private string GetKey(string areaId, string type, string date, string appId, string privateKey)
        {
            var hmac = HMACSHA1.Create();
            var publicKey = GetPublicKey(areaId, type, date, appId);
            var data = System.Text.Encoding.UTF8.GetBytes(publicKey);
            //密钥
            var key = System.Text.Encoding.UTF8.GetBytes(privateKey);
            hmac.Key = key;

            //对数据进行签名
            var signedData = hmac.ComputeHash(data);
            return Convert.ToBase64String(signedData);
        }

        private string GetPublicKey(string areaId, string type, string date, string appId)
        {
            return string.Format("http://open.weather.com.cn/data/?areaid={0}&type={1}&date={2}&appid={3}", areaId, type, date, appId);
        }

        /// <summary>
        /// 获取请求URL
        /// </summary>
        /// <param name="areaId">地区ID，例：北京=101010100</param>
        /// <param name="type">预警:alarm 实况:observe 指数:index 常规预报(24小时):forecast1d</param>
        /// <param name="date">日期,年月日时分,例：201410091040</param>
        /// <param name="appId"></param>
        /// <param name="privateKey"></param>
        /// <returns></returns>
        public string GetUrl(string areaId, string type, string date, string appId, string privateKey)
        {
            string key = GetKey(areaId, type, date, appId, privateKey);

            return string.Format("http://open.weather.com.cn/data/?areaid={0}&type={1}&date={2}&appid={3}&key={4}",
              areaId, type, date, appId.Substring(0, 6), HttpUtility.UrlEncodeUnicode(key));
        }