- Ceiling: what is the ceiling? Why do they need to know the ceiling?
- Zulu time: what is the zulu time?
- Dew point: why do they need to know the dew point?
The ceiling is the lowest altitude where clouds cover more than half of the sky. This is important because climbing above that altitude means you will most likely have to fly through clouds. This makes navigation more difficult and pilots are required to have special training to fly in low visibility. If the ceiling is too low, pilots can't be at a safe altitude above the ground and out of the clouds at the same time. Low ceilings are also critical for landing, and may require pilots to make an instrument approach and landing.
Zulu time refers to UTC time, which is the universal coordinated time. Zulu or UTC time is helpful as a worldwide reference in fields like aviation, to avoid issues like dealing with changing between local time zones. The ATIS will contain the Zulu time that it was updated so pilots know how recent the information is.
The dew point in relation to the temperature gives the pilots information about the humidity, and can affect visibility. If the dew point is close to the temperature, humidity is high, which can cause hazy conditions, or even fog. As with the ceiling, this can warn pilots of possible changes in conditions that will make it harder to see the ground or other aircraft while in flight. In terms of aircraft performance, the dew point is also needed to determine the true density altitude, when combined with the pressure and temperature. A high dew point means a higher density altitude, which reduces aircraft performance.
Dew point is also very important in certain helicopters with carburetors, like the Robinson R-22 and R-44 helicopters, which are subject to carb icing even during take-off because they only use as-required power rather than full throttle. In this case, carb heat needs to be applied whenever the temp/dew point spread is 15C or less. Here is the citation from the R-22 POH:
Dew point can most certainly be used for performance. High humidity can cause a longer take off roll. When combined with a high altitude airport and hot temps, it can be a struggle to get airborne. Cooler, less humid air is more dense--there's more "air" for the wing to bite into. So, a pilot listening to ATIS would want to know the temp, dew point, and temp for the airport to determine (in this example) takeoff roll distance.
To add to the above, in all carbureted A/C dew point is needed for indication of carburetor icing.
Dew Point relative to temperature is a useful indication of weather conditions .