This is important since the region that is less dense will be more inclined to rise. This is one ingredient that can lead to the initial burst point for convection. Moisture content influences the density of air and when these air parcels approach each other the less dense parcel will tend to ride over the denser parcel instead of thoroughly mixing with it. Wind direction has an important influence on the moisture content of the air. Air flowing from an oceanic source will often have a higher moisture content while air blowing from the continent interior will tend to be drier. Moisture content can vary locally by other factors such as the influence from wet vs. drier soils, lakes, urban vs. nonurban, vegetation type and irrigation practices.

When air streams converge, the moisture within those air streams will converge also and this leads to dewpoints increasing. Boundaries such as fronts, outflow boundaries and drylines are locations convergence can occur. Moisture convergence can increase the dewpoint of the air and thus increase the instability of the air. Lifting, instability increase and moisture increase can all occur along a boundary. This makes a boundary a prime candidate for a burst point somewhere along the line (often where best convergence takes place).

The previous Haby Hint focused on temperature convergence. Both temperature and moisture convergence will contribute to increasing the Theta-E of the air. The intersections of a higher temperature axis with a higher dewpoint axis will have higher Theta-E values. Increasing Theta-E contributes to increasing instability since it results from a combination of temperature and dewpoint increasing. This location called a Theta-E ridge can be a burst point for convection.