Why Your Weather App Is Wrong About Your Balcony
Your weather app isn't wrong. It's just answering a different question. It gives the air temperature for your area, measured in the shade about 2 meters off the ground. Your balcony has a wall in the sun, a hard floor, pots, wind and open sky, so it runs its own weather. By the model behind Balcony Plants (balconyplants.co), a south-west balcony can run up to about 12°C (22°F) above the forecast on a hot, still afternoon, and an exposed pot can drop about 5°C (9°F) below the forecast low on a clear, calm night.
What your forecast actually measures
A forecast describes the weather for a whole area. It comes from weather stations that stand out in the open, in the shade, over grass and away from buildings. That's on purpose: it's the fairest way to describe the weather for everyone.
It just isn't where your plants live. Your plants live next to a wall that's been in the sun since noon, on a floor that holds heat, in pots that warm up from the outside in.
The 4 things that give your balcony its own climate
1. Which way it faces
This decides how much direct sun your balcony gets, and sun is what heats it up.
- South, south-east and south-west: about 6 or more hours of direct sun a day
- East and west: about 4 to 6 hours
- North, north-east and north-west: under 3 hours
A south-west balcony gets strong sun right through the hottest part of the day, which is why it's the one that runs furthest above the forecast. (In the southern hemisphere, swap north and south.)
2. Which floor you're on
The one that surprises people: a higher floor isn't simply hotter. The wind is stronger up there, and wind sweeps away the warm air that builds up around your pots. On a breezy afternoon, a top-floor balcony can stay closer to the forecast than a sheltered one lower down.
What height does change is drying and cold. Wind pulls water out of pots faster, even on mild days. And a high balcony sees more open sky, so it loses more heat at night. A ground-floor balcony gets a little extra warmth from the sun-heated pavement around it.
3. Wall or railing
Where a pot sits matters almost as much as the balcony itself. Against the wall is the warmest spot: the wall soaks up sun all day and gives some of that heat back at night. Out on the railing is the most exposed: more wind, more sun, more open sky.
That's why moving a pot is often the cheapest fix there is. Back from the railing on a hot afternoon, against the wall on a cold night.
4. Wind and open sky
At night your balcony stops gaining heat and starts losing it. A pot with a clear view of the sky gives off heat into it, and on a clear, still night it can end up colder than the air around it. That's how a pot gets frost on a night the forecast said 3°C (37°F).
Clouds and wind cancel most of this. Clouds work like a lid, and a breeze keeps mixing warmer air back in. The worst nights for balcony plants are the clear, calm ones.
The pot itself plays a part too: a dark plastic pot in full sun runs much hotter than a light terracotta one. There's more on that in the heatwave guide, and the cold side is in the frost guide.
Same plant, different balcony
Take verbena, a sun-loving summer flower. We ran it through Balcony Plants for every direction a balcony can face:
- Facing south, south-east or south-west:a good fit. “This much sun keeps it at its best all summer, but grow it as a summer plant, or bring it in over winter.”
- Facing east or west:“Just shy of the sun it wants, so expect a lighter show.”
- Facing north, north-east or north-west: “There is nowhere near enough direct sun here for it.” The suggestion is to swap it for something that copes with less sun, like a hosta.
Same plant. Only the direction changed, and the answer went from a good fit to a swap.
Tell it your city, which way your balcony faces and which floor you're on, and it does this for your plants. Not sure what to grow there yet? The free plant picker starts from your balcony too.
The words, in plain English
- Microclimate: the weather in one small spot, like your balcony, which can be different from the weather for your area.
- The warm air pocket: the layer of air around your pots that heats up when the sun hits the wall, the floor and the pots themselves.
- Night sky cooling: on a clear night, anything with a view of the sky gives off heat and can end up colder than the air.
- Wind drying:wind pulls water out of pots and leaves faster, even when it isn't hot.
FAQ
What is a balcony microclimate?
It's the weather on your balcony itself, which can be quite different from the forecast for your area. The sun on the wall and floor, the wind, which floor you're on and how much open sky your pots see can make a balcony hotter than the forecast by day and colder than it at night. That's why two balconies in the same building can need different plants and different care.
How much hotter is a balcony than the forecast?
It depends on which way it faces, the sun and the wind. By the model behind Balcony Plants, a south-west balcony can run up to about 12°C (22°F) above the forecast on a hot, still summer afternoon. On a breezy day, or on a north-facing balcony, the gap is much smaller. It can also go the other way: on a clear, calm night an exposed pot can drop about 5°C (9°F) below the forecast low.
Does floor height change balcony temperature?
Less than people think, and not always upward. Higher floors are windier, and wind carries away the warm air around your pots, so a top floor isn't automatically hotter. What height does change is drying and night cold: wind dries pots faster, and a high balcony sees more open sky, so it cools more on clear nights.
Does it matter if my pots are against the wall or on the railing?
Yes. Against the wall is the warmest spot: the wall soaks up sun during the day and gives some of that heat back at night, so pots there stay a few degrees warmer on a cold night. Out on the railing, pots get more wind, more sun and more open sky, so they heat up faster on a hot afternoon and cool down faster at night. Moving a pot between the two is often the cheapest fix on a balcony.
Why did my balcony plants get frost when the forecast said above freezing?
Because the forecast low is the air temperature, and a pot with a clear view of the sky can get colder than the air. On a clear, calm night an exposed pot can drop a few degrees below the forecast low, which is enough for frost on a night forecast at 2 or 3°C (36 or 37°F). Pots against the wall, or under the balcony above, stay warmer.
Is my weather app wrong for my balcony?
It's accurate for what it measures: the air in the shade, for your area. It just doesn't know which way your balcony faces, which floor you're on or where your pots sit, and those can move the temperature at your plants by several degrees either way.
Before you trust tomorrow's forecast
Your weather app will keep answering the question it was built for. The question you actually have is which of your plants is at risk tomorrow, and that one needs your balcony, not your city.