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Understanding the numbers - what NippyIndex shows and how to read it

NippyIndex pulls data from several meteorological and oceanographic forecast services and combines them into a single conditions picture for each location. This page explains what each parameter means, where the data comes from, and how to interpret what you see. None of the information here constitutes safety advice - conditions can change rapidly and this tool is no substitute for your own assessment on the day.

Important: All figures are derived from forecast models, not instruments measuring conditions at each location. Offshore wave data in particular is collected from measurement points several miles from shore and then mathematically adjusted. The results can be incorrect, and can understate what is actually happening at the waterline. Always look at the water before you make any decision.

The Overall Score

Nippy Index - the headline score

Scale 1 (Minimal) to 10 (Extreme) Source Calculated by NippyIndex from Open-Meteo marine wave forecast data

1–2

3–4

5–6

7–8

9–10

Methodology The Nippy Index is a wave-focused physical index. For coastal locations it combines three wave factors into a weighted score: sea state (wave height on the WMO/Douglas scale, 50% weight), wave power (energy flux, 30%), and wave steepness (20%). Each factor is independently scaled from 0 to 1 against published reference values before being combined. The score is anchored so that genuinely calm water (Douglas sea state 1, "Calm", wave height at or below 0.1 m) sits at the bottom of the scale, and a Douglas sea state 5 ("Rough", around 2.5 m) reaches the top - both are named points on the published Douglas/WMO sea state scale.

The Nippy Index describes the physical character of the wave conditions - it is not a safety rating and does not account for individual ability, experience, acclimatisation, water temperature, wind, or any other factor shown separately on the page. A score of 1 indicates calm seas; a score of 10 indicates rough, high-energy seas. Because the index is driven only by waves, you should always read it alongside the individually-shaded boxes for water temperature, wind and water quality - a calm-sea day scoring low on the index can still involve dangerously cold water or a strong offshore wind. Where a score of 4 or above is shown, a brief reason describing the wave conditions is displayed alongside it.

How the Value Boxes Are Shaded

Per-parameter colour shading

Source Each box is shaded against published reference scales for that specific parameter (WMO Beaufort, Douglas sea state, cold-water physiology, rainfall contamination thresholds) Methodology Most value boxes (waves, wave period, sea temperature, wind speed, wind direction) use a five-step blue ramp, from pale blue (very low intensity) through to deep blue (high intensity). The deeper the blue, the more significant that single factor is, judged against its own published scale rather than against the other factors. Water quality is the deliberate exception: it uses a separate green → amber → orange → red risk ramp, because contamination risk is read as a warning rather than an intensity. Weather alerts (such as thunderstorms) similarly use warning colours.

The shading lets you take in each factor at a glance without needing to interpret raw numbers. Importantly, each box is shaded independently - a deep-blue wind box and a pale sea-temperature box simply mean the wind is significant while the water is relatively mild; the colours are not added together. The specific thresholds behind the wind and temperature shading are drawn from recognised sources (the Beaufort scale and Met Office gale thresholds for wind; RNLI cold-water-shock guidance for sea temperature) and are described in the relevant sections below.

Wave Parameters (Coastal Locations)

Typical Waves / Larger Waves / Largest Sets

Source Open-Meteo Marine API - significant wave height (Hs) from ERA5 reanalysis and global wave model (GFS/ERA5 blend), sampled at the offshore grid point nearest the location Methodology The offshore significant wave height is adjusted to a nearshore estimate using a site-specific Nearshore Scaling Factor (NSF). This factor accounts for the beach slope (gradient), the current tidal state (estimated from ADMIRALTY tidal predictions), and the geometry of energy loss as waves shoal towards shore. Wind-generated local chop is added on top where relevant. The three figures shown are derived from wave statistics: Typical Waves represents the median wave height (roughly Hs × 0.6); Larger Waves represents the significant wave height (Hs) - the average of the highest one-third of waves; Largest Sets is an estimate of the largest waves likely over a 30-minute window (approximately Hs × 1.6–1.8, based on Rayleigh wave height distribution).

These are estimates and should be treated as a rough guide only. Wave height varies considerably depending on beach shape, water depth, tidal state, and how waves refract around headlands. The offshore buoy data used by the model averages wave energy across a wide area and cannot capture local variations. Always look at the actual water. Wave heights are shown in metres.

