Why Do Bridges Freeze Before Other Roads?
― A bridge has no "warmth of the ground" beneath its feet
On a winter morning, an ordinary road may be merely wet while the bridge is covered in ice. The bridge isn't in an especially cold place. The difference is that there is no ground under it. It loses heat from above and below, and nothing makes up the loss.
It is a clear winter morning and you leave home on your bike. The road is a little wet and gleams dark, but your tyres grip the surface firmly.
Then you reach the bridge over the river, and suddenly your front wheel slides sideways. The surface looked exactly like the wet road you were just riding on.
The roadside sign saying "Bridge may be icy" exists for moments like this. Same morning, same air temperature. So why does only the bridge freeze?
There are just two reasons a bridge freezes first
An ordinary road cools only from its top surface. A bridge hangs in the air like a board, so its underside is exposed to cold air too. It is like having twice as much surface to cool from.
Deep ground holds heat stored during the day and over the summer, so even on a winter night it stays warmer than the air. An ordinary road gets that heat from below. A bridge cannot.
In short, a bridge is a place where "a lot of heat goes out and none comes in." Let's take it step by step.
On a clear night, the road throws its heat into the sky
Clear, windless nights get especially cold. The ground and the road surface keep sending heat toward the night sky as invisible light (infrared). When there are clouds, they send some heat back. A clear night sky returns almost nothing. This is called radiative cooling.
Look at Figure 1. On the bridge at left, heat escapes to the sky through the upward arrows, and also through the downward arrows. Below the bridge is a river or valley, where cold air flows.
On the ordinary road at right, heat also escapes upward. But directly under the road lies a thick layer of soil. It holds heat stored up through autumn, and sends it up to the road surface, shown by the upward arrows.
Compare the cooling: the bridge is about 2.5 times faster
So how big is the difference? Suppose a 20 cm thick concrete bridge and a road on the ground are both at 5℃ at sunset. Use the slider in Figure 2 to move the time after sunset.
Both start at 5℃, but the bridge falls below 0℃ around midnight and reaches about −3℃ by morning. The ground-level road is still 1 to 2℃ at the same hour. One is icy and the other is merely wet. The bike scene at the start comes down to exactly this difference.
The temperature in a weather forecast is the air temperature about 1.5 metres above the ground. On a clear night the road surface throws heat into the sky, so it can be several degrees colder than the air. Even with a forecast low of 3℃, you can't relax on a bridge.
Elevated roads and pedestrian footbridges are open underneath, so they behave like bridges. Tunnel entrances and roads shaded by buildings or hills can't warm up in the midday sun, so ice tends to linger there well into the morning.
So what should we do?
- On a cold, clear, windless morning, cross a bridge assuming it is icyEven if it looks merely wet, you can't tell the difference
- Slow down before the bridgeDon't brake or swerve suddenly on the bridge itself
- When walking, take short steps and land with your whole foot flatWear gloves and keep your hands out of your pockets
A thin layer of ice is clear, so the dark road shows through it. It looks the same as a wet road. If someone falls and hits their head hard, seems confused, or can't stand up, call 119 (emergency services in Japan) without hesitation.
Summary
An ordinary road gets through the night with heat shared from the ground beneath it. A bridge has no such warmth, and it also sheds heat from both top and bottom. That is why, on the same morning, only the bridge drops below 0℃ first.
The ground is the road's hot-water bottle.
A bridge doesn't have one.
For why frozen roads are so slippery, see "Why Are Icy Roads So Slippery?". For how ice grows up out of the soil, see "Why Do Frost Columns Grow Out of the Soil?".
- On a clear, cold morning, compare a nearby footbridge or small bridge with the ground-level road just before it.
- Compare how frost forms on things that are open underneath, such as a car roof, a bridge railing or a bicycle saddle, and on things close to the ground.
- On a night likely to be cold, put two identical boards in your garden or on your balcony. Lay one directly on the ground and prop the other up on blocks. Compare the frost the next morning.
Observe icy roads only from a pavement where you can stand safely, and never step into the roadway. You can see with your own eyes that things open underneath collect thicker frost.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Physics" and "Basic Earth Science"
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity-level courses not taught in high school (heat transfer engineering, meteorology)
- ResearchNot yet taught as settled fact even at university; researchers are still investigating it
Junior highTerms: this phenomenon has names
- Radiative cooling: an object cools by giving off infrared light and losing heat. It is strongest on clear, windless nights.
- Heat capacity: the amount of heat needed to raise or lower an object's temperature by 1℃. Heavier things that store more heat have a larger value and cool more slowly.
- Ground heat (heat flow in the ground): heat that soaked into the ground during the day or summer and returns to the surface at night or in winter.
Junior highHigh schoolChecking with a formula: how much does a bridge cool in one night?
