Dew factor cricket explained: why chasing gets easier

The floodlights have been on for an hour, the temperature has dropped four or five degrees since the innings break, and a fast bowler at the top of his mark is wiping his hand on a towel that a team-mate has been carrying in his pocket for three overs. It has not rained. The dew factor in cricket explained in one image is that towel: the ball has picked up a film of water from grass that nobody watered, the bowler cannot hold it the way he did at six o'clock, and every plan the fielding captain made at the toss is now being renegotiated a ball at a time.

Dew is condensation. When the sun goes down, the grass blades of an outfield radiate heat away to a clear sky far faster than the air above them cools, and because a blade of grass is thin and poorly connected to the warm soil beneath it, its surface temperature falls below the dew point of the air around it. Water vapour then condenses directly onto the blade. That is the entire mechanism, and every consequence in a day-night cricket match follows from it: a wet ball skids instead of gripping, a wet outfield runs faster, and the side bowling second is asked to defend a total with equipment it can no longer control.

The physics: why grass gets wet when nothing falls on it

The common description of dew as moisture that falls out of the air is wrong in a way that matters, because it hides the variable a team can actually forecast.

Dew forms by radiative cooling. Every surface exposed to the night sky emits infrared radiation upwards and receives less back, so it loses heat. A thick concrete slab loses heat slowly, because it is drawing warmth from a large mass behind it. A grass blade is thin, has a large surface area for its volume and sits on a cushion of still air, so it cools quickly and can drop several degrees below the temperature of the air around it. Once the blade is colder than the dew point, the air touching it can no longer hold its water as vapour, and the excess condenses onto the leaf.

Three conditions decide how much of this happens.

A clear sky is the first. Cloud absorbs and re-emits infrared radiation back towards the ground, which slows the cooling of the surface and can stop it reaching the dew point at all. A humid but overcast evening often produces far less dew than a slightly drier clear one.

Still air is the second. A breeze mixes the cooled, saturated layer at grass level with warmer, drier air from higher up, which keeps the blade above the dew point. Heavy dew is a phenomenon of calm nights, and a wind that arrives at nine o'clock can dry an outfield without anybody touching it.

A small gap between the air temperature and the dew point is the third, and it is the number a support staff actually watches. If the evening air is at twenty-six degrees with a dew point of twenty-four, the surface only has to lose a couple of degrees before condensation starts. If the dew point is at fourteen, the grass has a long way to fall.

Why humid evening cricket gets it worst

The grounds where dew decides matches are not the wettest grounds, and that is the point people miss.

Heavy dew requires humid air, a large drop in surface temperature after sunset, and calm, clear conditions. That combination is characteristic of the subcontinent outside the monsoon, of coastal grounds in the Gulf, of the Caribbean, and of parts of southern Africa and Australia in particular months. What those places share is not rainfall but a large amount of water vapour in the air combined with skies that clear at night.

Seasonal timing does most of the work. In northern India in the weeks after the monsoon, daytime heat gives way to sharply cooler nights while absolute humidity stays high, which is close to a laboratory setup for dew. In the deep subcontinental summer the nights are hot enough that the surface does not fall far below the dew point, and dew is lighter. That is why sides who have played at the same venue in different months describe entirely different conditions.

The stadium itself contributes. A ground enclosed by tall stands has less air movement at grass level than an open one, which favours dew. An outfield cut long holds more surface area for condensation and more water within the sward than one cut short. Grounds with heavy irrigation in the days before a match start the evening with more moisture available to give up.

Two more factors sit outside the ground's control. The first is start time, since a match beginning in mid-afternoon and finishing at ten in the evening runs its entire second innings through the worst of the condensation window. The second is the calendar, because whether a series lands in a dew month at all is a scheduling decision taken years in advance, discussed in the piece on how the international cricket calendar is assembled.

• Air temperature
• Grass surface temperature
• Dew point

Illustrative profile of an evening, drawn to show the mechanism rather than measured at any venue. Dew begins once the grass surface, which cools faster than the air, falls below the dew point.