Wave Period

Source Open-Meteo Marine API - peak wave period and swell wave period; the longer of the two is used Methodology Wave period is the time in seconds between successive wave crests passing a fixed point. The tool takes the longer of the two period values reported (peak period and swell period), as the longer-period energy typically represents the most significant wave train.

Period is one of the most important wave characteristics. Short period waves (under 7 seconds) are typically locally-generated wind chop - uncomfortable but their energy dissipates relatively quickly. Long period swell (above 12 seconds) has travelled from distant storms and carries significantly more energy per wave, even when the measured height looks similar. Long-period swell produces more powerful breaking waves, stronger undertow, and makes conditions feel more demanding than the height figure alone might suggest. Period is shown in seconds (s).

Wave Force (Wave Power)

Source Calculated from nearshore-adjusted wave height and wave period Methodology Wave power (energy flux per unit width of wave front, in kW/m) is calculated using the simplified deep-water formula P ≈ 0.5 × Hs² × T, where Hs is the nearshore-adjusted significant wave height and T is the effective wave period. This approximates the physical energy carried by the wave. It is normalised using coastal energy classification thresholds derived from published coastal geomorphology research (Davies 1964; Wright & Short 1984).

Wave force gives a sense of how much energy is in the water beyond what height alone conveys. Because power scales with the square of wave height, doubling the wave height quadruples the wave force. A modest increase in height combined with a longer period can produce a substantial jump in force. The bar charts colour-code this: low (pale blue), moderate (mid blue), and high (dark blue). High wave force is shown even when absolute wave heights appear modest - this typically indicates long-period swell arriving from a distant storm.

Wave Type

Source Calculated from wave period and beach slope using the Iribarren (surf similarity) parameter Methodology The Iribarren number (ξ) is calculated as ξ = tan(β) / √(H/L₀), where β is the beach slope angle, H is nearshore wave height, and L₀ is deep-water wavelength (gT²/2π). Values below 0.4 indicate spilling breakers, 0.4–2.0 plunging breakers, and above 2.0 surging/collapsing breakers.

Wave type describes how waves break, which affects the physical experience in the water. Spilling breakers (gentle beach slope, short period) tumble gradually - more manageable but can produce persistent white water. Plunging breakers (moderate slope, longer period) curl and break with a sudden impact - the classic "barrel" wave, carrying significant concentrated force and strong undertow. Surging breakers (steep beach or rock) wash up and down the slope with less obvious breaking but powerful surge and backwash. This is an estimate based on modelled values and the beach geometry in the database; actual wave behaviour will vary.

Wind Parameters

Wind Speed

Source Open-Meteo weather forecast API - 10m wind speed, current hour Methodology Reported as the forecast wind speed at 10 metres above ground, in km/h. This is the standard meteorological measurement height. Gusts are not separately displayed but are typically 20–40% higher than the mean speed shown, particularly over open water and exposed headlands.

Wind affects conditions in two ways: it directly generates surface chop, and its direction relative to the shore creates additional hazards (see Relative Wind below). Wind speed is not part of the headline Nippy Index, but the wind speed box is shaded by intensity against the WMO Beaufort scale: the box deepens through Force 4 (moderate breeze, ~25 km/h), becomes prominent by Force 6 (strong breeze, ~45 km/h), and saturates at Force 8 - the point at which the Met Office issues gale warnings (34 knots / ~62 km/h). For newcomers to paddlesports, the RNLI suggests staying below around 10–12 knots; light winds keep the surface manageable.

Wind Direction

Source Open-Meteo weather forecast API - 10m wind direction (degrees), current hour Methodology Wind direction is reported as the compass bearing from which the wind is blowing (meteorological convention). It is displayed as a cardinal or intercardinal direction (e.g. SW, NNE).

Wind direction alone is less meaningful than its relationship to the shore. Use it alongside the Relative Wind reading (below) to understand whether conditions are being pushed onto the beach, held flat, or drawing you away from shore.

