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.

Morning dew is one of the most underrated distance killers in golf. A ball rolling through dewy grass picks up a thin film of water that adds spin and reduces both carry and roll. Testing by MyGolfSpy and Trackman fitters shows a wet driver face and dew-coated ball can strip 5-15 yards of carry off a typical drive, mostly by knocking down spin efficiency at impact — the water disrupts the friction between clubface and ball. Iron shots off a dewy lie behave like a slight flyer: less backspin, less stopping power on greens, and unpredictable dispersion.

Cool morning conditions stack additional effects on top of dew: cold-dense air (already worth ~2 yards per 10°F below 75°F), tight, dew-slicked greens that release less because the mowers haven't run yet, and a cold golf ball that compresses less at impact. Rule of thumb for a dawn tee time: take one extra club on approaches, expect greens to be slow and grainy for the first few holes as dew burns off, and wipe your ball and clubface on every shot until you stop seeing water on the towel. GWI weights morning humidity, dew point, and cloud cover into the hourly score so you can see when a dew-affected early window climbs into playable territory around 9-10 AM as temperatures rise.

A wet golf ball is measurably harder to control and flies noticeably shorter. When water sits between the clubface and the ball at impact, it acts as a lubricant that disrupts the friction the club needs to impart proper backspin. Trackman and independent lab testing consistently show water on the ball causes a “flyer” effect: reduced spin, lower launch, less carry, and dramatically less predictable dispersion. Expect roughly 5-10 yards of lost carry on iron shots and 10-15 yards on drives, plus significantly less stopping power on greens because the ball lands with less spin.

Controllability suffers even more than distance. A wet ball off an iron often flies straighter than intended (a slice or draw won't curve as hard because there's less spin to work with), then rolls out unpredictably on landing. Around the greens the problem inverts: a wet ball on a wedge shot can grab the grooves inconsistently, causing chunked or bladed contact. Practical fix: carry a dedicated dry towel just for the ball and clubface, wipe both between every shot in wet conditions, and take one extra club on approaches to accept the flyer effect rather than fight it. GWI factors precipitation intensity and ground moisture into the score, so hours after heavy rain typically drop several tiers even when the sky has cleared.

Wet fairways can strip 20-40 yards of total distance off a driver even when your carry number is unchanged — the ball simply plugs or skids where it lands instead of releasing. On a firm summer fairway, a 250-yard drive might carry 235 and roll out to 260. On a saturated fairway after overnight rain, that same 235-yard carry produces almost zero rollout, leaving you 25 yards shorter on every hole. Over 18 holes that adds up to two extra clubs into greens for the entire round.

Iron shots are less affected because they're mostly a carry number to begin with — but you'll notice fairways playing longer, greens holding more (which is good), and mud sticking to the ball on any shot from soggy turf. Mud on the ball creates unpredictable dispersion, often 10-15 yards offline in the direction opposite the mud. USGA rules typically allow you to lift, clean, and place in wet conditions during casual play — use it. GWI's precipitation and ground-moisture inputs pull the score down for several hours after significant rainfall, even under clearing skies, so you can see when the fairways have had time to drain.

Wet greens are almost always slower, not faster — the intuition that “water is slick” doesn't hold on turfgrass. A saturated green typically loses 1-2 feet of stimpmeter reading. A green that rolls a fast 12 on stimp in normal conditions may drop to 10-10.5 after heavy rain, and a moderate 10 can drop to 8-8.5. The water fills the microscopic gaps in the grass blades and makes the surface tacky rather than slick. It also softens the underlying soil, so ball-in-motion friction increases.

Putts break less on wet greens for the same reason: the ball is slower off the putter face, gravity works on it longer, but the increased friction damps out the sideways component of the break. Rule of thumb: after significant rain, hit putts 15-20% harder than you would in dry conditions, play less break than the line suggests, and expect the first putt of the day to run short until you calibrate. Approach shots into wet greens are the good news — balls stop faster, spin is preserved, and you can attack pins that would be dangerous on firm surfaces. GWI factors green firmness (via precipitation and soil moisture) into the hourly score alongside the more obvious wind and temperature terms.

Yes — the ball flies about 2% farther for every 1,000 feet above sea level. That's the widely cited Trackman and USGA rule, and it holds up well in practice. Sea-level golfers visiting Denver (5,280 ft) see roughly 10-11% more carry across the bag — a 150-yard 7-iron plays like a 165, and a 250-yard drive stretches to about 275. Reno (4,500 ft), Albuquerque (5,300 ft), Mexico City (7,300 ft), and Park City (~7,000 ft) sit in similar or higher air. The cause is lower air density: thinner air means less drag on the ball, so it holds its velocity longer and carries farther.

