How Base 65 counts, and where every number comes from

Heating degree days

For each day: take the day's high and low, average them, and subtract the average from 65°F. A positive result is that day's heating degree days; a day that averages 65°F or warmer counts zero. This is the National Weather Service's definition (NWS: heating and cooling degree days). Base 65 keeps one decimal place.

The heating-degree-day season runs from July 1 to June 30, so the season total starts again every July 1.

Normals

"Normal" is NOAA's 1991-2020 daily normal heating degree days (base 65°F) for each reference station, from the NOAA NCEI U.S. Climate Normals (dataset "normals-daily", element DLY-HTDD-NORMAL). NOAA marks some stations' normals as "representative": their records had gaps, and NOAA estimated the normal with help from nearby stations. Normals to date are the sum of the daily normals over the same dates.

Observed weather

Daily highs and lows come from automated airport weather stations (ASOS and AWOS), as summarised by day by the Iowa Environmental Mesonet at Iowa State University. Base 65 fetches them twice a day and keeps them from July 1, 2024 on. It skips any reading that is missing or impossible (a high below the low, or outside -50 to 125°F), and any day out of line with the nearest stations that reported it: a high and low less than a fifth as far apart as theirs on a day when theirs were at least 10°F apart (the station reported only part of the day), or a daily average more than 12°F from theirs. A skipped day is read from the county's second-nearest station. Readings are usually complete by the morning after; the newest day may be revised at the next fetch.

Counties and stations

Each county uses the reference station nearest its center (the internal point in the U.S. Census Bureau 2024 Gazetteer); a day that station missed is taken from the second-nearest, and a day both missed is counted at normal and labelled "estimated". County outlines are from the Census Bureau's 2024 cartographic boundary file (1:500,000), simplified. 32 stations are airports whose own NOAA normals exist; six small Ohio airports use the NOAA normals of a nearby cooperative weather station, 2.9 to 8.0 km away (for Albany, CARPENTER 2 S in Meigs County, 7.3 km; every distance is in the table below). Thirty-two of the 38 stations are in Ohio; six just over the line serve border counties.

From your bills to a season

A bill covers the days from its first read date up to the day before its second. For each bill, Base 65 adds up that period's degree days for your county. Then it fits two numbers to your bills:

fuel used = (used anyway per day × days) + (heating use per degree day × degree days)

With two or more bills from different weather, both numbers come from the bills (least squares, neither allowed below zero). With one bill, or bills from very similar weather, the "used anyway" part is set to zero unless you enter it, and the whole bill counts as heating. The season ahead is the same formula applied to each month: observed degree days for days already past, normals for days ahead. Cost = fuel × your price + the monthly customer charge. "10% colder" adds 10% to the degree days still ahead.

The tank clock starts from the gallons in the tank after the last fill and subtracts each day's estimated use (observed weather to yesterday, normal after) until the tank reaches your alert level.

What it cannot know

• The weather at your house: a station can be up to 72 km away (shown for every county), and valleys, lakeshore and city centers differ.
• Changes to the home or its habits since the bills: new windows, a new furnace, a different thermostat setting, a new household member.
• Price changes. Use your current all-in price; supplier contracts, budget plans and taxes vary.
• Heat pumps: their efficiency falls in deep cold, so a straight line through mild-weather bills can understate the coldest months. For electric heat, take the "used anyway" figure from a spring or fall bill with no air conditioning.
• Well-insulated homes start heating below 65°F; the fit absorbs part of that, but mild-month estimates are the least certain.

When to ask someone

• Several bills in a row well above what their weather explains: have the heating system checked by a licensed heating contractor, and ask your utility to confirm the meter reads were actual, not estimated. With propane, using more gas than you expect can also mean a leak (Propane Education & Research Council): call your propane retailer.
• Budget billing, payment plans and energy assistance: your utility or fuel dealer.
• A propane or oil tank getting low: the tank clock is an estimate from your bills and the weather, not a reading. Check the tank's gauge and order before it runs low. If a propane tank runs out, your propane retailer or a qualified professional must check the system for leaks before the gas is turned back on; never turn the gas on at the tank yourself (Propane Education & Research Council, Safety Guide for Propane Users).
• You smell natural gas: do not try to find the leak, do not switch anything on or off, and do not smoke or use a flame; leave the house and, from a safe distance, call your gas utility and 911 (Public Utilities Commission of Ohio, Natural gas safety). You smell propane: get everyone out, avoid making sparks, and once safely away call your propane retailer, 911 or the fire department (Propane Education & Research Council).

Independence

Base 65 is made by WeldonPC. It is not NOAA, the National Weather Service, Iowa State University, the Census Bureau or any utility, and none of them reviews or endorses it. gallivantohio.com is this site's address only: Base 65 is not affiliated with GALLIVANT (Uncrate LLC and ZombieCorp LLC) or with Gallivant LLC, and it publishes no travel information.