NetworkOnMainThreadException

package com.example.kris.stormy;

        import android.content.Context;
        import android.net.ConnectivityManager;
        import android.net.NetworkInfo;
        import android.os.Bundle;
        import android.support.v7.app.ActionBarActivity;
        import android.util.Log;
        import android.widget.Toast;

        import com.squareup.okhttp.Call;
        import com.squareup.okhttp.Callback;
        import com.squareup.okhttp.OkHttpClient;
        import com.squareup.okhttp.Request;
        import com.squareup.okhttp.Response;

        import org.json.JSONException;
        import org.json.JSONObject;

        import java.io.IOException;


public class MainActivity extends ActionBarActivity {

    public static final String TAG = MainActivity.class.getSimpleName();

    private CurrentWeather mCurrentWeather;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String apiKey = "27974c4bc33201748eaf542a6769c3b7";
        double latitude = 37.8267;
        double longitude = -122.423;
        String forecastUrl = "https://api.forecast.io/forecast/" + apiKey +
                "/" + latitude + "," + longitude;

        if (isNetworkAvailable()) {
            OkHttpClient client = new OkHttpClient();
            Request request = new Request.Builder()
                    .url(forecastUrl)
                    .build();

            Call call = client.newCall(request);
            call.enqueue(new Callback() {
                @Override
                public void onFailure(Request request, IOException e) {

                }

                @Override
                public void onResponse(Response response) throws IOException {
                    try {
                        String jsonData = response.body().string();
                        Log.v(TAG, jsonData);
                        if (response.isSuccessful()) {
                            mCurrentWeather = getCurrentDetails(jsonData);
                        } else {
                            alertUserAboutError();
                        }
                    }
                    catch (IOException e) {
                        Log.e(TAG, "Exception caught: ", e);
                    }
                    catch (JSONException e) {
                        Log.e(TAG, "Exception caught: ", e);
                    }
                }
            });
        }
        else {
            Toast.makeText(this, getString(R.string.network_unavailable_message),
                    Toast.LENGTH_LONG).show();
        }

        Log.d(TAG, "Main UI code is running!");
    }

    private CurrentWeather getCurrentDetails(String jsonData) throws JSONException {
        JSONObject forecast = new JSONObject(jsonData);
        String timezone = forecast.getString("timezone");
        Log.i(TAG, "From JSON: " + timezone);

        return new CurrentWeather();
    }


    private boolean isNetworkAvailable() {
        ConnectivityManager manager = (ConnectivityManager)
                getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = manager.getActiveNetworkInfo();
        boolean isAvailable = false;
        if (networkInfo != null && networkInfo.isConnected()) {
            isAvailable = true;
        }

        return isAvailable;
    }

    private void alertUserAboutError() {
        AlertDialogFragment dialog = new AlertDialogFragment();
        dialog.show(getFragmentManager(), "error_dialog");
    }
}