The NWS Web site features a handy on-line calculator for determining the heat index based on a combination of either temperature and relative humidity or temperature and dew point.

And the bubble line. Sometimes, instead of having a nice separation between the bubble and the dew line, there's a special point which for some reason those two curves meet. Non-ideality.

The sensor is composed of pyroelectric refrigerator and semiconductor detector, and controlled by a operator without model, In this way the air dew-point can be measured on line.