An Arduino + Ethernet shield example on how to get data from Open WeatherMap

openweather.ino

/*
 Web Client to consume Open WeatherMap web service
 
 This sketch connects to a website (http://api.openweathermap.org)
 using an Arduino Ethernet shield and get data from site.
 
 Circuit:
 * Arduino MEGA 2560 R3 Board
 * Ethernet shield attached to pins 10, 11, 12, 13
 
 created 24 May 2015
 by Erico Porto

 Based on the WeatherUnderground version from Afonso C. Turcato
 */

#include <ArduinoJson.h>
#include <SPI.h>
#include <Ethernet.h>

#define RBUFFSIZE 600

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

const char server[] = "api.openweathermap.org"; 

// Set the static IP address to use if the DHCP fails to assign
IPAddress ip(192,168,1,202);

EthernetClient client;

char responseBuffer[RBUFFSIZE];
int  rbindex = 0;

boolean startCapture;

void setup() {
  Serial.begin(9600);

  // start the Ethernet connection:
  if (Ethernet.begin(mac) == 0)
  {
    Serial.println("Failed to configure Ethernet using DHCP");
    Ethernet.begin(mac, ip);
  }
  
  // give the Ethernet shield a half-second to initialize:
  delay(500);
  Serial.print("My IP Address is: ");
  Serial.println(Ethernet.localIP());
  Serial.println("Connecting...");
  
  // if you get a connection, report back via serial:
  if (client.connect(server, 80))
  {
    Serial.println("Connected!");
    
    const String html_cmd1 = "GET /data/2.5/weather?q=SaoPaulo,BR HTTP/1.1";
    const String html_cmd2 = "Host: api.openweathermap.org";
    const String html_cmd3 = "Connection: close";
    
    //You can comment the following 3 lines 
    Serial.println(" " + html_cmd1);
    Serial.println(" " + html_cmd2);
    Serial.println(" " + html_cmd3);
    
    // Make a HTTP request:
    client.println(html_cmd1);
    client.println(html_cmd2);
    client.println(html_cmd3);
    client.println();
    
    responseBuffer[0] = '\0';
    rbindex = 0;

    startCapture = false;   
  } 
  else
  {
    // if you didn't get a connection to the server:
    Serial.println("Connection failed!");
  }
}

void loop()
{
  // if there are incoming bytes available 
  // from the server, read them and buffer:
  if (client.available())
  {
    char c = client.read();
    if(c == '{') {
      startCapture=true;
    }
    
    if(startCapture && rbindex < RBUFFSIZE) {
      responseBuffer[rbindex] = c;
      rbindex++;
    }
  }
  
  // if the server's disconnected, stop the client:
  if (!client.connected()) {   
      
    
    Serial.print("Received bytes");
    Serial.print(strlen(responseBuffer));
    Serial.println("Disconnecting.");
    client.stop();
    client.flush();
    
    Serial.println(responseBuffer);
    
    Serial.println();
        
    StaticJsonBuffer<500> jsonBuffer;
    
    JsonObject& root = jsonBuffer.parseObject(responseBuffer);

    if (!root.success()) {
      Serial.println("parseObject() failed");
    }  else {
    
      //Now, some examples of how to use it!    
      Serial.print("Current Temperature: ");
      Serial.print((double)root["main"]["temp"]);
      Serial.println(" K\n");
      
      Serial.print("Rain: ");
      Serial.println((double)root["rain"]["3h"]);
      Serial.println();
      
      Serial.print("Wind: ");
      Serial.println((double)root["wind"]["speed"]);
      Serial.println();   
    }
    // do nothing forevermore:
    while(true);  
  }
}

Thanks for sharing the code. I guess it won't work as it is, unless you provide your APPID at the end of GET request.