Why Is a Forest Cooler Than a City?
― Trees Cool the Air by "Sweating"
On a hot summer day, stepping off asphalt pavement into a park or forest, you can sometimes feel the air turn noticeably cool — cooler than shade alone could explain. That's because trees don't just block sunlight; they constantly evaporate water, drawing heat straight out of the surrounding air itself.
On a summer afternoon, after walking along a stretch of glaring, sun-baked asphalt, stepping into a tree-lined park or a dense forest can make the cool you feel change quite noticeably.
Stepping into a building's shadow does cool you down, but the coolness inside a forest often feels a level deeper than that.
Both are blocking direct sunlight, so where does this difference come from?
A building's shadow and a forest's shade both block direct sunlight in the same way. Yet the coolness still differs.
A tree draws water up through its roots and continuously releases it as water vapor from its leaves. This evaporation draws heat from its surroundings.
The process of "transpiration" we saw in how trees pull water up to great heights acts here as a cooling system.
Shade alone doesn't explain it
Shade from a building and shade from street trees both block direct sunlight. But a building's walls and asphalt absorb sunlight in the sunlit areas and keep storing it as heat. As we saw in how cities become heat islands, they keep releasing that stored heat into the surroundings, day and night. Even in the shade, you're still affected by heat radiating from around you.
In a forest, meanwhile, less sunlight reaches the ground and trunks to begin with, and there are far fewer "hard, reflective surfaces" that drive heat storage. But that alone still doesn't explain the extra layer of coolness in a forest.
Trees are constantly "sweating"
A tree draws water up through its roots and constantly releases it as water vapor through tiny pores in its leaves called stomata. This is called transpiration. The amount of water a single tree releases in a day varies with the tree's size, species, and the weather, but for a large tree it can exceed 100 liters.
This transpiration works on basically the same principle as a person sweating to cool their body. Both rely on the fact that water absorbs heat from its surroundings as it changes from liquid to gas (water vapor).
it comes from evaporation actively "carrying heat away."
Evaporating water draws heat from its surroundings
For water to become water vapor, it needs enough energy to break free of the forces holding its molecules together. This energy is called the latent heat of vaporization.
The chill you feel right after getting out of a pool is the same effect — water on your skin evaporating and drawing heat from your body as it does. When water vapor is released from a tree's leaves, the exact same mechanism draws heat from the surrounding air.
Across a whole forest, countless leaves are constantly repeating this "cooling by evaporation." On top of the shade effect, this is thought to be what makes a forest even cooler than a city. In the next collapsible section, we'll put a number on how much heat this actually removes.
Differences from asphalt matter too
A forest's ground and leaves don't absorb and store as much solar radiation as asphalt does. A forest is also thought to block wind or alter airflow near the ground in ways that somewhat soften the inflow of warmed city air. It seems closer to the truth to say that shade, transpiration, low heat storage, and changes in airflow all combine to create a forest's coolness.
Urban green spaces such as parks and street trees are reported to cool their surroundings through the same transpiration mechanism, though on a smaller scale than a forest. This effect is thought to be part of the reasoning behind preserving green spaces and maintaining street trees as a measure against summer heat.
Try it yourself
- Put a small amount of water on the back of one hand (leave the other dry)
- Hold both hands still, without any breeze from a fan, for a few tens of seconds
- Check that the wet hand feels noticeably cooler
- Fan it to speed up evaporation, and check that the coolness increases further (the faster the evaporation, the faster heat is drawn away)
When water vapor is released from a tree's leaves, this same "latent heat" mechanism draws heat from its surroundings.
Summary
A forest is thought to be cooler than a city because trees don't just block sunlight — they constantly evaporate water (transpiration), drawing heat from the surrounding air. This is the same latent-heat mechanism that lets sweat cool your body. On top of that, not storing heat the way asphalt does, and altering airflow, are also thought to play a part in the coolness.
A forest isn't just blocking sunlight.
It's constantly evaporating water, cooling the air itself.
Want to know more? ― Terms, numbers, and links to the textbookWe've marked which level each part belongs to, from middle-school science to open research questions
- MSCovered in middle-school science
- HSCovered in high-school "Basic Chemistry" / "Basic Physics"
- HS+Covered in high-school "Biology," or advanced/sidebar content in textbooks
- Univ.Content from a university specialty course (environmental science, meteorology) not taught in high school
- ResearchNot even taught as settled fact at university — something researchers are actively investigating
MSTerms: words behind a forest's coolness
- Transpiration: the process by which plants release water as vapor through the stomata in their leaves.
- Latent heat of vaporization: the heat a liquid absorbs from its surroundings as it turns into gas.
- Heat island: the phenomenon where a city's temperature runs higher than that of surrounding areas.
