How soil temperature is measured and modelled

Every soil temperature figure you see online for your postcode is modelled. There is no network of probes in suburban lawns. Understanding what that means is the difference between using the number sensibly and over-trusting it.

What is actually measured

Real soil temperature observations come from a relatively small number of weather stations, where thermometers sit at standard depths — commonly 5cm, 10cm, 20cm, 50cm and 100cm — under a defined surface, usually bare soil or short grass.

These are good measurements at those specific points. There are just not very many of them, and yours is not one of them.

How the figure for your postcode is produced

Weather models solve for the land surface as well as the atmosphere. A land surface scheme tracks the energy balance at the ground — incoming radiation, outgoing longwave, sensible and latent heat, and conduction downwards — and steps soil temperature and moisture forward at several depths across a grid.

That grid is coarse: a cell is typically a few kilometres across in the better models. The number you get for your postcode is an interpolated value from the cell containing it.

So it carries three sources of error:

The model’s own error. The land surface scheme is a simplification, and it inherits any error in the forecast driving it.

Spatial averaging. One value represents a few square kilometres containing roads, buildings, woodland, water and your lawn.

Surface assumptions. The model assumes a generic land surface for the cell. Your lawn has its own aspect, shading, drainage, thatch layer and soil texture.

Which depth to use, and why

Mowdar reports 6cm because that is germination depth: roughly where sown seed sits and where new roots start working. Most published germination thresholds are quoted at 5–10cm, so the figure and the threshold refer to the same thing.

Depth matters more than people expect:

• The surface (0cm) swings wildly — it can be 15°C above air temperature in full sun and below freezing the same night. Almost useless for decisions.
• 5–10cm is the seeding and rooting zone. Damped, lagging air temperature by a couple of days, and the right depth for germination questions.
• 20cm and deeper lags by weeks and is essentially the season’s average. It tells you about the trend, not this week.

What makes your lawn differ from the model

Fairly predictable things, and they mostly push in one direction:

• Aspect. A south-facing slope runs warmer than the cell average in spring; a north-facing one lags, sometimes by a week or more.
• Shade. Under a tree or on the north side of a fence, soil warms much later.
• Drainage. Wet soil warms slowly — water has a high heat capacity and evaporation removes energy. A poorly drained lawn can sit several degrees behind a well-drained one nearby.
• Thatch. A thick thatch layer insulates, slowing spring warming.
• Hard surfaces. A lawn ringed by paving and walls runs warmer, as does anything in a town centre.

The practical consequence: treat the modelled figure as a good estimate for your area, and expect your shaded, badly drained back corner to be behind it.

Checking it yourself

A soil thermometer costs very little and settles the argument for your specific lawn.

1. Push the probe to about 6cm — most have a depth marking.
2. Take it at the same time each day. Mid-morning is a reasonable proxy for the daily mean at that depth; early afternoon reads warm.
3. Take it in the part of the lawn you actually care about, and take a second reading in the worst corner.
4. Compare with the modelled figure over a week or two. Once you know your lawn runs, say, 1.5°C behind, you can apply that offset from then on.

The honest summary

The modelled figure is genuinely useful and far better than guessing from air temperature or the calendar. It is not a reading from your lawn, and anyone presenting it as one is overselling it.

See the modelled figure and trend for your area.