Show the numbers

Why the gap between air temperature and dew point matters

Item	Air temperature	Grass surface temperature	Dew point
17:00	33C	34C	23C
18:30	30C	29C	23C
20:00	27C	25C	23C
21:30	25C	22C	23C
23:00	24C	21C	23C

What dew does to the cricket ball

A cricket ball is a piece of engineering that depends on its surface, and water changes every property that surface provides.

The seam is the first casualty. A bowler grips a ball by placing fingers on or beside the seam, and the seam's raised stitching is what gives the hand purchase. Wet, the leather and the thread both become slippery, and the fingers slide at the moment of release rather than holding until the last instant. The result is a ball that leaves the hand earlier and with less imparted rotation than the bowler intended.

Swing goes next. Conventional swing depends on air flowing differently over the two sides of the ball, which requires one side to be kept smooth and shiny and the other to roughen. A ball that is wet cannot be polished, because the shine a fielding side spends an innings building is a thin, dry film. Reverse swing, which needs one side to become genuinely rough and dry while the other stays smooth, is close to impossible to generate in heavy dew, and the mechanics of why are set out in how reverse swing works.

Seam movement suffers for a different reason. Off the pitch, deviation depends on the seam landing upright and biting into the surface. A wet ball is less likely to be released with a stable seam position, and a damp surface offers less grip when it lands.

There is a fourth effect that batters feel more than bowlers do. A wet ball is heavier and comes on to the bat faster off the pitch, because it skids rather than gripping. That combination, more pace and less deviation, is exactly the ball a batter wants. It is why scoring rates in the second innings of a dewy match tend to rise even against bowling that has not obviously deteriorated.

Grip, spin and the bowler who suffers most

Every bowler is worse in dew, and finger spinners are worse by a wide margin.

A finger spinner generates revolutions by rolling the ball out across the fingers with the seam cutting into the air and then into the pitch. That transfer depends entirely on friction between skin and leather. Wet the leather and the ball simply leaves the hand without taking the spin with it, arriving as a slow, straight delivery at a length the batter has already picked. There is no technique that compensates, which is why captains routinely take a finger spinner out of the attack when dew arrives rather than trying to bowl through it.

A wrist spinner loses slightly less, because the ball is flicked from a position where the fingers are underneath it and the release is less dependent on drag along the seam. The loss is still severe, and the drift and dip that make a leg spinner dangerous both depend on air interacting with a spinning, dry surface. The differences between the two families are covered in the piece on types of spin bowling.

Fast bowlers keep more of their craft but lose the parts that need precision. A yorker is the delivery most affected, because it is aimed at a target a few centimetres deep and a ball that slips from the hand becomes either a low full toss or a half volley. Slower balls, which depend on a deliberately unusual grip, become unreliable in the same way. The consequence for the last overs of a limited-overs innings is severe, and the plans that fall apart are the ones described in death bowling in T20.

The fielding side loses two more things. Catching a wet, heavier ball under lights is harder, particularly in the outfield where the ball has run through soaked grass. And throwing suffers, because a wet ball leaves the hand unpredictably and a fielder aiming at the stumps from the rope will lose accuracy at exactly the moment a run-out is available.

Why dew distorts the toss

The toss is meant to be a coin. In a heavy-dew fixture it is closer to a lottery for a large and known advantage, and that is the strongest argument against the way night cricket is currently played.

The reasoning available to a captain is simple. If dew is expected, the side bowling second will bowl with a wet ball and field on a wet outfield, and the side batting second will face bowling that has lost most of its variation. A captain winning the toss in those conditions bowls first almost automatically, and the decision requires no judgement at all. A toss that removes the need for judgement has stopped being a tactical moment and become a transfer of expected value.

The general evidence on toss effects is more modest than the commentary suggests. A study by Gaurav Sood and Derek Willis covering 44,224 matches found that winning the toss raises the chance of winning by a small but real margin of roughly 2.8 per cent overall, and that the effect is larger in conditions where the toss confers a greater advantage, day-night matches among them. The broader question of how much the coin decides is examined in does the toss matter in cricket.

