HOTTER, MORE HUMID, AND MORE ACTIVE MEANS MORE CANS. UV 3 AND UP MEANS PACK SUNSCREEN. THAT'S THE WHOLE APP. EVERYTHING BELOW IS THE MATH, ONLY KEEP READING IF YOU WANT IT.
YOU TELL THE MACHINE WHERE YOU ARE AND HOW LONG YOU'RE GONNA BE OUTSIDE. IT TELLS YOU HOW MANY VENDING MACHINE CANS YOU'RE GONNA SWEAT OUT, PLUS HOW MUCH SUNSCREEN YOU'LL BURN THROUGH. BIG SHOUTOUT TO THE CIGGIE INDEX FOR THE IDEA. THAT ONE TURNS AIR POLLUTION INTO CIGARETTES.
THE WEATHER COMES STRAIGHT FROM OPEN-METEO: TEMP, HUMIDITY, FEELS-LIKE TEMP, AND UV. HUMIDITY IS THE SNEAKY PART. WHEN IT'S HUMID YOUR SWEAT CAN'T EVAPORATE, SO 33°C AT 64% HUMIDITY FEELS MORE LIKE 39°C. THAT'S THE NUMBER WE USE.
SWEAT (L/HR) = (FEELS-LIKE °C − 20) × 0.065 × ACTIVITY
ACTIVITY IS 0.5× IF YOU'RE JUST CHILLING, 1× IF YOU'RE WALKING AROUND, 1.6× IF YOU'RE REALLY MOVING. WE CAP IT AT 2 LITERS AN HOUR BECAUSE THAT'S ABOUT THE MOST A BODY CAN PUMP OUT. MULTIPLY BY YOUR HOURS, DIVIDE BY A 500ML TALL CAN (ロング缶), DONE.
FOR REFERENCE, A FULL 24 HOURS OUTSIDE ON A HOT TOKYO SUMMER DAY USUALLY LANDS SOMEWHERE AROUND 20–40 CANS. AN OVERCAST WINTER DAY MIGHT NOT EVEN HIT 5.
WE MADE UP THE 0.065 NUMBER, BUT WE TUNED IT SO THE RESULTS LAND WHERE THE REAL RESEARCH SAYS THEY SHOULD. US WORKPLACE HEAT-SAFETY RESEARCH (THE AGENCIES NIOSH AND OSHA) SAYS PEOPLE WORKING IN THE HEAT SWEAT AROUND 1–1.5 L/HR. SPORTS SCIENCE RESEARCHERS (ACSM, SAWKA) PUT THE NORMAL RANGE AT 0.5–2.0 L/HR. SAME ENERGY AS THE CIGGIE INDEX'S PM2.5 ÷ 22 RULE, A ROUGH BUT HONEST SHORTCUT.
BURN TIME (MIN) ≈ 200 ÷ UV INDEX
THAT'S THE CLASSIC RULE OF THUMB FOR FAIR SKIN WITH NO PROTECTION. UNDER UV 3 THE WHO SAYS YOU'RE FINE, SO THE MACHINE GIVES YOU NOTHING. UV 3 AND UP, DERMATOLOGISTS SAY REAPPLY EVERY 2 HOURS, SO YOU GET ONE TUBE FOR EVERY 2 HOURS YOU'RE OUT THERE.
FOR REFERENCE, UV 1–2 IS TYPICAL BEFORE 8AM OR AFTER 5PM, UV 3–5 IS A MILD OR OVERCAST DAY, AND UV 8+ SHOWS UP AROUND MIDDAY IN JAPANESE SUMMER.
THIS ASSUMES YOU KEEP THE SAME PACE THE WHOLE TIME, AND IT DOESN'T KNOW YOUR BODY SIZE, HOW USED TO THE HEAT YOU ARE, WHAT YOU'RE WEARING, OR WHETHER YOU FOUND SHADE. IT ALSO GRABS THE WEATHER RIGHT NOW AND PRETENDS IT STAYS THAT WAY, EVEN THOUGH NOON UV DOESN'T LAST ALL DAY. REAL SWEAT RATES SWING BY ±50% FROM PERSON TO PERSON. NONE OF THIS IS MEDICAL ADVICE, JUST A VENDING MACHINE GUESSING.
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