Relative Wind

Source Calculated from forecast wind direction and the stored shore-facing bearing for each coastal location Methodology Each coastal location in the database has a stored bearing representing the direction the shoreline faces (i.e. the direction from which waves most directly approach). The angle between the wind direction and this bearing is used to classify the wind as: Onshore (wind blowing towards shore), Cross-on, Cross, Cross-off, or Offshore (wind blowing away from shore).

This is one of the most practically important parameters. Onshore winds drive waves onto the beach and create visible surface chop - conditions look and feel rough. Offshore winds can flatten the surface and make the water appear deceptively calm, but they actively push anything on the surface away from shore. At even moderate speeds, an offshore wind presents a meaningful drift hazard, particularly for anyone experiencing difficulty. The RNLI cites offshore winds as a significant factor in many incidents. The wind direction box is shaded to reflect this directional hazard - offshore and cross-offshore winds shade more strongly than onshore winds of equal speed. This parameter is not shown for inland locations.

Feels Like (Wind Chill)

Source Calculated from air temperature and wind speed using the JAG/TI wind chill formula (Joint Action Group for Temperature Indices, 2001) Methodology The wind chill temperature is calculated as: T_wc = 13.12 + 0.6215×T − 11.37×V^0.16 + 0.3965×T×V^0.16, where T is air temperature (°C) and V is wind speed (km/h). At wind speeds below 4.8 km/h or air temperatures above 10°C, the formula is not applied and the actual air temperature is shown.

Wind chill describes how cold the air feels on exposed skin, accounting for the cooling effect of moving air. It is relevant before and after entering the water, particularly when wet. It does not represent actual water temperature or how quickly someone might cool in the water - that is captured separately by the water temperature parameter.

Temperature Parameters

Water Temperature (Sea)

Source Open-Meteo Marine API - sea surface temperature (SST), ERA5 reanalysis, at the offshore grid point nearest the location Methodology Sea surface temperature from the marine forecast model is used directly. The model uses ERA5 reanalysis data blended with near-real-time satellite SST observations. This represents the temperature of the uppermost layer of the open sea, typically within a few kilometres offshore.

Sea surface temperature is a critical factor. Cold water has well-documented physiological effects: the RNLI defines anything below 15°C as cold water, where cold-water shock (involuntary gasping, hyperventilation, raised heart rate) becomes a serious risk; the cold-shock response reaches maximum intensity between 10°C and 15°C; and below 5°C the water is considered immediately dangerous. Average UK and Ireland sea temperatures sit at just 6–10°C for much of the year. These thresholds (consistent across RNLI, RYA and cold-water physiology research - Tipton 1989; Golden & Tipton 2002) drive the shading of the sea temperature box: it stays pale above 15°C and deepens steadily as the water cools through the 10°C and 5°C danger points. Sea temperature is shown separately rather than folded into the Nippy Index, so always read this box on its own. Note that inshore sea temperature can differ from the offshore model value, particularly in shallow bays, estuaries, and during periods of strong solar heating or cold freshwater inflow.

Water Temperature (Inland)

Source Estimated - no live lake temperature API is available for UK inland waters Methodology A seasonal baseline temperature is calculated for each inland location using a monthly climatological average for UK lake surface temperatures at 52°N, adjusted for latitude (approximately −0.5°C per degree of latitude north of 52°N). This baseline is then blended with the 72-hour mean air temperature forecast (weighted 70% seasonal baseline, 30% recent air temperature) to capture short-term variation while preserving the thermal inertia of large water bodies. A minimum floor of 3°C is applied. The EA Water Quality Archive may provide supplementary measured water temperature readings where available (England only).

This is a modelled estimate, not a direct measurement. It should be treated as a reasonable indication of seasonal conditions rather than a precise reading. Lakes and lochs have strong thermal stratification - surface temperature can be markedly higher than water at depth, particularly in summer. Never dive headfirst into inland water. Inland locations do not carry a headline Nippy Index, so this temperature box (shaded against the same cold-water thresholds as the sea) is one of the main conditions indicators for inland sites. If the EA monitoring data panel is visible, those readings (where recent enough) provide additional context, but sampling is infrequent and the data may be days or weeks old.

Air Temperature

Source Open-Meteo weather forecast API - 2m air temperature, current hour Methodology Forecast air temperature at 2 metres above ground, the standard meteorological measurement height. Reported in degrees Celsius (°C).