Two qualifications keep this honest. Dew does not appear in every day-night match, or even in most of them, and averaging across all such fixtures dilutes an effect that is severe in a subset. And a captain who wins the toss and bowls first is choosing conditions rather than being handed a result, so the advantage is an increase in probability rather than a guarantee.

• 44224Matches analysed
• 2.8Overall lift in win probability from winning the toss, per cent

From Sood and Willis, an analysis across formats and levels. The study reports a larger effect where conditions favour one innings, day-night matches included.

The countermeasures, and how well each one works

Grounds fight dew with three tools and a lot of labour, and none of the three is a solution.

Dragging a rope across the outfield. Two members of the groundstaff walk the width of the field holding a long, weighted rope so that it brushes the tops of the grass blades. The droplets are knocked off the leaves and fall into the sward where they no longer wet the ball. It is the most effective of the three in the short term and the least durable, because condensation resumes the moment the rope has passed. In a heavy-dew match the rope is dragged during every drinks break, every strategic timeout and often between overs.

Super soppers. These are motorised rollers with absorbent barrels, built for clearing rainwater, and they lift standing moisture rather than droplets clinging to blades. They are used when dew has become heavy enough to pool, and they are slower to deploy, so they belong to the scheduled breaks rather than to the gaps between overs.

Surfactant sprays. An agricultural product is applied to the outfield hours before play. Reporting on its use in Indian cricket has identified it as a surfactant borrowed from farming, and the principle is that it lowers the surface tension which allows water to sit as beads on a leaf, so the moisture spreads and drains rather than collecting where the ball will pick it up. It is preventative rather than corrective, it has to go on well before the first ball, and its effect diminishes as the evening goes on.

Three more measures sit around the edges. Grass is cut shorter before a dew match, because a shorter sward holds less water and presents less surface for condensation. Irrigation is stopped earlier in the week. And in some competitions the fielding side is given latitude to change the ball if it becomes unfit, though that is a playing-conditions matter decided by the umpires rather than a dew remedy.

20%45%20%15%

• Surfactant spray, applied before play20%
• Rope dragging during breaks in the second innings45%
• Super soppers at scheduled intervals20%
• Shorter cut and irrigation management in the days before15%

Illustrative distribution of groundstaff effort across an evening, drawn from how the measures are described in practice rather than from recorded timings.

Show the numbers

When each countermeasure is applied through a dewy evening

Item	Value
Surfactant spray, applied before play	20%
Rope dragging during breaks in the second innings	45%
Super soppers at scheduled intervals	20%
Shorter cut and irrigation management in the days before	15%

How a fielding captain actually defends a total in dew

Knowing dew is coming changes the order in which a captain spends bowlers, and the good sides plan the whole innings backwards from it.

The first decision is to front-load the spinners. A finger spinner who will be unusable at nine o'clock is worth bowling out at seven, even if the match-up is less favourable then. Sides defending in expected dew routinely complete a spinner's full allocation inside the first half of the chase and accept a worse distribution of overs in exchange for having them bowled while the ball still grips.

The second is to protect a specialist. Holding back the bowler with the best wet-ball skills, usually a quick with a strong seam-up stock ball rather than one who relies on variations, gives the captain something to bowl at the end when everything cute has stopped working.

The third is to change the length. As grip goes, the yorker becomes a gamble and the hard length becomes the safer option, because a ball banged into the pitch does not need a fine release and it takes the pace off the shot into the longer part of the ground.

The fourth is to accept a different kind of field. A wet outfield is quicker, so the value of a saving fielder in the ring falls and the value of one at the rope rises. Sides that keep an aggressive ring in heavy dew tend to be the ones conceding boundaries through it.

And the fifth is a matter of expectation setting. A total that would be defendable in the dry needs a premium in a dew match, and a batting side that knows the second innings will be easier has to bat with that in mind rather than to the ordinary par score. The mechanics of pacing an innings around a known condition are covered in how to pace a run chase.

What the chasing side has to do differently

Batting in dew is easier, and it is not free. Sides that treat a wet second innings as an automatic win give matches back in three specific ways.

