How to Build a Weather API with FastAPI and Vercel

Provide real-time weather data to apps and websites with a single FastAPI route.

Avatar for ricardo-gonzalezRicardo Gonzalez Software Engineer
Avatar for ismaelrumzanIsmael RumzanDX Engineer
Guides/Functions
2 min read
Last updated October 17, 2025

To provide real-time weather data, you need to handle API integration, error handling and automatic scaling. With Vercel functions and FastAPI, you can build a production-ready weather API in minutes.

In this tutorial, you will build and deploy a weather API route using the Open-Meteo API that:

  1. Accepts a city name and geocodes it to coordinates
  2. Fetches current weather data from the Open-Meteo API
  3. Returns temperature, humidity, wind speed, and other weather metrics with optional metric/imperial unit conversion
  • Python 3.10+ installed locally
  • A Vercel account
  • Basic understanding of FastAPI and async/await concepts

Start with the template Deploy FastAPI Boilerplate. Once your the project is deployed on Vercel, clone the repository locally.

You can also create the project locally using the Vercel CLI with the following command

terminal
vc init fastapi

Update your requirements.txt to include the httpx library for making async HTTP requests:

requirements.txt
fastapi
uvicorn
httpx

Open main.py and add the necessary imports at the top of the file:

main.py
from fastapi import FastAPI, HTTPException, Query
from fastapi.responses import HTMLResponse
import httpx

Then add the weather endpoint after your existing routes:

main.py
@app.get("/api/weather/{city}")
async def get_weather(city: str, units: str | None = Query(default="metric")):
    """
    Get current weather for a city.
    
    Args:
        city: City name (e.g., "London", "New York")
        units: Temperature units - "metric" (Celsius) or "imperial" (Fahrenheit)
    
    Returns:
        Weather data including temperature, humidity, wind speed, and location info
    """
    # Normalize units parameter
    normalized_units = "imperial" if units == "imperial" else "metric"
    
    async with httpx.AsyncClient() as client:
        # Step 1: Geocode city to get coordinates
        try:
            geo_response = await client.get(
                "https://geocoding-api.open-meteo.com/v1/search",
                params={
                    "name": city,
                    "count": 1,
                    "language": "en",
                    "format": "json"
                }
            )
            geo_response.raise_for_status()
            geo_data = geo_response.json()
            
            if not geo_data.get("results"):
                raise HTTPException(status_code=404, detail=f"City '{city}' not found")
            
            location = geo_data["results"][0]
            name = location["name"]
            country = location["country"]
            latitude = location["latitude"]
            longitude = location["longitude"]
            
        except httpx.HTTPError as e:
            raise HTTPException(status_code=500, detail=f"Geocoding API error: {str(e)}")
        
        # Step 2: Fetch current weather data
        try:
            weather_params = {
                "latitude": latitude,
                "longitude": longitude,
                "current": "temperature_2m,relative_humidity_2m,apparent_temperature,wind_speed_10m",
                "timezone": "auto"
            }
            
            # Add unit parameters for imperial if needed
            if units == "imperial":
                weather_params["temperature_unit"] = "fahrenheit"
                weather_params["wind_speed_unit"] = "mph"
            
            weather_response = await client.get(
                "https://api.open-meteo.com/v1/forecast",
                params=weather_params
            )
            weather_response.raise_for_status()
            weather_data = weather_response.json()
            
            # Return structured weather data
            return {
                "city": name,
                "country": country,
                "latitude": latitude,
                "longitude": longitude,
                "units": normalized_units,
                "current": weather_data["current"]
            }
            
        except httpx.HTTPError as e:
            raise HTTPException(status_code=500, detail=f"Weather API error: {str(e)}")

Install the dependencies and launch the application in dev mode:

terminal
pip install -r requirements.txt
vercel dev

Use curl to test the API route:

terminal
# Test with default metric units
curl http://localhost:3000/api/weather/london


# Test with imperial units
curl "http://localhost:3000/api/weather/san%20francisco?units=imperial"

You can also test the endpoint in your browser using the interactive API documentation at http://localhost:3000/docs.

  • Push the changes to your remote repository or run the vercel cli command
  • Vercel will create a new preview deployment for your to test
  • Merge to main branch to deploy to Production
When you deploy a FastAPI app to Vercel, the application becomes a single Vercel Function and uses Fluid compute by default.

In this tutorial, you’ve built a real-time weather API using FastAPI on Vercel.

You learned to:

  • Structure a dynamic API route and integrate external APIs
  • Deploy the app to Vercel as a function for automatic scaling

For a production application, make sure that you use a weather API that will not be rate limited based on the amount of traffic that you are expecting.

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