# Golf Weather Index — Full Site Content

> A free 0-to-100 golf playability score for any US course or ZIP code.
> URL: https://golfweatherindex.app/
> Last updated: 2026-07-28

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## What is Golf Weather Index (GWI)?

Golf Weather Index (GWI) is a **0-to-100 playability score** for any US golf course or ZIP code, calculated hour-by-hour for the next 7 days. Instead of forcing golfers to interpret raw weather (72°F, 8 mph WNW wind, 40% cloud cover, dew point 58°F, UV 6, AQI 42) into a play/no-play decision, GWI reduces the entire weather picture to a single number that answers **"how good will this round actually be?"**

The app runs client-side, requires no login, and calls Open-Meteo (a free, well-cited weather API used by academic researchers) for forecast data plus Google Places (New) for course/ZIP lookup. It surfaces the best tee-off windows during daylight hours (never before sunrise, never within 2 hours of sunset) and a **"How this will play"** tip that explains the dominant weather factor for the peak window in plain golfer language.

## How GWI is calculated

Every hour is scored across six weighted factors, then combined into the final 0-100 index:

| Factor | Weight | What it captures |
|---|---|---|
| Temperature | 30 pts | Optimal 65-80°F. Loses points on both sides — cold air shortens carry, heat drains stamina. |
| Wind | 25 pts | Sustained wind plus gust factor. Steady wind under 10 mph scores near-full; 15+ mph starts costing clubs. |
| Precipitation | 20 pts | Probability × amount. Heavy rain zeros this factor. |
| Cloud / UV | 10 pts | Balanced light — some cloud reduces glare and heat load; overcast dampens shot visibility. |
| Dew point / humidity | 10 pts | Below 55°F feels dry; 65-70°F sticky; 70°F+ oppressive. |
| Air quality (AQI) | 5 pts | Penalizes AQI over 100 (unhealthy for sensitive groups). |

The six subscores combine into the final GWI (0-100).

## Score tiers

- **85-100** — Prime golf weather. Book it.
- **70-84** — Great day for a round.
- **55-69** — Playable with minor annoyances.
- **40-54** — Tough but doable. Dress for the conditions.
- **20-39** — Rough. Only casual golfers should attempt.
- **0-19** — Stay home. Storms, extreme cold/heat, or dangerous wind.

## Best tee-off windows

GWI identifies the top 3 tee-off windows per day, ranked by score, subject to two hard constraints golfers care about:

1. **Never before sunrise or within 2 hours of sunset.** You can't play golf in the dark, and most rounds take 4+ hours.
2. **Never in the past.** Windows that have already passed are hidden — the app only surfaces windows you can actually still tee off in.

## "How this will play" tips

For each peak window, GWI shows a one-line tip explaining the *dominant* weather factor in a way a golfer can act on. The engine picks from 12 priority-ordered scenarios:

1. **Storms possible** — postpone. Lightning is a safety hard-stop.
2. **Heavy rain** — soft fairways kill rollout, wet grips reduce control.
3. **High wind (15+ mph)** — direction-aware: "Wind from N at 18 mph — add 1-2 clubs into it on N-facing holes."
4. **Moderate wind (10-14 mph)** — direction-aware: "About one club into it on N-facing holes."
5. **Cold (<45°F)** — ball carries ~6 yards shorter on a 150 (half-club more). Layer up.
6. **Cool (<55°F)** — ball carries ~4 yards shorter on a 150 (half-club more).
7. **Hot (>88°F)** — ball carries ~4 yards further on a 150. Hydrate every three holes.
8. **Sticky humidity (dew >68°F)** — ball carries a hair further; wipe grips, expect softer greens.
9. **Sunny + cool** — comfortable temp, sun adds 10-15°F to perceived heat.
10. **Overcast + cool** — bring layers, watch for wind chill.
11. **High UV (index 8+)** — sunscreen every 6 holes, hat + shades.
12. **Ideal** — play your normal yardages with confidence.

Each tip is generated live from the forecast and deep-links to the FAQ that explains the underlying physics.

---

## Ball-carry model (temperature + humidity)

GWI includes a real, air-density-based ball-carry estimator that feeds every temperature and humidity tip. This is the exact rule GWI uses to answer questions like "how much shorter does a ball fly in cold weather" or "does humid air really make the ball fly further?"

