Hi, welcome to the forum.
Which HTU21D module do yo have and how did you connect the HTU21D to the Arduino Nano ? Because the HTU21D is a 3.3V chip and the Nano is 5V board, so there is a voltage conflict on the I2C bus.
Which sketch do you use ?
It measures the "Relative Humidity", not the amount of water in the air. It is not milli-liters water per liter air. It is a number that tells the percentage of the amount of water that the air can hold for that temperature.
At 67°C, the hot air can hold more water. With the same amount of water in the air, the Relative Humidity decreases.
If you wrap the sensor in a damp towel (not dripping wet, but no dry parts), then it should be near 100% Relative Humidity for all temperatures.
You may not hold the sensor in direct sunlight. There are almost no sensor that allow to be in direct sunlight.