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Hydration and Heat Planner

Prepare for outdoor activity using temperature, humidity, activity level, and general hydration guidance.

Plan for the Conditions

Free · No account needed · General guidance, not a medical protocol

Show your working

How this tool reaches its answer.

A tool that returns a number owes you three things: what it looked at, what it took for granted about you, and where its answer stops being reliable. All three are below.

What it measures

The inputs

  • Expected temperature and humidity, because humidity decides how well sweating cools you. You can type them in or let an optional lookup fill them from the forecast.
  • Planned duration and intensity of the activity.
  • Whether the session is in direct sun, and whether you are acclimatised to the conditions.

What it assumes

Taken for granted

  • You are a healthy adult undertaking recreational activity, not competing in extreme heat.
  • Drinking to thirst is the strategy the 2017 consensus on exercise-associated hyponatremia recommends, and over-drinking is the failure mode it describes.
  • Sodium replacement becomes relevant during prolonged heavy sweating rather than on every session.

Where it stops

Known limits

  • Sweat rate varies widely between people and this planner cannot measure yours. Weighing yourself before and after one session is how you get your own figure.
  • It does not account for medication, pregnancy, or a condition affecting fluid balance or heat tolerance.
  • It states no fluid volume, in a session or in a day. Drinking well beyond thirst can be dangerous, and fixed fluid recommendations are what the consensus warns against.
  • The heat index is measured in the shade, at rest. It knows nothing about your pace, your clothing, or your sleep.
Planner

Plan the session around the conditions

Seven questions about the weather and the session you had in mind. The answer is a heat index band, what to change about the session, a fluid strategy, and the signs that mean stop. It never contains a volume to drink.

The heat index is computed in this browser. Nothing you type here leaves the page, and the optional forecast lookup sends a rounded latitude and longitude and nothing else.

Fill the two figures from the forecast, if you want

Your browser asks you for permission, then sends a latitude and longitude rounded to about a kilometre to Open-Meteo, which needs no key and no account. It receives nothing else: not the session, not how hard you plan to work, not one word you typed about yourself. The planner works the same whether you use it or type the figures in.

Open-Meteo forecast API

The conditions

Take these from the forecast for the hour you plan to be out.

The forecast figure for when you will be outside, not the daily high.

In Celsius or Fahrenheit?

Humidity decides how much of your sweat evaporates, which is the part that cools you.

per cent
How exposed is the route?

Heat index figures describe shade. Open sun raises what the same air feels like.

The session you had in mind

Answer for the session as planned. The planner may tell you to change it.

minutes
How hard?
How used to the heat you are

Acclimatisation is gained over days and lost over days. The first week back counts as a first week.

Which of these describes you?

What heat does to a session

Heat raises the cost of the same work. Humidity compounds it, because sweat that does not evaporate does not cool you. It only costs you fluid. A cool day at ninety per cent humidity can be harder than a warm dry one.

The planner turns the forecast into a plainer question: should today’s session be shorter, easier, earlier, or moved indoors?

Why there is no number here

Ask most hydration calculators how much to drink and they will tell you. The 2017 consensus on exercise-associated hyponatremia will not, and says why: losses through sweat and urine vary so much between people that recommending fixed ranges of fluid intake is not appropriate. Its recommendation is to drink when you are thirsty, and the condition it is warning about comes from drinking too much rather than too little.

So this planner names a strategy instead of a volume. Carry fluid once the session runs long or the conditions turn. Drink to thirst. Do not drink past what you lost, which the NATA position statement puts plainly. If you want a personal figure, weigh yourself before and after one session and work out your own sweat rate, which takes an hour and beats any average.

Before, during, after

Arrive already hydrated rather than trying to catch up mid-session. For anything under about an hour in mild conditions, water and thirst are usually the whole plan. For longer or hotter work, carry fluid so that drinking to thirst is possible, and include sodium once you have been sweating heavily for an extended period. Sodium blunts the fall in blood sodium; it does not license drinking more.

Afterwards, replace steadily over the following hours. Pale urine is a reasonable check.

Warning signs to stop

Stop and get out of the heat if you develop tiredness, dizziness, nausea, cramp in the arms, legs or stomach, or a headache that comes on during the session. Those are the NHS signs of heat exhaustion.

Call the emergency services for confusion, slurred speech, hot skin that is not sweating, a very high temperature, a seizure, or loss of consciousness. Heat illness is a medical emergency and it can progress quickly. Call for help rather than pushing on.

One caution that reads like the others and is not: nausea, headache and confusion late in a long effort you have been drinking steadily through can be over-drinking. Drinking more makes that worse.

Sources

What this page draws on

  1. The Heat Index Equation, the Rothfusz regression (NOAA Weather Prediction Center)
  2. NWS Heat Index chart and classifications
  3. NWS heat index, the full sunshine adjustment
  4. Heat index and its limits as a measure of heat stress (OSHA)
  5. Heat, protecting new workers and the Rule of 20 Percent (OSHA)
  6. Heat-related illnesses and first aid (OSHA)
  7. Heat exhaustion and heatstroke (NHS)
  8. Exercise-Associated Hyponatremia: 2017 Update
  9. NATA Position Statement: Fluid Replacement for the Physically Active (McDermott et al., 2017)
  10. Open-Meteo forecast API

Endurvity provides educational information, not medical advice. Always consult a qualified professional before changing your training, nutrition, or medical care.

Know the method before you trust the number.

Every Endurvity tool publishes its assumptions. If a health tool will not show you its working, that is the answer.

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