Treat the road as a single slab that stores heat, and write down how outgoing heat relates to the temperature drop. The bridge and the ground-level road are compared by changing only the net heat they lose.
| In symbols | ΔT ＝ ( q × t ) ÷ ( ρ × c × d ) |
| In words | Temperature drop ＝ net rate of heat loss × time ÷ heat capacity of 1 square metre of slab |
| Where it comes from | Conservation of energy. The heat that leaves the slab equals its heat capacity times its temperature drop |
| Symbol | Meaning and unit |
| ΔT | Temperature drop (℃) |
| q | Net rate of heat loss per square metre (watts per square metre) |
| t | Time (seconds) |
| ρ, c, d | Concrete density (kilograms per cubic metre), specific heat (joules per kilogram per ℃), thickness (metres) |
| Density of concrete | About 2400 kg per cubic metre |
| Specific heat of concrete | About 880 J per kg per ℃ |
| Thickness of the bridge deck | Assumed 0.2 m (20 cm) |
| Heat escaping from the top surface on a clear night | Assumed about 70 W per square metre (said to be tens to 100 W) |
| Heat escaping from the bridge's underside | Assumed about 30 W → bridge total about 100 W |
| Heat the ground-level road receives from the soil below | Assumed about 30 W → net about 40 W |
| Mass of 1 square metre of slab | 0.2 × 2400 ＝ 480 kg |
| Heat capacity of 1 square metre of slab | 480 × 880 ＝ 422400 J/℃ |
| Heat the bridge loses in 1 hour | 100 × 3600 ＝ 360000 J |
| Bridge temperature drop per hour | 360000 ÷ 422400 ≒ 0.85 ℃ |
| Heat the ground-level road loses in 1 hour | 40 × 3600 ＝ 144000 J |
| Ground-level road temperature drop per hour | 144000 ÷ 422400 ≒ 0.34 ℃ |
| Ratio of cooling speeds | 0.85 ÷ 0.34 ＝ 2.5 times |
| Bridge temperature drop in 10 hours | 0.85 × 10 ＝ 8.5 ℃ |
| Ground-level road temperature drop in 10 hours | 0.34 × 10 ＝ 3.4 ℃ |
| Time for a bridge at 5℃ at sunset to reach 0℃ | 5 ÷ 0.85 ≒ 5.9 hours |
A bridge that is 5℃ at sunset drops below 0℃ after about 6 hours, and by morning, 10 hours later, it is about −3.5℃. The ground-level road is about 1.6℃ on the same morning and does not freeze. These numbers are estimates based on assumptions, but they show that whether there is ground underneath is enough to decide between freezing and not freezing.
High schoolHigh school+Thinking of heat flow as a "balance"
High schoolIn Basic Physics, you learn that the heat an object gains is "mass × specific heat × temperature change." The formula in this article uses the same relation on the "losing" side. The road's temperature is set by the difference between outgoing and incoming heat, that is, by the heat balance.
High school+The heat given off by radiation is proportional to the fourth power of the object's absolute temperature (the Stefan–Boltzmann law). Seen from the ground, a clear night sky behaves like an object tens of degrees below zero, so the road keeps losing heat on balance. When clouds are present they send heat back, and the cooling weakens.
UniversitySurface energy balance and the heat conduction equation
In meteorology, the heat entering and leaving the ground surface is handled with a surface energy balance equation, split into net radiation, sensible heat, latent heat and ground heat flux. For a ground-level road, the ground heat flux cancels part of the night-time radiative cooling. The temperature distribution inside the ground is described by the heat conduction equation. Surface temperature swings with day and night and with the seasons, but the deeper you go, the smaller the swing and the later it arrives. That is why deep ground stays warmer than the surface even on a winter night. A bridge deck is treated as a slab of finite thickness whose lower face touches air, solved with heat escaping from both faces.
📖 Derivations and further reading: Heat equation (Wikipedia, in Japanese) ／ Radiative cooling (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- Predicting which bridge freezes, and when Road temperature changes a great deal with a bridge's orientation, height, humid air from the river and how shade falls. Systems that predict road icing several hours ahead are still being improved in many regions.
- Frost or ice? Whether the water on a road is frost deposited directly from water vapour in the air, or rain or condensation that has frozen, is thought to change how slippery it is. The exact conditions at the boundary are not yet well understood.
- How well anti-icing measures work Many methods are being tried, such as systems that carry heat from underground to the road surface and pavements that resist freezing. The balance of cost and benefit is said to need checking site by site.
In other words, this article too is "an explanation within what is known today." The numbers in ② in particular are simplified estimates meant to show how the mechanism works.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Weather and its changes" | Radiative cooling on clear nights |
| High school | Basic Physics: "Heat" | Heat capacity and the temperature-change formula |
| High school+ | Physics: "Thermal radiation" | The Stefan–Boltzmann law |
| University | Meteorology, heat transfer engineering | Surface energy balance, heat conduction equation |
| Research | Road meteorology, snow and ice science | Predicting and preventing road icing |
| ― | Link to daily life | How to cross bridges, overpasses and shaded roads on winter mornings |
- Kondo Junsei, Chihyōmen ni chikai taiki no kagaku ― rikai to ōyō (近藤純正『地表面に近い大気の科学 ― 理解と応用』), University of Tokyo Press (on surface energy balance and radiative cooling)
- Japanese Society of Snow and Ice (日本雪氷学会), ed., Shinpan Setsuhyō Jiten (『新版 雪氷辞典』), Kokon Shoin (on road icing and frost)
- Ministry of Land, Infrastructure, Transport and Tourism (国土交通省), regional development bureaus' winter road warning materials (on bridges, tunnel entrances and shaded spots freezing easily)
- Heat capacity – Wikipedia (Japanese)
* This article is a popular-science explanation. The figures given are rough guides to help you understand the mechanism. Actual road conditions vary greatly with place and weather. On winter roads, follow information and instructions from road administrators, local governments, the police and other authorities.