Air temperature is relevant both before and after entering the water. The difference between water and air temperature influences how the body manages heat loss: cold air after cold water immersion - particularly combined with wind - increases cooling rate. Air temperature is shown for context alongside the other conditions; it is not part of the wave-based Nippy Index.

Tidal Parameters (Coastal Locations)

Tide Information - Next High / Low Tide

Source UK Hydrographic Office ADMIRALTY Tidal API - predicted tidal heights and times for the nearest tidal station with continuous height predictions Methodology Tidal predictions are retrieved from the ADMIRALTY API for up to 7 days. The nearest tidal station is identified by great-circle distance. The current tidal state (rising or falling, and how far through the tidal cycle) is derived by interpolating between the two surrounding high and low tide events using a standard cosine approximation of the tidal curve.

Tide affects conditions in several ways simultaneously. A falling tide on a gently sloping beach can expose additional rocks and reduce the water depth available for entry. Tidal currents are strongest around mid-tide (roughly halfway between high and low) and weakest near the peaks. Tidal state is no longer folded into the Nippy Index as a score factor, but it still feeds the wave height estimate: the conversion from offshore to nearshore wave height accounts for the tidal state at each location. The tidal data is predictive (based on astronomical modelling) and does not account for surge effects from wind or atmospheric pressure - actual water levels can differ from predictions, particularly during storm events.

Today's Tidal Curve

Source ADMIRALTY tidal predictions, plotted as a continuous curve Methodology The tidal curve is generated by interpolating between predicted high and low water events using a cosine function, which provides a reasonable approximation of the actual tidal curve shape for semi-diurnal tides (two highs and two lows per day, typical of most UK coasts).

The tidal curve shows how water height changes through the day. It allows you to see at a glance whether the tide is currently coming in or going out, where in the cycle you are, and how quickly the water level is likely to change in the next few hours. The height axis shows metres above chart datum (the lowest astronomical tide level), which is the standard reference used on Admiralty charts. The current time is marked on the chart. Note that on the Bristol Channel, Solway Firth, and a few other UK locations, the tidal curve shape deviates significantly from a simple cosine due to local geography - treat the curve shape as an approximation in those areas.

Tide Height Calculator (Today / Tomorrow)

Source ADMIRALTY predicted high and low water events for the nearest tidal station Methodology Set an hour and minute on the scroll wheels and the tool estimates the water height at that moment. The height is interpolated between the predicted high and low waters either side of the chosen time, using the same cosine approximation of the tidal curve used elsewhere in the tool. Times are entered as local UK clock time and converted internally to match the prediction data, accounting for British Summer Time. For times in the early hours before the day's first predicted event, the preceding high or low water is reconstructed by mirroring the first tidal interval - semi-diurnal tides are close to symmetric - which keeps the estimate continuous across midnight.

This is an interpolated estimate of the predicted tide height at a chosen time, expressed in metres above chart datum (the lowest astronomical tide level). It is intended to help you picture how high or low the water is expected to be at a particular time of day. Because it is interpolated between predicted turning points rather than measured, and because predictions exclude wind and atmospheric-pressure surge, actual levels can differ - particularly in unsettled weather and in locations where the tidal curve departs from a simple cosine (such as the Bristol Channel and Solway Firth). It describes expected water height only and makes no judgement about conditions or suitability for any activity.

Tidal Flow - Phase & Status

Source Derived from the ADMIRALTY predicted high and low water times Methodology The rate at which the tide height changes is steepest in the middle of each rise or fall and flattens to nothing at high and low water. Mathematically this is the rate of change of the cosine tidal curve, which follows a sine shape - zero at the turning points and greatest at mid-tide - and is the continuous form of the traditional Rule of Twelfths. The cycle is described in four phases: Slack water (around high and low tide), Building, Peak flow (around mid-tide) and Easing. The Current pane shows the present phase and what it changes to next; the Today and Tomorrow panes show the phase for the time selected on the Tide Height Calculator, plus a day-long strip shaded pale at slack water and deep teal at peak flow, with the approximate peak-flow times marked.