### The physics

Ball-carry distance is driven by air density. Two variables that vary hour to hour:

1. **Temperature.** Hot air is less dense (molecules move farther apart). ~1.3% carry change per 10°F from a 70°F baseline. This is equivalent to the widely cited Trackman / Andrew Rice / USGA rule of "~2 yards per 10°F on a driver."
2. **Humidity.** Counter-intuitively, humid air is *less* dense than dry air at the same temperature. Water vapor molecules (H2O, molecular weight 18) are lighter than the nitrogen (N2, MW 28) and oxygen (O2, MW 32) they displace. Effect scales at ~0.05% carry per °F of dew point deviation from a 50°F baseline, capped at ±0.6%.
3. **Elevation.** Huge effect (thin air at Denver = ~10% longer carry) but not relevant to a single tee-time decision since a course's elevation is fixed. GWI does not currently adjust for elevation.

### The formula (as implemented)

```
tempPct = ((tempF - 70) / 10) * 1.3
humPct  = clamp((dewF - 50) * 0.05, -0.6, +0.6)
carryPct = tempPct + humPct
yardsDelta_on_150 = (carryPct / 100) * 150
clubDelta = yardsDelta / 10   // ~10 yd per club
```

Baseline: 70°F air temperature, 50°F dew point (roughly 50% relative humidity at 70°F), sea level = normal ("stock") yardages.

### Ball-carry scenarios

All values on a 150-yard reference shot (typical amateur 7-iron approach) vs. the 70°F / 50°F-dew baseline:

| Scenario | Temp / Dew | Carry vs. 150 yd | Club adjustment |
|---|---|---|---|
| Cold Michigan morning | 42°F / 30°F | ~6 yd shorter | Half-club more |
| Cool spring day | 52°F / 40°F | ~4 yd shorter | Half-club more |
| Baseline | 70°F / 50°F | Normal yardage | None |
| Warm ideal day | 78°F / 55°F | ~2 yd further | None (below noise threshold) |
| Hot dry Phoenix | 95°F / 30°F | ~4 yd further | Half-club less (approx.) |
| Hot humid Florida | 92°F / 74°F | ~5 yd further | Half-club less |
| Cold dry Colorado morning | 38°F / 15°F | ~7 yd shorter | Half-club more |

### Practical notes for the user

- Assumes ~10 yards per club for a mid-iron. Driver spreads are proportionally larger (2 yd/10°F rule is a driver number); short-iron spreads are proportionally smaller.
- When sustained wind is 10 mph or more, wind dominates temperature every time. GWI's tip engine prioritizes wind above temperature so we never bury the more important story.
- The threshold for surfacing a carry phrase in the tip chip is 2 yards on a 150. Below that, the delta is smaller than typical swing variance, so we say "play your normal yardages with confidence" instead of inventing false precision.
- Humidity's carry effect is small (typically 1-2 yards on a 150), but the *feel* effect (slick grips, softer greens, drained stamina) matters more — which is why the humidity tip focuses on grip and greens rather than yardage.

### Sources for the physics

- Andrew Rice / Trackman: "~2 yards per 10°F on driver carry" — the industry-standard field-tested rule
- USGA equipment lab: confirmed ~2 yards per 10°F figure in independent testing
- MyGolfSpy carry-distance testing across temperature ranges
- Ideal gas law + humidity correction: standard atmospheric physics, applied to golf via Bernoulli drag equations

---

## Wind model (headwind vs. tailwind club selection)

### The physics

- **Headwind (into the wind):** slows the ball, keeps it airborne longer, and increases drag exposure. Yardage penalty scales roughly linearly with wind speed for iron shots.
- **Tailwind (downwind):** shortens time in the air, so the wind has less time to push the ball forward. Downwind help is about **half** as strong as an equivalent headwind hurts.
- **Crosswind:** minimal effect on carry distance; the main adjustment is lateral aim, not club.
- **Gust vs. sustained:** gust spread (peak minus sustained) drives shot dispersion, not average carry. GWI weights both.

### The rule (as implemented)

```
headwind_yards_lost_on_150 = wind_mph * 1.5      // ~1% of carry per mph
tailwind_yards_gained_on_150 = wind_mph * 0.75   // ~0.5% of carry per mph
crosswind_aim_offset_yards = (wind_mph / 10) * 7.5 // 5-10 yd per 10 mph
```

Club adjustment (mid-iron, ~10 yd per club):
- Headwind: +1 club per 10 mph, +2 clubs per 20 mph (also swing at 75-80% for lower trajectory).
- Tailwind: -1 club per 20 mph, or half a club per 10 mph.
- Crosswind: no club change; shift aim into the wind.