Running between the wickets becomes hazardous rather than helpful. A soaked outfield is slippery at the turn, and the third run in an over of hard running is where batters pull up. Sides chasing in heavy dew tend to take fewer twos and threes than the conditions appear to offer, and the ones that ignore that lose a batter to a hamstring or a slip at the crease.

Bat grip suffers too, and it is rarely mentioned. A batter's hands and the rubber on the handle get wet from gloves, from sweat and from the ball itself, and a top hand that slips at the point of contact turns a controlled swing into a leading edge. Chasing sides go through gloves at a rate that has nothing to do with sweat alone.

And the ball, though it does less, is still hard and now quicker off the surface. A short ball that skids is more dangerous, not less, because the batter has less time to adjust and the ball holds its line at throat height. Sides that bat as though the bowling has become harmless get caught by exactly that delivery.

The correct approach is unglamorous: take the boundaries the conditions offer against the bowlers who have lost their variations, run in ones rather than chasing twos, and treat the short ball with more caution rather than less. A chase in dew should look boring, and the ones that go wrong are the ones that do not.

What dew is not, and why the distinction matters

Three different wet conditions get called dew in commentary, and only one of them is.

Rain wets the ball from above and stops play. Its effects on the surface, the outfield and the covers are different from condensation, and the playing conditions have a whole apparatus for dealing with it. A rain-affected outfield is usually wetter in patches, where dew is close to uniform across the ground.

A damp pitch is a separate matter again. Moisture in the top of the pitch, whether left by preparation or by weather, helps a seam bowler by giving the ball something to grip, and it is one of the few forms of wetness that favours the bowling side. Dew does not soak into a rolled pitch to any useful degree; it sits on the grass around it. A commentator describing a green, damp surface and a dewy outfield is describing two conditions pulling in opposite directions.

Humidity on its own is the third, and the most frequently confused. High humidity in daylight is often invoked to explain swing, and the physics of that claim is contested. Whatever its merits, it is not dew, because no condensation has occurred and the ball is dry. Dew requires a surface that has cooled below the dew point, which is why it belongs to the evening and not to a muggy afternoon.

Getting these apart matters for prediction. A side that reads a humid afternoon as a guarantee of evening dew will be wrong whenever the cloud stays in or the wind gets up, and both of those are common.

Dew and the pink ball

Day-night Test cricket put a new ball into the same conditions and produced a problem that red-ball cricket had not had to solve.

A pink ball is manufactured to stay visible under floodlights across a long innings, which means a different finish and, in most versions, an extra layer of surface treatment. That treatment changes how the ball responds to moisture. Reports from sides playing under lights have consistently described the pink ball behaving differently in the evening session from the afternoon, and dew is one of the variables in that, alongside the change in light and the difference in the ball's construction.

The complication for a Test is duration. A limited-overs side deals with dew for a couple of hours in one innings. A five-day match under lights meets the same conditions in the final session of each day, repeatedly, with a ball that is at a different age each time. That produces a session that behaves unlike any other in the match, which is a large part of why the format's evening period has the reputation it does. The ball itself, its construction and how it differs is covered in the pink-ball Test explainer.

There is a fielding consequence too. Catching a wet pink ball under lights against a dark sky is a genuinely harder task than catching a red one in daylight, and sides preparing for a day-night Test spend disproportionate time on evening catching practice for exactly that reason.

