Why do farmers spray water on their fields on frosty nights?
― Not to stop it freezing, but to make it freeze
In early spring, on nights when frost is forecast, sprinklers sometimes run all night long over orchards and tea fields. Spraying water on a cold night sounds like a recipe for even more freezing. Yet in practice, this water protects the buds — and it does so precisely because it's allowed to freeze.
It's a clear, windless night in early spring. The evening forecast has just warned: "Tomorrow morning will be cold, with a risk of late frost."
That night, the sprinklers in the orchard start turning. The next morning, the branches — and even the swelling buds — are completely wrapped in clear ice.
To look at, these branches seem the most frozen and damaged of all. But once the ice melts, those buds are alive. It's the buds in the neighbouring field, the one that wasn't sprayed, that have turned brown and died.
Why "freezing" protects the buds: two reasons
When water turns to ice, it gives off heat to its surroundings. As long as the sprayed water keeps freezing on the bud's surface, that heat keeps welling up. This heat warms the bud back up against the cold trying to take hold.
As long as unmelted ice and unfrozen water sit side by side, the temperature can't move from 0°C. The bud gets pinned at 0°C. Even if the air drops to −4°C, the bud inside the ice stays at 0°C.
What actually decides whether a bud dies isn't "did it freeze," but "how low did it get." For many fruit trees, a bud that has begun to swell is said to be damaged somewhere around −2°C to −3°C. If it's held at 0°C, it never crosses that line. Let's look at this step by step.
What does it mean that freezing releases heat?
When you cool water down, it freezes at 0°C — but while it does, the thermometer sits at 0°C for a while without moving. There's a stretch of time when you're cooling it, yet the temperature isn't dropping. That "missing cooling power" is really the heat that water gives up as it turns to ice.
In terms of quantity, freezing one gram of water is said to release about 334 joules of heat. That's as much heat as it takes to warm that same gram of water by roughly 80°C — hardly a small amount. Keep spraying water over an entire field, and this heat keeps being released across the whole field. Take a look at Figure 1: it shows the difference between what happens on the same night, on the left and on the right.
The mechanism behind staying locked at 0°C
If freezing only released heat, you'd expect it to eventually run out and stop. But a mix of ice and water has a special property: its temperature can't move. It's the same reason a drink with ice in it stays cold for as long as the ice lasts.
On the bud's surface, fresh water keeps falling and freezing, over and over. As long as there's still unfrozen water present, the temperature can't drop below 0°C. Whether the air is at −4°C or −5°C, the inside of the ice shell stays at 0°C regardless. For the bud, it's like sitting in a small room cut off from the cold outside.
The single most important rule with this method is: don't stop spraying until the ice has fully melted in the morning. Stop, and no fresh water freezes, so no more heat is released. Worse, as the remaining ice then melts, it draws heat back out of its surroundings. Cut it off halfway, and the bud can end up colder than if it had never been sprayed at all.
This method only works on clear, windless nights. On windy days, the sprayed water gets blown about and evaporates more easily. Evaporation draws heat away — the opposite effect — and that heat loss can outweigh the heat gained from freezing. That's why growers say "don't do this on a windy night": it comes down to that balance.
Summary
Spraying water on a frosty night isn't a trick to stop things freezing. It's a way of letting water freeze in the bud's place, and putting to full use both the heat that releases and the fact that a mix of ice and water stays locked at 0°C. What looked like the very cause of the cold turns out to be the bud's protector.
Water leaves heat behind as it freezes.
That heat is what holds the bud at 0°C.
For more on "freezing" seen from a different angle, see Why do needle ice columns grow up out of the soil? and Why do icicles form on the "slightly milder" day, not the coldest one?. For why the ground cools so much on a clear night, see Why is the morning grass wet when it hasn't even rained?.
- Fill a cup generously with ice, then add water until it comes about halfway up the ice. Stir occasionally, with a thermometer left in the cup.
- Leave it in a warm room and read the temperature every 10 minutes, writing it down. Check that the reading barely moves while ice remains.
- The moment the very last piece of ice melts, the temperature starts to climb. That's the point where the "ice-and-water coexistence" ends. In the field, the bud stays inside that unmoving stretch of time until morning.
No thermometer? Just dip a finger in the ice water and compare how it feels while ice remains versus after it's gone — you'll still notice the difference. Don't leave your finger in too long.
Want to go deeper? — Terms, formulas, and how this connects to textbooksClearly labelled by level, from lower-secondary science to university-level specialist courses
- Lower sec.Covered in lower-secondary school science
- Upper sec.Covered in upper-secondary "Basic Physics / Basic Chemistry"
- Upper sec.+Advanced upper-secondary content, or textbook sidebar material
- UniversityNot covered at school — university-level specialist content (thermodynamics, plant physiology)
- ResearchNot yet settled even at university level — an active area of current research
Lower sec.Terminology: this phenomenon has a name
- Sprinkler freeze protection (overhead irrigation frost protection): a frost-protection method in which water is continually sprayed onto crops on nights when frost is expected, using the heat released on freezing to hold the temperature at 0°C. Used in orchards and tea fields.