The tradeoff is spin and shape control. In thinner air a ball spins the same but reacts less — a 10-degree draw at sea level curves maybe 6-7 degrees at 6,000 feet, so shots don't shape as much and mis-hits stay straighter but travel farther offline. Wedge shots also don't check up the same way. Practical adjustment: subtract the elevation percentage from your normal club yardages (a mid-iron plays roughly one club shorter at 5,000+ feet), aim tighter to your target because dispersion widens, and expect firm-fast conditions to compound the effect on approach shots that don't hold. GWI's temperature and dew-point terms already reflect local air density, but elevation is a fixed course-level factor you should learn once for any high-altitude course you play.

Barometric pressure changes ball flight by roughly 1 yard per 0.2 inches of mercury (inHg) on a driver, which is smaller than most golfers think. A typical high-pressure system reads around 30.20 inHg and a stormy low around 29.60 inHg — a 0.6 inHg swing that translates to about 3 yards of carry on a full driver, and roughly 1 yard on a mid-iron. The mechanism is the same as elevation: higher pressure = denser air = more drag = shorter carry. Lower pressure lets the ball fly a hair farther.

In practice, pressure alone is rarely the biggest factor in a round because temperature, humidity, and wind swings dominate the density calculation. Where pressure matters is as a leading indicator of incoming weather: a falling barometer usually means an approaching front, and with it wind shifts, precipitation, and drops in cloud cover. When GWI's score changes significantly between hours on an otherwise stable day, it's often because the underlying pressure trend is pulling wind speed, temperature, and humidity together in the same direction. Watch the daily average score alongside your peak window — a widening gap usually means weather is on the move.

The sweet spot for a comfortable, high-performance round is 65-80°F. In that range the golf ball performs close to its rated compression, muscles stay loose without needing constant warm-up, air density is neutral (no distance penalty or bonus), and heat stress is minimal even over 4-plus hours on the course. Most tour events schedule tee times to keep players in this band, and it's why spring and fall shoulder seasons produce so many career rounds — the weather itself is out of the way.

Above 85°F, physiological cost compounds fast: heart rate rises, focus fades on the back nine, and dehydration silently drops club-head speed by 3-5 mph if you don't drink water aggressively. Above 90°F with a dew point over 70°F, cardiovascular strain is real and shot quality suffers noticeably. Below 55°F, the ball itself compresses less and cold muscles need dedicated warm-up; the same swing produces 3-5% less carry, and grip pressure creeps up in cold fingers. Below 40°F, most amateurs lose 10+ yards across the bag and shot dispersion widens. GWI's temperature factor peaks in the 65-80°F window and drops symmetrically as you move away from it — that's why an 82°F round often scores higher than an 88°F or 60°F one, even with identical wind and sky.

UV exposure over a 4-plus-hour round is a real performance factor, not just a skin-cancer risk. A UV index of 8 or higher — common on clear summer days from 10 AM to 4 PM — delivers a sunburn dose in under 25 minutes of unprotected exposure, but the more immediate golf impact is eye fatigue, glare-driven focus loss, and heat stress. Golfers regularly report their swing tempo suffering by holes 13-14 on high-UV days even when the temperature itself feels manageable. The sun does the damage; heat just makes it obvious.

Practical steps: SPF 30+ on face, ears, neck, and forearms before the first tee (re-apply at the turn); a wide-brim or bucket hat rather than a visor if UV is 9+; polarized sunglasses to cut glare on greens and lakes; and one bottle of water every three holes minimum, more if the dew point is above 65°F. Cart-only courses ease the physical load but don't reduce UV exposure much. GWI factors UV index directly into the score, so an 82°F day at UV 10 will score lower than an 82°F day at UV 5 — the sun is doing more work on your body in the higher-UV window, and the score reflects that.

Lightning is the only weather factor that turns golf from “play through it” into “stop playing immediately.” The National Weather Service reports golf courses among the most dangerous places to be caught in a thunderstorm because they combine wide open terrain, elevated metal objects (clubs, flagsticks, carts), and long exposure time. The safety rule is well-established: if you can hear thunder, you're within lightning range, even if the sky above you is clear. Head to a permanent building or a lightning-safe shelter immediately — not under a tree, not in a golf cart, not in an isolated rain shelter. Wait 30 minutes after the last thunder before returning to play.

Beyond the safety hard-stop, thunderstorms usually bring the full deck of golf-hostile conditions in quick succession: wind gusts of 30+ mph, driving rain, hail on occasion, and rapid temperature drops of 10-20°F as the storm passes. Course superintendents commonly close play for 60-90 minutes after any nearby lightning strike. GWI treats lightning probability as a safety override, not a comfort factor — when the hourly thunderstorm probability is elevated, the score drops sharply regardless of how the other conditions look, and the Caddie's Read swaps to a storm-warning beat. Check GWI's hourly view before booking a tee time within 24 hours of a forecast front, and always re-check within an hour of teeing off if storms are in the area.

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.