What's in a graph? A lot!

I promised to occasionally write about the value of an automatic weather station. This is one of those occasions.

The first two graphs below show the wind speed and wind direction, in that order, from our weather station in Cedar Falls. The dates are March 6th and 7th. The 2 minute average wind speeds on both days were mostly less than 10 mph with a few periods exceeding 10 mph (blue line). But there were higher gusts (stars). The gusts exceeded 20 mph

Note a couple of things. First the winds were gusty during the evening of the 5th and early morning of the 6th (stars) before dying down during the day on the 6th. After the Sun came up on the 7th wind speeds increased again and became gusty (stars). Winds were from the WNW on the 6th but gradually decreased and became southeasterly during the day of the 6th. On the 7th winds increased from the south, becoming gusty after sunrise. This is a common pattern for wind speeds. Stronger winds aloft are not always felt at the surface at night because nighttime cooling separates the near surface air from the strong winds aloft. If winds (as in this case) begin to suddenly increase after sunrise it is likely that solar heating is mixing the air allowing stronger winds aloft to drop to the surface.

Let’s look at the temperature (red), dew point (green), and relative humidity (blue) lines on the graph below and the pressure change on the bottom chart. Lets just look at March 7th beginning at 8:00 a.m. Notice how the temperature (red) started rising quickly after sunrise. The sharp rise was due to solar heating but enhanced by the mixing down of air aloft. As the air sank toward the ground it warmed at 5.5 degrees F for every 1,000 feet it descended. Temperatures warmed to near 60 in the afternoon.

At the same time the relative humidity decreased. As air warms its capacity for water vapor increases. Since more water could be in the air at 60 degrees than at 40 degrees, the relative humidity decreased from 60% in the morning to less than 30% in the afternoon.

Finally, the dew point increased slight during the day. The dew point is one way to measure the amount of water vapor in the air. It increase slightly because the southerly winds were bringing slightly higher dew points into the area.

While all of that was going on the air pressure was decreasing as chilly high pressure was moving away from Iowa and lower pressure to the west was inching eastward. The pressure change chart shows this clearly.

All of the factors measured and shown on these charts are related to each other and to changes in the atmosphere over the United States. Forecasters use these changes, along with other data, to create daily weather forecasts. Data from automatic weather stations is one tool that is used to make those forecasts.