- Latent heat of fusion: the heat released to the surroundings when a liquid turns into a solid. Because it moves in and out without changing the temperature, it's sometimes called "hidden heat."
- Radiative cooling: on clear, windless nights, the ground and leaves keep radiating heat away into the sky, cooling below the air temperature. This is usually what's happening on a frosty night.
Lower sec.Upper sec.Checking with a formula: how many watts of heating does spraying provide?
Suppose 3 millimetres' worth of water is sprayed per square metre of field per hour. Assume all of it freezes, and convert the heat released into an amount per second. The unit is the watt, the symbol for the amount of heat released per second.
| Symbol: heat released when 1 gram of water freezes | said to be 334 joules |
| Water sprayed per square metre per hour | 3 litres |
| Weight of 1 litre of water | 1000 grams |
| Seconds in 1 hour | 3600 seconds |
| Weight of water sprayed in 1 hour | 3 × 1000 ＝ 3000 |
| Heat released as a result (joules) | 3000 × 334 ＝ 1002000 |
| Converted to per second (watts) | 1002000 ÷ 3600 ≒ 278 |
About 278 watts per square metre. The heat that the ground gives up to the sky on a clear night is said to be somewhere from a few tens of watts to around 100 watts per square metre, so the sprayed water works out to more heating than that loss. Think of it as placing a small electric heater over every square metre of the field.
Upper sec.Upper sec.+A tug-of-war between warming heat and heat loss
Upper sec.Basic Physics teaches that heat added during a change of state doesn't go into raising the temperature. Sprinkler freeze protection is exactly this "unused heat," put to use from the releasing side. Looked at in reverse, you also learn that melting ice draws in exactly as much heat again.
Upper sec.+In an actual field, while latent heat of fusion is coming in, latent heat of evaporation is going out. The heat that evaporating 1 gram of water draws away is said to be about seven times the heat released when it freezes. Even a small fraction of the sprayed water evaporating is enough to flip the balance. This is why spraying is considered risky on a dry, windy night — it comes down to the size of that ratio.
UniversitySupercooling and whether an ice nucleus forms
Pure water can stay unfrozen below 0°C. This is called supercooling, and freezing needs something to act as an ice nucleus to get started. Plant sap also supercools, but once ice begins forming outside the cells, water is drawn out of the cells, dehydrating them — this is thought to be the main cause of freeze damage. What sprinkler freeze protection actually guards against, strictly speaking, is not "freezing" itself but "cooling low enough for ice to form outside the cells." In some plants, whether ice-nucleating bacteria are present on the leaf surface is known to affect how readily freeze damage occurs.
ResearchWhat isn't fully understood yet
- The optimal spray rate. How much water is enough depends on the combination of temperature, wind speed, and humidity. Growers rely heavily on experience, and research is still working out numerical guidelines for different conditions.
- The mechanism of cell damage. How much freeze damage comes from dehydration versus ice crystals rupturing membranes varies by plant and by conditions, and is still debated.
- Alternatives that don't use water. Fans, covers, and chemical treatments have all been tried as frost-protection methods that don't use water, but none is as reliable as spraying, and this remains a challenge in areas short on water.
In other words, this article too only explains things as far as they're currently understood. For what to actually do in your own field, follow the advice of your local agricultural extension service.
How this connects to textbooks, by level
| Level | Subject/unit | Where in this article |
|---|---|---|
| Lower sec. | Science — states of matter and temperature | How a mix of ice and water stays locked at 0°C |
| Upper sec. | Basic Physics — heat and temperature / Basic Chemistry — states of matter | The latent-heat calculation and the 278-watt estimate |
| Upper sec.+ | Physics — heat balance | The tug-of-war with heat lost to evaporation |
| University | Thermodynamics / plant physiology | Supercooling and ice forming outside the cell |
| Research | Agricultural meteorology | The optimal spray rate and the mechanism of freeze damage |
| ― | Everyday connections | Why ice water stays cold, and how to read spring frost news reports |
- Japan Meteorological Agency, "Past Weather Data Search" (気象庁「過去の気象データ検索」) (records of minimum temperatures and frost)
- Ministry of Agriculture, Forestry and Fisheries, "Agricultural Production, Environment and Technology" (農林水産省「農業生産と環境・技術」) (materials on countermeasures against weather-related damage)
- Japanese Society of Agricultural Meteorology (日本農業気象学会) (ed.), The Science of Agricultural Meteorology (『農業気象の科学』) (chapter on frost-protection methods and heat balance)
- Levitt, J. "Responses of Plants to Environmental Stresses" (chapter on cold stress and freeze damage)
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. Whether actual frost-protection measures are appropriate depends on equipment and local conditions. Follow the advice of your local authority or agricultural extension service before carrying them out.