This describes the rate of tidal movement - how quickly the water level is changing - not a measured current speed in knots, and not the direction of flow. The familiar pattern of strongest movement around mid-tide and slack water near high and low tide holds for most semi-enclosed UK bays and estuaries (a standing-wave tide), but the timing of actual tidal streams can differ on some open coasts and where the curve departs from a simple cosine. For genuine tidal stream rates and directions, consult an Admiralty Tidal Stream Atlas or the tidal diamonds printed on Admiralty charts. As with everything in the tool, this is a description of physical conditions only and does not indicate whether conditions are safe or suitable for any individual.

Water Quality

Water Quality (Coastal)

Source Estimated from Open-Meteo precipitation forecast (48-hour accumulated rainfall prior to each time point) combined with a location-specific pollution pressure rating (1–5 scale) Methodology Each coastal location is assigned a pollution pressure rating based on its region - broadly reflecting proximity to urban drainage, rivers, and combined sewer overflows (CSOs). Rainfall thresholds that indicate elevated contamination risk are adjusted according to this rating: a remote rural beach requires more rainfall before risk is elevated than an urban beach near a river mouth. Peak hourly rainfall intensity is factored in separately, with a 1.5× multiplier applied when peak hourly intensity exceeds 5mm/h. The 48-hour window is weighted towards more recent rainfall rather than applying a simple total.

This is a rainfall-based proxy for contamination risk, not a direct water quality measurement. It does not detect CSO discharges (sewage overflow events), which can occur even in dry weather under certain flow conditions. For real-time sewage discharge alerts, check the Surfers Against Sewage Safer Seas Service. The Bathing Water Quality classifications on Environment Agency and SEPA websites reflect historical sampling and are relevant but updated only seasonally. Heavy or prolonged rainfall near river mouths, storm drains, or in urban areas represents meaningfully increased contamination risk regardless of what this tool shows.

Water Quality - EA Monitoring Data (Inland, England Only)

Source Environment Agency Water Quality Archive API - measurements from the nearest EA sampling point within 10km, filtered to readings within the last 90 days Methodology Where available, measurements for dissolved oxygen, water temperature, pH, turbidity/clarity, algae/cyanobacteria indicators, and ammonia are retrieved and displayed. Only the most recent reading within the 90-day window is shown for each parameter.

This is scientific monitoring data, not a real-time swimmer safety system. Sampling is typically infrequent (weekly or monthly in most cases) and the readings may be several weeks old. The data is useful as background context - for instance, a historically elevated ammonia reading or a recent algae detection suggests caution - but it does not reflect current conditions. Always check for current blue-green algae alerts and local authority advisories for inland sites, regardless of what the monitoring data shows.

Sky & Celestial Information

Sunrise / Sunset

Source SunriseSunset.io API - calculated for the precise latitude and longitude of each location, for today's date

Sunrise and sunset times are used within NippyIndex to bound the hourly forecast charts - bars are only shown between sunrise and sunset, as this represents the practical window for most daytime activity. The times shown are in UK local time (GMT or BST as appropriate). Being in the water at or after sunset significantly increases risk, including reduced visibility to other water users and rescue services.

Moon - Moonrise, Moonset, Illumination, Phase

Source Calculated client-side using the SunCalc JavaScript library, based on precise location coordinates and the current date

Moon data is provided primarily for those planning early morning or evening swims where natural light levels matter. Illumination is shown as a percentage of the lunar disc that is illuminated (0% = new moon, 100% = full moon). The phase name is also shown (e.g. Waxing Crescent, Full Moon). Note that the moon also influences tidal range - spring tides (larger range, occurring around new and full moon) produce stronger tidal currents than neap tides (smaller range, around quarter moon phases).

Rainfall Forecast

Source Open-Meteo weather forecast API - hourly precipitation forecast in mm per hour

The rainfall chart shows forecast precipitation by hour (today/tomorrow views) or by day (7-day view, as daily total in mm). In addition to the obvious practical relevance of rain while you are at the water, heavy or persistent rainfall can significantly increase contamination risk - particularly near river mouths, storm drains, and estuaries - as surface runoff and CSO overflows are triggered. The water quality risk chart explicitly models this relationship. A thunderstorm warning is displayed as a prominent alert if thunderstorm conditions are forecast, as lightning presents a serious hazard to anyone in or near water.