HSChecking with a formula: how much heat does one tree remove in a day?
By multiplying the amount of water a tree transpires in a day by water's latent heat of vaporization, we can estimate how much heat a single tree draws from its surroundings.
Heat removed = mass of water evaporated × latent heat of vaporization
| Mass of water evaporated | In kg. 1 liter of water is about 1 kg |
| Latent heat of vaporization | A well-known reference value is about 2.26×10⁶ J/kg (a typical figure at 100°C) |
| Heat removed | In J (joules) |
The actual latent heat varies somewhat with temperature, but here we'll use a typical figure often used in textbooks to get a sense of scale.
Let's take the water a large tree transpires in a day as a representative figure of 100 liters (100 kg).
| Heat removed | 100 × 2.26×10⁶ = 2.26×10⁸ |
| Heat removed per day | About 2.26×10⁸ J |
That's hard to picture on its own, so let's convert it into units of electric power (kilowatt-hours, kWh). 1 kWh is 3.6×10⁶ J.
| Converted to kWh | (2.26×10⁸) ÷ (3.6×10⁶) ≒ 62.8 |
| Heat removed per day (in kWh) | About 62.8 kWh |
If we take a home air conditioner's cooling capacity as a representative 2.2 kW, this converts as follows.
| Converted to AC running time | 62.8 ÷ 2.2 ≒ 28.5 |
| Equivalent running time | About 28.5 hours |
The math shows that a single large tree's transpiration in just one day removes about as much heat from the surrounding air as running a home air conditioner for more than a full day straight. Across a whole forest, this effect adds up over countless trees.
※ The transpiration rate, latent heat, and AC capacity are all representative approximate figures. Actual values vary widely by tree species, size, weather, and product.
HS+Transpiration isn't always the same amount
A tree's transpiration rate changes with temperature, humidity, sunlight, wind, and soil moisture, among other conditions. When the soil is dry, trees are known to close their stomata to hold back transpiration and prevent water loss. Environmental science distinguishes between how much a tree could theoretically transpire given the weather conditions (potential evapotranspiration) and how much it actually transpires (actual evapotranspiration) — the two don't necessarily match.
Univ.The idea of "allocating" the energy the ground receives
In meteorology and environmental science, we consider how the solar energy the land surface receives gets allocated. The ratio between "sensible heat," which directly warms the air, and "latent heat," which goes into evaporating water, is expressed by a measure called the Bowen ratio. Places with rich vegetation and ample moisture tend to send more of their energy into latent heat (evaporation), lowering the share that goes into sensible heat (directly warming the air), and this is one of the quantitative underpinnings for why forests and green spaces feel cool.
ResearchWhat's still unclear
- How to arrange forests and green spaces within a city to spread their cooling effect most efficiently across the whole urban area is a topic in urban climatology. The effect is said to depend on a green space's shape, size, and the layout of surrounding buildings, but optimal design guidelines are still a work in progress.
- How rising temperatures and drying conditions from climate change will affect the transpiration capacity of forests and urban green spaces themselves is also an active area of research. As drying progresses, transpiration is suppressed, and some point to the possibility that the cooling effect itself could weaken, though the scale of the impact is said to vary by region and tree species.
- Precisely separating out how much of a forest's cooling effect comes from shade, transpiration, changes in airflow, and low heat storage, respectively, by location and season, is also a research theme in environmental science.
The sense that forests are cool is familiar to everyone, but accurately measuring that effect and applying it to urban planning is a field where research is still ongoing.
Links to the textbook (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science: states of matter and plant functions | Basic terms for transpiration and latent heat |
| HS | Basic Chemistry/Physics, states of matter | Estimating heat removed by a tree from latent heat |
| HS+ | Biology: plant water physiology | Stomatal opening/closing and changes in transpiration rate |
| Univ. | Meteorology, Environmental science | Sensible/latent heat allocation, the Bowen ratio |
| Research | Urban climatology (ongoing research) | Optimizing green space layout, climate change and transpiration capacity |
- Explanatory material from Japan's Ministry of the Environment (環境省) on "the effects of greening in heat-island countermeasures."
- Forestry and forestry-related textbook explanations of transpiration and forests' climate-moderating effects.
- Meteorology textbook explanations of sensible/latent heat allocation (the Bowen ratio).
- Research reviews in urban climatology (on the cooling effects of urban green space and their contributing factors).
- Basic chemistry textbook entries on representative values for water's latent heat of vaporization.
※ Figures such as transpiration rate, latent heat, and air conditioner cooling capacity are representative approximations; actual values vary widely by condition.
※This article is a general-audience science explainer. For specific figures and effects related to green-space development or urban planning, please check information from official sources such as government ministries or local authorities.