using UnityEngine;
using System.Collections;
using System;

[Serializable]
public class WeatherData
{
    public Main main;
}

[Serializable]
public class Main
{
    public float temp;
    public float pressure;
}

public class Weather : MonoBehaviour
{
    IEnumerator Start()
    {
        string url = "api.openweathermap.org/data/2.5/weather?q={0}&APPID={1}&units=metric";
        string city = "Moscow";
        string appid = ""; // API key
        url = string.Format(url, city, appid);

        WWW www = new WWW(url);
        yield return www;

        //print(www.text);
        var w = JsonUtility.FromJson<WeatherData>(www.text);
        print("Temp: " + w.main.temp + ", Pressure: " + w.main.pressure);
    }
}

{
    "coord": {
        "lon": 37.62,
        "lat": 55.75
    },
    "weather": [
        {
            "id": 803,
            "main": "Clouds",
            "description": "broken clouds",
            "icon": "04n"
        }
    ],
    "base": "cmc stations",
    "main": {
        "temp": 14.72,
        "pressure": 1005.2,
        "humidity": 72,
        "temp_min": 14.72,
        "temp_max": 14.72,
        "sea_level": 1024.71,
        "grnd_level": 1005.2
    },
    "wind": {
        "speed": 3.8,
        "deg": 143.002
    },
    "clouds": {
        "all": 64
    },
    "dt": 1463515258,
    "sys": {
        "message": 0.0073,
        "country": "RU",
        "sunrise": 1463447555,
        "sunset": 1463506831
    },
    "id": 524901,
    "name": "Moscow",
    "cod": 200
}