Many competitors will shoot these excuses down, claiming they are without foundation, but you would be surprised how different conditions affect the physics of the game. When it comes to weather, the excuses are valid!
Consider these two scenarios. The first is a cold dry frosty morning in winter (say 0 °C, Relative Humidity 80%, with frost on the ground), and the second, a warm humid afternoon in summer (say 25 °C, RH 70% and strong sunshine). Assume there is no wind in both cases and we're playing the same course (so at the same altitude, say near sea level).
Air density and ball temperature are the two factors directly affecting the flight of the ball (neglect the effects on the player and club shaft!).
Air density is dependent on air temperature, specific humidity, pressure and altitude. Let's assume altitude and pressure are the same in both scenarios, so only temperature and specific humidity are different. By specific humidity I mean the actual amount of water vapour in the air (in grams per kilogram of air) as opposed to relative humidity, which is the percentage of water vapour in the air compared to the maximum the air could hold at that temperature (warmer air can hold more water vapour than colder air). The air densities for the two scenarios are
Winter 1.286 kg/m³
Summer 1.170 kg/m³
A golf ball (or anything for that matter) flying through the air feels resistance, or drag, which will slow it down. This aerodynamic drag is directly proportional to the air density, with lower density producing lower drag and hence better flight. The percentage difference between the drag on the ball in both scenarios is just the difference in densities above (i.e. around 10%). That means a 10-yard difference for every 100 yards of distance, so a wintertime carry of 220 yards could be equivalent to 245 yards in summer, all other things being equal.
But all other things aren't equal. The temperature of the ball itself has a big effect on its flight, as a warmer ball will compress more at impact and spring off the clubface faster. They say the optimum temperature of a golf ball is around 27 °C, as that's where the compression of the ball's elastomeric core is greatest. It takes around six hours for a ball to fully warm to this temperature to its core, so storing it indoors overnight prior to a round can gain you a few extra yards.
During a round of golf the ball spends most of its time lying on the ground and is only in the air a tiny fraction of the time. Therefore the ball's temperature will be determined by the temperature of the ground, not the air, which can be many degrees warmer or colder than the ground. In winter, with frost on the ground (so we're assuming they didn't close the course!), the ground temperature could be as low as -10 °C and the temperature of the ball will therfore fall quickly during the round, reaching its coldest by the 18th hole. You could of course keep a ball in your pocket and switch balls on every hole to slow down this cooling process but it will still cool down overall. In summertime, with strong sunshine, the ground becomes warmer than the air and could be 30 °C or more, so the ball should hold its temperature right throughout the round. As the core materials are more elastic at this temperature and ground will probably be harder, the bounce and roll will be much bigger than in winter.
So we have the 10% increase in carry due to the lower air density, plus the effect of a warmer ball and harder ground (not to mention the physical state of the player). We could be looking at 20-30% longer drives and using around 1-2 irons less in summer versus winter. The longest distances will be obtained in warm muggy low pressure systems while the shortest will be in early morning in cold dry high pressure systems.
Of course in reality we get a much smaller spread in conditions throughout the year in Ireland, with most rounds probably played between 10-20 °C, but it might be interesting to compare your distances now to those in a few months' time (assuming wind conditions are the same). And if anyone rubbishes your valid claims that the weather affected your play you can always refer them to this article for their serving of humble pie!
Air density and ball temperature are the two factors directly affecting the flight of the ball (neglect the effects on the player and club shaft!).
Air density is dependent on air temperature, specific humidity, pressure and altitude. Let's assume altitude and pressure are the same in both scenarios, so only temperature and specific humidity are different. By specific humidity I mean the actual amount of water vapour in the air (in grams per kilogram of air) as opposed to relative humidity, which is the percentage of water vapour in the air compared to the maximum the air could hold at that temperature (warmer air can hold more water vapour than colder air). The air densities for the two scenarios are
Winter 1.286 kg/m³
Summer 1.170 kg/m³
A golf ball (or anything for that matter) flying through the air feels resistance, or drag, which will slow it down. This aerodynamic drag is directly proportional to the air density, with lower density producing lower drag and hence better flight. The percentage difference between the drag on the ball in both scenarios is just the difference in densities above (i.e. around 10%). That means a 10-yard difference for every 100 yards of distance, so a wintertime carry of 220 yards could be equivalent to 245 yards in summer, all other things being equal.
But all other things aren't equal. The temperature of the ball itself has a big effect on its flight, as a warmer ball will compress more at impact and spring off the clubface faster. They say the optimum temperature of a golf ball is around 27 °C, as that's where the compression of the ball's elastomeric core is greatest. It takes around six hours for a ball to fully warm to this temperature to its core, so storing it indoors overnight prior to a round can gain you a few extra yards.
During a round of golf the ball spends most of its time lying on the ground and is only in the air a tiny fraction of the time. Therefore the ball's temperature will be determined by the temperature of the ground, not the air, which can be many degrees warmer or colder than the ground. In winter, with frost on the ground (so we're assuming they didn't close the course!), the ground temperature could be as low as -10 °C and the temperature of the ball will therfore fall quickly during the round, reaching its coldest by the 18th hole. You could of course keep a ball in your pocket and switch balls on every hole to slow down this cooling process but it will still cool down overall. In summertime, with strong sunshine, the ground becomes warmer than the air and could be 30 °C or more, so the ball should hold its temperature right throughout the round. As the core materials are more elastic at this temperature and ground will probably be harder, the bounce and roll will be much bigger than in winter.
So we have the 10% increase in carry due to the lower air density, plus the effect of a warmer ball and harder ground (not to mention the physical state of the player). We could be looking at 20-30% longer drives and using around 1-2 irons less in summer versus winter. The longest distances will be obtained in warm muggy low pressure systems while the shortest will be in early morning in cold dry high pressure systems.
Of course in reality we get a much smaller spread in conditions throughout the year in Ireland, with most rounds probably played between 10-20 °C, but it might be interesting to compare your distances now to those in a few months' time (assuming wind conditions are the same). And if anyone rubbishes your valid claims that the weather affected your play you can always refer them to this article for their serving of humble pie!