Weather Links/Resources

What does Observation Time Mean?

Much of the data at the Midwestern Regional Climate Center comes from National Weather Service cooperative observing stations where an observer records the temperature and precipitation at the same time every day. A common observation time is 7:00 a.m. Precipitation measured at that time would be the total amount for the 24-hour period from 7:00 a.m. the day before to 7:00 a.m. the day of the observation. However, not all stations have a 7:00 a.m. observation time. Some record from midnight to midnight, producing a true daily record, while others may record in the evening.

Cooperative station data from the Midwestern Regional Climate Center will be stamped with an observation time of "AM", "PM", or "MID" to indicate the time of day when the observation was recorded. Whenever possible, the actual time of observation will be noted on the data.

What is the difference between dew point temperature and wet bulb temperature? What is the relative humidity?

In the past, relative humidity was difficult to measure directly. An easier parameter to measure was the wet bulb temperature. This was found by placing a wet cloth over a thermometer, and then passing air over the cloth to evaporate the water. Initially, the evaporation of the water causes the temperature to decrease but the temperature eventually becomes constant. This steady temperature is called the wet bulb temperature and it relates to the amount of moisture in the air. It is then possible to calculate the dew point and relative humidity using the dry bulb and wet bulb air temperatures. These days, there are better methods for measuring relative humidity directly and there are few uses for the wet bulb temperature. In fact, it is unusual for the wet bulb to be measured directly anymore. It is now calculated from the air temperature and dew point or relative humidity.

What is the difference between dew point temperature and wet bulb temperature? What is the relative humidity?

The dew point temperature, wet bulb temperature and relative humidity all relate to the amount of moisture in the air. The dew point is the temperature to which the air must be cooled to reach saturation. The difference between the air temperature and the dew point temperature is proportional to the relative humidity. The closer the two temperatures are, the higher the relative humidity. When the air temperature becomes the same as the dew point temperature, the relative humidity reaches 100% and fog will develop and dew will form on surfaces.

A common misconception is that the relative humidity cannot exceed 100%. In fact, this does occur but it is a temporary condition. The temperature can actually drop below the dew point, causing the air to become super-saturated. This condition is corrected quickly as moisture condenses into liquid to form fog or dew. As the water vapor in the air decreases, the dew point temperature decreases and the relative humidity returns to 100%.