Should you play golf today?

You lose roughly 2 yards of carry per club for every 10°F drop below 75°F, and gain about the same going the other way. That's the widely cited Andrew Rice / Trackman rule, and the USGA confirms the ~2 yards per 10°F figure. The cause is air density: cold air is denser, so the ball fights more drag and the ball itself compresses less at impact. On a 45°F morning versus a 75°F afternoon, expect your driver to carry about 6 yards shorter and your 7-iron about 4-5 yards shorter. GWI's temperature factor scores this comfort-and-distance penalty into every hour so you can see when carry-friendly conditions line up with your tee time.

Humidity plays a small supporting role. Contrary to the popular myth that humid air is “heavy,” humid air is actually less dense than dry air at the same temperature — water vapor molecules are lighter than the nitrogen and oxygen they displace. On a hot, humid day the ball can carry ~1-2 yards further than the temperature alone would predict, but grips get slick and greens get softer, which usually matters more than the tiny distance bonus.

Here's what GWI's carry model predicts across common playing conditions (all values on a 150-yard reference shot vs. the 70°F / 50% humidity baseline):

Assumes ~10 yards per club for a mid-iron. Wind, elevation, and lie will shift these further — when the wind is 10 mph or more, wind dominates temperature every time.

Direct sunlight can make the air feel 10-15°F warmer than the thermometer reads, because the sun's radiation heats your skin directly on top of the air temperature. Air temperature is measured in the shade — step into full sun and you add solar radiation to what your body feels. A clear, calm 70°F round can feel like 80-85°F on an exposed fairway, while the same 70°F under heavy overcast feels like an actual 70°F. That's why GWI blends cloud cover into the score alongside raw temperature: an 85°F day with 60% cloud cover often plays more comfortably than a 78°F day under a cloudless sky, especially in July and August.

The golfer's rule of thumb is one extra club for every 10 mph of headwind, and one less club for every 20 mph of tailwind. Tailwinds help only about half as much as an equivalent headwind hurts — the ball is airborne for less time downwind and the wind can't push it as far as it can hold it back. A 150-yard 7-iron shot into a 10 mph headwind plays closer to 160-165 yards, so take a 6-iron. Into a 20 mph headwind, add two clubs and swing at 75-80% for a lower, more controllable ball flight. Crosswinds don't change club much but shift your aim 5-10 yards into the wind for every 10 mph of crosswind.

Here's what to expect on a 150-yard reference shot at common wind speeds:

Headwind rule: ~1% of carry per mph, so ~1.5 yd per mph on a 150. Tailwind helps about half as much (~0.75 yd per mph). Gusts matter more than sustained wind for dispersion — GWI weights both. For crosswinds, hold aim 5-10 yd into the wind per 10 mph.

Six factors matter beyond temperature and wind, and GWI scores all of them: precipitation (soft fairways kill rollout, wet grips reduce control), humidity and dew point (a dew point above 65-70°F makes 85°F feel oppressive and drains stamina by the back nine), cloud cover (glare, visibility, and perceived heat), UV index (sunburn risk and eye strain over 4-plus hours), lightning and thunderstorm probability (a safety hard-stop, not a comfort factor), and gust spread (steady 15 mph plays fine; 8 mph gusting to 22 wrecks tempo). Humidity actually adds slightly to ball distance (humid air is less dense) but the effect is under 1-2 yards and swamped by how it makes you feel walking 18 holes.

Dew point is the single best number for how a humid round will feel. Below 55°F it feels dry and comfortable; 55-65°F is pleasant; 65-70°F feels sticky; above 70°F is oppressive and drains stamina fast by the back nine. Humid air is actually slightly less dense than dry air, so the ball technically carries 1-2 yards farther — but that gain is completely swamped by the physical toll: slick grips, sweat in your eyes, softer greens that hold shots but produce less roll on fairways, and a heavier feel through the swing. Wipe your grips between shots, drink water every three holes, and take one extra club on approaches to compensate for softer landing areas.

GWI scores each hour from 0 to 100 across six tiers. 85-100 is prime golf weather — book it. 70-84 is a great day for a round. 55-69 is playable with minor annoyances. 40-54 is tough but doable, so dress for the conditions. 20-39 is a rough round only casual golfers should attempt. 0-19 means stay home and practice indoors. Most golfers plan around windows scoring 70 or higher on the day-of forecast. On a hot, humid, or windy afternoon you may only get one or two hours above 70 all day, which is why the “Best windows to tee off” recommendation surfaces the top hours regardless of the daily average.

Yes, always re-check GWI on the morning of your round. Forecast accuracy degrades with distance — day-one and day-two scores are typically within a few points of what actually happens, but by day five through seven the underlying weather models can swing 10-20 points as fronts, wind shifts, and pop-up storms come into focus. The 7-day view is best used to pick the most promising day of the week, not to lock in a specific tee time. Once you're inside 24-48 hours, GWI's hourly view sharpens considerably and the "Best windows to tee off" recommendation becomes reliable. If today's GWI looks worse than what the 7-day showed yesterday, trust today's number — it's built on fresher observations.

The Caddie’s Read is the single narrative card that appears in the Conditions section whenever you have a peak tee-off window. Instead of three separate chips, it reads like the briefing a real caddie would give you on the first tee — usually two to four sentences covering (1) a weather headline in plain language (temperature feel, sky, wind character, direction), (2) a ball-flight and club math beat that combines the temperature and dew-point carry effect with a headwind club adjustment for holes facing into the wind, (3) a course-management beat that reads the greens and fairways for the day (soft and receptive, firm and fast, muggy grips, bright sun on cool ground, high UV, storm risk), and (4) a playability drift note when the peak score is much higher or lower than the daily average, so you know whether to tee off close to your window or whether the whole day plays about the same.

The Caddie’s Read never adds new API calls — it’s built from the exact same hourly numbers powering the score. Beats drop out automatically when the data doesn’t warrant them, so a mild, uneventful window may only show a sentence while a stormy or windy one gets the full four. If lightning risk is high, the storm warning trumps every other beat.

A Caddie’s Read is the practical playing advice a real caddie gives you based on the conditions — not just a temperature and a wind speed, but what that combination means for your club selection, ball flight, and course strategy. A good caddie translates raw weather into decisions: “cold air is dense, so hit one more club into greens”; “the ball is carrying long on a hot muggy afternoon, take one less”; “wind is 15 mph out of the SW, add a club and knock it down on any south-facing hole”; “greens are soft after last night’s rain, attack the pins but expect less rollout off the tee”; “storm risk on the back nine, plan to walk in if you hear thunder.” GWI’s Caddie’s Read card synthesizes that same read from the hourly forecast for your peak tee-off window, so you walk to the first tee with the same information a real caddie would give you.