What effect does the weather have on a cricket ball?

The effects of weather on our summer sport are well documented but with Twenty20 cricket we might have to consider more subtle meteorological influences.

The quick-fire nature of the game and the need to accumulate runs rapidly means that a higher premium is set on hitting sixes.

A strong enough wind will, of course, have an influence on a ball’s carry but temperature and humidity are important factors. Contrary to instinct, humid air is less dense than dry air, and a lofted ball should travel further in warm, humid air than when it is chilly and dry.

Set against that, very high humidity can reduce distance a smidgen by making the ball less elastic and a little bit heavier - from the air’s moisture and from sweat imparted by the hands of bowlers and fielders.

And a difference of 25C only adds about six metres (20 feet).

Even so, a muggy day usually just about favours the batsman - perhaps only by a metre or two but that might be the difference between hitting a six or holing out at long-off.

Across the Atlantic, baseball has been subject to intricate and scholarly studies in this regard, given the high significance of the home run - an importance that the six in Twenty20 has rapidly attained.

And it is not only a batted ball that comes under consideration. A pitcher may find, for example, that his fastball has more zip when it is warm and humid.

It seems, though, that the long-held belief that high humidity aids the swing of a cricket ball may have been overstated. A study conducted by scientists at AUT University in New Zealand casts doubt on this received wisdom.

A humid day is often a cloudy one, and their hypothesis is that it is cloud cover, or rather the consequent lack of sunshine, that is the culprit. A ball’s movement through the air depends on differential air flow across two sides of the ball - the side roughened through use has turbulent flow across it while one kept shiny has smooth “laminar” flow.

When sunshine heats the ground it creates small, localised convection currents which might sometimes negate the useful turbulence created by the differential flow around the ball. These currents are absent under thick cloud cover on a cool day.

In baseball as in cricket, much of a pitched or bowled ball’s movement through the air is imparted by the grip and release of the pitcher or bowler. Pitchers, like spin bowlers, hold the ball in various ways before delivery to impart spin through their fingers and wrists.

This used to be a problem at Coors Field in Denver, Colorado - Mile High City - the home of the Colorado Rockies. The altitude makes the atmosphere great for batters and a graveyard of pitchers, but this has been addressed to a degree.

Nothing can be done about the thinner air at this altitude which allows balls to fly for home runs far more easily than at sea-level, even given the much lower humidity. There used to be two runs more scored per game than the average at other stadia, a notable difference given that is a relatively low scoring sport.

We come back initially to elasticity, and at extreme low humidity balls are far bouncier and a shade lighter, and spring off the bat with alacrity. More than that, pitchers complained that the extreme aridity that sometimes typifies Denver made the dry balls harder to grip and control. Curve balls and sliders would be straighter than they intended.

So in the winter of 2002 the Rockies contrived a walk-in humidor - a giant version of the boxes used to keep cigars moist.

This keeps the game balls at 50% humidity which lowers their bounciness and aids the pitchers’ grips.

The effects may be small and subtle - but in games like baseball and cricket that can turn on tiny margins the implications are significant. Since the humidor, pitchers are on average giving up one less run per game, making baseball at Coors Field a bit less of a game of roulette.

By Stephen Davenport, MeteoGroup UK