Jules Stichbury, of Waikawa Beach, asks :-
I was watching USA golf and the commentator said that US golfers aren't used to playing in rain as there is a huge chance of lightning when it rains in the USA. The same seems to happen in Australia and parts of Europe. In NZ we have lots of rain and few lightning storms. So I was wondering if the reason has something to do with islands and continents as it seems to me the bigger the land mass the more lightning?
Rebekah LaBar, a meteorologist with the MetService, responded.
More lightning does tend to occur over continents than over the ocean.
Lightning is part of the atmosphere's continual balancing act; in this case, the discharge of an electric field. The formation of this electric field requires a strong updraft, or rising air. If the inside of the cloud becomes turbulent enough, collisions between small hail, ice, and water particles transfer one charge to one particle (for example, the ice crystal may become positively charged) and leave the opposite charge on the other. Eventually a strong electric field builds up and lightning then discharges until the electric field weakens and dissipates.
In order to understand how such a strong thunderstorm updraft can form, we have to take a look at parcel theory.
As the surface of the earth warms unevenly, pockets of air that are warmer than the air around them begin to rise (think of a hot air balloon). The air "parcel" will cool as it rises, but as long as the surrounding environment is cooler, it will continue to rise. If the air parcel holds enough water vapour, it will eventually cool to the point of saturation and form a cloud.
Temperature generally decreases with height through the troposphere, and the faster it decreases the more "unstable" we say the atmosphere is. A very unstable atmosphere has the potential for air parcels to rise very rapidly.
The most unstable atmosphere will have very hot and moist air near the surface and very cold and dry air aloft.
Land has a lower specific heat than water, meaning it doesn't take as much solar radiation to warm up as water does. So the larger the land mass, the greater the potential to heat up quickly, leading to greater instability and more rapidly rising air parcels.
Not only is the heat important, but also the moisture content; for example, the Gulf of Mexico provides a source of surface moisture to the central and southeastern United States while the Saharan Desert remains hot and dry. Without sufficient water vapour, the air parcel will rise without forming a cloud, or at least a low enough cloud for a thunderstorm.