Measuring atmospheric moisture at the surface

When you listen to a weather observation it sounds so simple. There are a number of automated meteorological observation units that take nearly continous weather observations. The official NWS automated observations are recorded and transmitted by ASOS-Automated Surface Observation System.

The FAA has augmented these observations with their own units known as AWOS-Automated Weather Observation Systems. Here is a link to the automated observation sites. In addition, the Minnesota Department of Transporation has some weather observing sites.

When I began my career with the NWS at Mitchell Field in Milwaukee I was a weather observer. Fortunately we had automated sensors already in the early 1970s. We were required to observe the cloud cover, along with the horizontal visibility on an hourly basis. This visual observation along with the temperature, dew point, wind and pressure was transmitted by teletype.

On a regular basis we would ensure the automated temperature and dew point sensor was reading accurately. Once a week the weather observer would stand in a shaded grassy area and swing a sling psychrometer that would measure the dry bulb and the web bulb temperatures. We would then calculate the dew point; ultimately deriving the relative humidity.

Here’s a photo of how they still determine relative humidity when on a forest fire incident.


Though not clearly viewed, there is a sling psychrometer in his right hand.

If you want to read more about the meteorological measurements you can check your knowledge at this NWS website.

I was partial correct when I said you would notice an increase in dew points today. The Twin Cities International Airport had a dew point of 51 degrees at this time on Wednesday. The dew point rose to 65 degrees when light rain fell at 6AM, but has since fallen back to 56 degrees. The dew point is in the lower 60s at Rochester this afternoon.

Check out the change in cloud cover from early this morning (see previous blog) to this afternoon on the visible satellite image.


Thunderstorms were firing up in portions of the Minnesota River Valley in west central Minnesota.

Latest computer model information still supports the best chance for thundershowers in central Minnesota on Saturday afternoon and evening.

  • Matt G

    Yep – I’ve been on a few “prescribed burns” with the USFS. The prescriptions have very specific criteria that must be met before ignition of the fire is allowed. In one case, the relative humiditiy was too high, but with all of the trucks and fire crews in place, the weather observer simply ‘spun the weather’ again and again until he got the reading he needed (there may have been some number fudging going on).

    It’s a pretty intense moment: millions of dollars-worth of resources (personnel and equipment) are in place and all their eyes are on a little device at the end of a string that someone is spinning around their finger….waiting in silence for the Kestral’s verdict!

  • Craig

    Matt, great commentary on the reality of working the weather on a fire fighting front. Thanks for sharing this insight.