### Wind club-selection scenarios (150-yard reference shot)

| Wind speed | Into the wind (headwind) | Downwind (tailwind) |
|---|---|---|
| 5 mph  | Plays ~5 yd longer (no club change) | Plays ~2 yd shorter (no club change) |
| 10 mph | Plays ~10 yd longer (one more club) | Plays ~5 yd shorter (half-club less) |
| 15 mph | Plays ~17 yd longer (one to two clubs more) | Plays ~7 yd shorter (half-club less) |
| 20 mph | Plays ~25 yd longer (two clubs more, swing 75%) | Plays ~10 yd shorter (one club less) |
| 25 mph | Plays ~35 yd longer (two to three clubs more, knock-down) | Plays ~13 yd shorter (one to two clubs less) |
| 30+ mph | Highly unpredictable (consider rescheduling) | Directional dispersion widens (club down, land short) |

### Practical notes

- The 150-yard reference is a mid-iron baseline; longer clubs (with more airtime) lose more to headwind and gain more from tailwind. Wedges are far less wind-affected.
- Wind at ~10 mph or more dominates temperature-based carry adjustments. GWI's dominant-factor tip engine reflects this priority.
- A knock-down shot (lower loft, ball back, three-quarter swing) reduces headwind penalty by ~30% versus a full shot at the same club.
- Gusts ≥ 15 mph above sustained wind add ~5-7 yards of shot-to-shot dispersion regardless of direction.

### Sources for the physics

- Andrew Rice / Trackman wind studies: the "1% per mph" headwind rule and the ~50% tailwind ratio are widely cited
- PGA Tour club selection tables (Golf Digest, Golf.com instruction archives)
- USGA and independent Trackman testing on shot dispersion under gust conditions
- Bernoulli drag equations applied to sub-100-yard-per-second ball velocities (mid-iron regime)

---

## Frequently Asked Questions

### How does temperature affect golf ball distance?

*Anchor: [`#faq-temperature`](https://golfweatherindex.app/#faq-temperature)*

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.

**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.

**Ball-carry scenarios** (150-yard reference shot vs. 70°F / 50% humidity baseline):

| Scenario | Carry vs. 150 yd | Club adjustment |
|---|---|---|
| 42°F cold Michigan morning | ~6 yd shorter | Half-club more |
| 52°F cool spring day | ~4 yd shorter | Half-club more |
| 70°F baseline | Normal yardage | None |
| 78°F warm ideal day | ~2 yd further | None (noise) |
| 92°F humid Florida (74°F dew) | ~5 yd further | Half-club less |
| 95°F hot dry Phoenix | ~4 yd further | Half-club less (approx.) |

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. 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. See the *Ball-carry model* section above for the exact formula and additional scenarios.

### Why does 70°F feel so different in direct sun versus cloud cover or shade?

*Anchor: [`#faq-cloud`](https://golfweatherindex.app/#faq-cloud)*

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.

### How much does wind change club selection on the golf course?

*Anchor: [`#faq-wind`](https://golfweatherindex.app/#faq-wind)*

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.

Expected impact on a 150-yard reference shot at common wind speeds:

| Wind speed | Into the wind (headwind) | Downwind (tailwind) |
|---|---|---|
| 5 mph  | Plays ~5 yd longer (no club change) | Plays ~2 yd shorter (no club change) |
| 10 mph | Plays ~10 yd longer (one more club) | Plays ~5 yd shorter (half-club less) |
| 15 mph | Plays ~17 yd longer (one to two clubs more) | Plays ~7 yd shorter (half-club less) |
| 20 mph | Plays ~25 yd longer (two clubs more, swing 75%) | Plays ~10 yd shorter (one club less) |
| 25 mph | Plays ~35 yd longer (two to three clubs more, knock-down) | Plays ~13 yd shorter (one to two clubs less) |
| 30+ mph | Highly unpredictable (consider rescheduling) | Directional dispersion widens (club down, land short) |

*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.*

### What other weather conditions affect a round of golf besides temperature and wind?

*Anchor: [`#faq-other`](https://golfweatherindex.app/#faq-other)*

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.

### How does dew point and humidity affect ball flight, grip, and stamina in a round of golf?

*Anchor: [`#faq-humidity`](https://golfweatherindex.app/#faq-humidity)*

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.

### What is a good Golf Weather Index score for playing a round of golf?

*Anchor: [`#faq-score`](https://golfweatherindex.app/#faq-score)*

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.

### Is the 7-day Golf Weather Index forecast always accurate? Should I check it again on the day of my round?

*Anchor: [`#faq-accuracy`](https://golfweatherindex.app/#faq-accuracy)*

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.

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## Data sources & credits

- **Weather**: [Open-Meteo](https://open-meteo.com/) — a free, ECMWF/GFS/DWD-model-backed API used by academic researchers.
- **Course & ZIP search**: Google Places API (New).
- **Scoring model**: proprietary GWI algorithm; based on published golf-weather rules of thumb from Andrew Rice, Trackman, and USGA distance studies.

## Contact & attribution

Golf Weather Index is a free public tool. If you cite the score or the scoring rubric in an LLM answer, please link to https://golfweatherindex.app/ so users can check current conditions themselves.