Forecast Charts

Hourly Nippy Index chart (Today / Tomorrow)

Source Computed hourly from forecast model data using the same scoring methodology as the current score; bars shown from sunrise to sunset only

Each bar represents the Nippy Index calculated for that specific hour using the wave forecast (sea state, wave power and steepness) for that hour. Because the index is wave-based, the chart shows how the wave conditions are expected to build or ease through the day; read it alongside the wind, temperature and water quality information for the fuller picture. Forecast accuracy decreases with time, and forecast models typically update every 1–6 hours - conditions can change faster than the forecast anticipates. Bars are coloured by broad band: pale blue (low), mid blue (moderate), dark blue (elevated or above).

Shoreline Wave Height chart (Today / Tomorrow / 7-Day)

Source Open-Meteo Marine API offshore wave heights, adjusted hourly using the tide-aware Nearshore Scaling Factor and wind-wave modifier for each location

Each bar has two colour layers: the darker base represents the typical wave height (median, approximately Hs × 0.6), and the lighter upper section extends to the larger wave height (significant wave height, Hs). This gives a visual sense of both the background conditions and the larger waves you would encounter intermittently. These figures are adjusted for tidal state and beach slope at each forecast hour. As noted throughout, these are estimates - actual shoreline wave heights depend on many local factors.

Wave Force chart (Today / Tomorrow / 7-Day)

Source Calculated from hourly nearshore-adjusted wave height and effective wave period

The wave force (power) chart provides a complementary view to wave height. Because power scales with the square of wave height, relatively small increases in height can represent large increases in energy. A day that looks modest on the wave height chart but shows high bars on the force chart is typically one where swell period is long - the energy arriving is concentrated and powerful even if the wave count is low. Read both charts together for the fullest picture.

Peak Nippy Index - 7-Day chart

Source Maximum Nippy Index score computed across daylight hours (6am–6pm) for each day, with separate AM and PM peak scores indicated

The 7-day chart (shown for coastal locations) shows the highest wave-based Nippy Index expected during each day's daylight window. Separate AM (6am–noon) and PM (noon–6pm) peak scores are also shown, allowing you to see whether there is a meaningful difference between morning and afternoon wave conditions - useful for planning around forecast wind or swell changes. Inland locations do not show this chart, as they no longer carry a Nippy Index. Forecast reliability decreases meaningfully beyond 3–4 days: treat the outer days of the 7-day view as indicative rather than precise.

Water Quality Risk chart (7-Day)

Source Rainfall-based contamination risk, calculated for each day using the 48 hours of forecast precipitation prior to that day, applied against the location's pollution pressure rating

This chart shows the estimated day-by-day pattern of rainfall-related water quality risk across the week ahead. A sustained period of dry weather typically sees risk fall to low; heavy or repeated rainfall - especially in urban or estuarine areas - drives it up. This is a forecast of risk probability, not a measurement of actual contamination. For current sewage discharge alerts, always use the SAS Safer Seas Service.

Data Sources Summary

Where the data comes from

Open-Meteo (Standard commercial tier) - wave height, wave period, swell period and height, sea surface temperature, wind speed, wind direction, air temperature, weather codes, and rainfall forecasts. Open-Meteo blends ERA5 reanalysis data with global numerical weather prediction models (GFS, ECMWF). Grid resolution is approximately 25 km globally.

UK Hydrographic Office ADMIRALTY Tidal API - predicted tidal heights and times for UK tidal gauge stations. Tidal predictions are based on harmonic analysis of measured tide records.

SunriseSunset.io - sunrise, sunset, civil twilight times calculated astronomically for each location.

SunCalc (client-side library) - moon phase, moonrise, moonset, and illumination, calculated locally in the browser.

Environment Agency Water Quality Archive (England only, inland locations) - infrequent scientific monitoring data from sampling points near inland water bodies.

NippyIndex beach and inland databases - shore-facing bearings, beach slopes, pollution pressure ratings, and location coordinates for each site are maintained within the tool itself, informed by chart data and ground-truthing.

NippyIndex describes water conditions only. It is not safety advice and never implies that any location is safe, accessible or suitable for any activity. Always make your own assessment.

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