The evening, step by step

1. 1Toss, an hour before playThe captains and support staff have already read the forecast dew point against the expected evening temperature. Where heavy dew is expected the captain winning the toss fields, and the decision is close to automatic.
2. 2First innings under sun and lightsConditions are dry and the ball behaves normally. Spinners grip, the outfield is slower, and the total posted is a fair reflection of the surface.
3. 3Sunset and the innings breakSurface cooling accelerates as soon as direct sun is off the ground. Groundstaff apply the first rope drag and the outfield is checked. Nothing has happened yet that a viewer would notice.
4. 4First ten overs of the chaseThe grass surface reaches the dew point and condensation begins on the blades. Bowlers start calling for a towel between deliveries and the shine on the ball becomes impossible to maintain.
5. 5Middle overs, grip failsFinger spin stops turning and slower balls become unreliable. The captain reorganises the attack around whoever can still control a wet ball, and the required rate falls without the batting side taking obvious risks.
6. 6Breaks are used for dew managementRope drags and super soppers run in every available interval. The relief lasts a few overs at most, and the frequency of the intervention rises as the evening goes on.
7. 7Death overs with a soaked ballYorkers become full tosses, the outfield runs quickly and boundaries come from shots that would have been fielded earlier. This is where the toss advantage is actually cashed.
8. 8Post-match discussionThe result is attributed to the toss, the schedule or the venue, and the argument about start times resumes without anything being measured.

The general sequence in a fixture where heavy dew arrives. Timings shift with latitude, season and start time.

What dew does to shortened matches and the rain rules

The interaction between dew and interrupted matches deserves its own note, because it produces outcomes that look unfair and are not errors.

The Duckworth-Lewis-Stern method calculates from overs remaining and wickets in hand. It has no input for the state of the ball, the wetness of the grass or the time of night, and it is not supposed to have one; adding a condition variable would require the method to make a judgement about the match rather than about the resources. The consequence is that a side chasing a revised target in heavy dew is treated identically to one chasing in the dry, even though the second is a materially harder job. The method's design and its acknowledged limits are set out in the article on how Duckworth-Lewis-Stern works.

There is a second interaction that cuts the other way. Because dew is worst late, a chase that finishes early through a reduction in overs can end before the conditions have fully turned. A rain break at the innings interval that removes overs from the second innings can therefore reduce the dew advantage rather than compound it, which is the opposite of what most viewers assume.

The scheduling argument dew keeps starting

Every proposed fix for dew is a scheduling fix, because the physics is not negotiable.

Earlier start times are the simplest. Moving a fixture forward by two hours pushes more of the second innings into the period before the surface has cooled to the dew point. The objection is commercial: prime-time broadcast slots are what pay for the competition, and moving away from them costs revenue that the fixture exists to generate.

Seasonal placement is the more powerful lever and the harder one to pull. Dew is worst in specific weeks of the year at specific latitudes, and a governing body that scheduled around them could remove most of the problem. That would mean handing those weeks to another format or another region, which the calendar is already too crowded to permit.

Playing knockout matches as day fixtures is a narrower proposal with real support, on the grounds that a tournament is entitled to protect its most consequential games from a condition it knows will distort them. The counter-argument is that it makes the knockouts play differently from the group stage that qualified the sides.

The last proposal is the most radical and resurfaces after every one-sided evening: allow a second toss at the innings break, or fix the batting order by a rule rather than a coin. It has never been adopted, and the objection is that it treats a symptom while making the sport's oldest ritual conditional. What is more likely, and is already happening quietly, is that venues and boards keep improving the countermeasures and keep saying nothing about how well they worked.

What to watch for on a dewy night

The signs are visible long before the commentary mentions them, and knowing them lets you read the second innings several overs early.

Watch the bowlers' hands between deliveries. A towel appearing in a fielder's waistband, a bowler drying the ball on a shirt, or the fourth official bringing a fresh towel to the boundary are the first indications, and they usually appear before the scoring rate moves.

Watch the wicketkeeper. A keeper standing up who suddenly retreats a couple of paces, or who starts taking the ball with two hands where one would normally do, is telling you the ball is coming through wetter and faster.

Watch which bowlers disappear. A frontline finger spinner with overs in hand who does not bowl again after the twelfth over of a chase has not been rested. And watch the outfield: a ball that used to pull up ten yards inside the rope now reaching it under the same shot is the clearest single sign that the grass has changed.

Then watch the required rate rather than the wickets. Dew rarely produces a collapse; it produces a chase that never looks difficult, where the rate drifts down without anybody playing a memorable shot. That quiet, uneventful second innings is what heavy dew actually looks like from the stands.

More on conditions, tosses and the tactics built around them is collected in the cricket section, and the full set of long-form explainers is on the blog.