Details from the National Weather Service
* WHAT...Temperatures as low as 35 will result in frost formation. * WHERE...Portions of northern Indiana and northwest Ohio. * WHEN...From 1 AM EDT /midnight CDT/ to 9 AM EDT /8 AM CDT/ Monday. * IMPACTS...Frost could harm sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered. Take steps now to protect tender plants from the cold.
Our physics model Experimental preview for tonight
Our own model of the heat budget at the station mast: the grass losing heat to the sky, clouds sending some back, the breeze stirring warmer air down, dew and frost releasing heat, warmth rising from the soil. Its constants were fitted to the station's own record. It runs beside our forecast and does not change it. Sunset 6:59 PM, sunrise 7:58 AM.
The physics expects 33.1° at the station by 7:59 AM, 2.5° colder than our forecast. Our low moved a quarter of the way toward it: 35.0°. Starts from our forecast for sunset (62.6°, dew point 35.4°).
The next two days, physics only from sunrise Sun 7:57 AM
| Period | Physics says | At about | Our forecast | Physics started from |
|---|---|---|---|---|
| Sun Oct 18 high | 64.0° | Sun 4 PM | 64.8° | the station's reading |
| Sun Oct 18 night low | 33.8° | Mon 7 AM | 35.6° | where the half before ended |
| Mon Oct 19 high | 67.8° | Mon 3 PM | 68.1° | where the half before ended |
| Mon Oct 19 night low | 35.9° | Tue 8 AM | 38.2° | where the half before ended |
The physics runs on its own: the first half starts from the station's reading at sunrise (31.8°), and each half after that starts where the one before ended, day into night into day, with clouds, wind and the air mass from the ECMWF and ICON forecasts. Violet is the physics, blue dashed our forecast, orange the station. Not blended into anything.
Tonight, sunset to sunrise preview, before sunset
What the physics model expects tonight, minute by minute, next to what actually happens. Before sunset this is a preview, started from our forecast for sunset. At sunset the model starts again from the station's own reading, and from then on the station's measured temperature and the grass probe are drawn on the chart as the night goes on, so you can watch the physics and the real night side by side.
Air is the temperature at the height the station measures (5 feet). The grass surface cools faster and further than the air above it on a clear night; that is where frost forms. The dew point is how low the air can cool before dew (or frost, below 32°) forms. The thin line marks 32°.
How tonight's run got its bearings
Before it runs, the model takes stock of the station, the ground and the forecasts. These are the values this run started from, and what it is being fed through the night.
| What it starts from | Value | Source |
|---|---|---|
| Air at the start | 62.6° | Starts from our forecast for sunset |
| Dew point at the start | 35.4° | the same reading |
| Grass surface at the start | 56.7° | the air reading, less the usual sunset gap to the grass |
| Soil below (warmth from underneath) | 53.4° | the 4 and 8 inch soil probes |
| Air pressure | 1,001 hPa | the station's barometer, at the station's height |
| Snow on the ground | none | From the forecast models (no recent report) |
What the forecasts feed it, every two hours
| Time | Air 2,000 ft up, brought to the ground | Cloud: low / mid / high | Wind (models) | Rain in the next 2 h |
|---|---|---|---|---|
| 6:59 PM | 61.6° | 0% / 0% / 0% | 4.8 mph | - |
| 8:59 PM | 62.1° | 0% / 0% / 0% | 4.0 mph | - |
| 10:59 PM | 62.8° | 0% / 0% / 0% | 4.3 mph | - |
| 12:59 AM | 63.4° | 0% / 0% / 0% | 4.5 mph | - |
| 2:59 AM | 63.4° | 0% / 0% / 0% | 4.0 mph | - |
| 4:59 AM | 62.2° | 0% / 0% / 0% | 3.3 mph | - |
| 6:59 AM | 61.9° | 0% / 0% / 0% | 3.0 mph | - |
Air 2,000 ft up: the temperature about 2,000 feet up in the ECMWF and ICON forecasts, worked out as it would be if that air were stirred down to the ground (it warms as it sinks; along the saturated rate when it rains). The breeze pulls the night toward it; on a calm, clear night the ground lets go of it and cools on its own. Cloud sends heat back down, low cloud the most. Rain cools the air toward its wet-bulb.
Night by night the run made at sunset
The first night finishes at sunrise; from then on every night is listed here, scored against the station. This is the live record for the lab's next test.
Today, sunrise to sunset from the sunrise reading
Violet is the day model, orange the station. Below it, how deep the sun mixes the air. Starts from the station's reading at sunrise (31.8°); ground moisture from ECMWF topsoil moisture. "Halfway to physics" is our forecast high moved halfway toward the physics high (plus a small fixed offset), using the version frozen for its test (that version's high: 64.5°). The afternoon update restarts the day model at 11 AM, noon and 1 PM with the sunshine the station actually measured.
How it works
At night. Every 5 minutes the model works out how much heat the grass loses to the sky and how much comes back (clear air sends little, low cloud a lot), how much the breeze mixes warmer air down, the heat released when dew or frost forms, and the warmth rising from the soil (measured live by the ground sensor). Clouds, wind and the air mass come from the ECMWF and ICON forecasts.
By day. The sun heats the ground; part of that evaporates water (following a crop calendar: bare fields in spring, a transpiring canopy in midsummer, drying crops in fall), part soaks into the soil, and the rest heats a layer of air that deepens through the morning. The sunshine is the forecast models', calibrated against the station's solar sensor.
In rain and on snow. In rain the air below the cloud is saturated and the ground runs a few degrees colder than on a dry night; snow insulates the ground from the soil's warmth and cannot warm past 32°. Both are built in, and snow depth comes from the snow estimate.
Fitted, not guessed. The night model's 13 constants were fitted to the station's minute-by-minute record on 474 nights in 2024 and 2025; the day model's 15 on 455 days.
What the tests found.
- Night: moving our low a quarter of the way toward the physics cut the average miss on 149 dry test nights from 1.52° to 1.38° Proven. Now being checked on new nights as they happen On trial.
- Day: with the crop calendar, the physics' average miss on months set aside for grading fell from 1.79° to 1.59°, better than our forecast alone (1.71°) Proven. Blending it with our high is on trial On trial.
- Rain: the wet-weather version brought the physics from 2.8° to 1.9° off on 30 rainy test nights, but added nothing to our low there Not proven.
- Snow: on last winter's nights with 2 inches or more, the warm bias fell from almost 6° to 2°, but the average miss is still 4.5°, behind the models' 2.8° Better, not good.
Not yet. Fog, clouds of its own and storms. Everything is in the Lab Notebook.