We hear it all the time, but do we really know what it means? The news anchor pans to the meteorologist/weather person and says something like, “Well, Dana, what is the weekend weather hold for us? Will this rain continue? And Dana answers, “Clint, we are in for a surprise. The skies will clear and temperatures on Saturday will be 15 degrees above normal. So, get out those shorts for at least one more great fall weekend.” When we hear this we conjure up our definition of normal: Normal (adjective): Conforming to a standard; usual, typical, or expected: it’s quite normal for puppies to bolt their food; normal working hours (Oxford English Dictionary).
So, the temperatures on the weekend will be above typical, higher than what is expected for this day in the fall. But, that is not exactly the meaning of “climate normals”, which is what the media meteorologist is referring to. Climate Normals are a calculation by the National Weather Service, and have a very precise meaning, different from the meaning we think of when we hear the term. Here is the definition according to the National Weather Service, which calculates normals for each weather station in the U.S.: Climate Normals (proper noun) are three-decade averages of climatological variables including temperature and precipitation. This product is produced once every 10 years.
So, the 1981–2010 U.S. Climate Normals is the latest release. (There are past Climate Normals for 1971-2000, 1961-1990, etc.) This dataset contains the calculated daily and monthly averages (mean of all the values for that day over those three decades) of temperature and precipitation, plus other climate parameters such as, snowfall, heating and cooling degree days, frost/freeze dates, and growing degree days. Climate Normals are provided for most of the weather stations across the country. Below is an example (Figure 1) of what these look like for temperature and precipitation for Hamilton, MT.
Climate Normals are calculations of the temperatures for the three decades, they are not the typical values and you may not even have experienced them for that day, even if you lived through all three decades. They are calculated averages, not measured values. This is something critically important about climate data: It is derived from calculations based on measured data, not the measurements themselves. So, for example, the Climate Normal for today in Hamilton is 64.1 degree F, while the high today was 75 degree F, about 10 degrees higher than the “normal” high temperature.
The plot below (Figure 2) shows how variable the climate is for any site. This is a plot of the daily maximum temperatures (Tmax) for Hamilton, MT for the 30 year of the last Climate Normal period (1981-2010).
The red pluses on the figure show the measured maximum temperature for each day over the three decades used to calculate the Climate Normals. The vertical line is October 6 and the horizontal line is the normal maximum temperature for that day; the grey diamond marks the intersection of the two, showing the calculated normal maximum temperature compared to the range of measured maximum temperatures for that day. You can see the huge range of data around the calculated normal compared to the measured Tmax. The normal TMax is not “typical” or “expected” in the normal sense of normal. That is, hardly any of the maximum temperatures measured for a particular day match the Climate Normal Tmax. So, normals are not really normal! But they are a convenient way to compare records to some average value. And there are some great ways to see these comparisons on the Weather Service sites. The graph below (Figure 3) is one of those plots. (See the note at the end of this post to see how to get these plots for your area. One catch is that these plots are not available for as many stations as the data I showed above. In chart below I have moved to a station 38 miles north of Hamilton, MT, Missoula, MT.
This chart will take some explaining, but it is well worth the effort. I will concentrate on temperature first (upper graph). The blue vertical bars (they are just very thin so look like a jagged line) are the maximum (top of bar) and minimum (bottom of bar) temperatures recorded at the Missoula weather station. The upper boundary of the green band is the Climate Normal Tmax and the lower boundary is the Climate Normal Tmin (minimum temperature). So, the green band represents the band of calculated normal temperatures and the blue “line” represents the measured temperatures throughout the year. The top of the pink, jagged band is the extreme temperature recorded for that day (Record Max) and the bottom of the blue band is the extreme minimum temperature recorded for that day (Record Min). So, this graph shows an incredible amount of information, allowing comparison throughout the year of measured data to calculated normals and recorded extremes. Precipitation graphs are simpler because they show only the measured data (the “stepped” line) and the “normal precipitation”. The graph shows cumulative precipitation through the year, so it is easy to see if precipitation is above the Climate Normal (dark green) or below normal (light brown). You can say a lot by looking at these plots about how temperature and precipitation in a specific year compares to the latest three decades used to calculate the normals (1981-2010 in this case).
The most obvious is how “un-normal” the normals are, or maybe a better way to look at it is how “un-normal” the daily records are. For example, for 2014 until October 6, there are many days that fall outside the normal range of temperatures. There are also very few days that were near or beat the extreme records—two maximum and two minimum out of just over nine months of records. But some months were really wild. Let’s zoom in on February using the figure below (Figure 4).
For the first third and last third of February, every day had minimum and maximum temperatures substantially lower than normal. During the middle of the month, it was the opposite with the temperatures substantially (but not as extreme) above normal. There is a clear step in the weather across the boundaries between these two conditions. Like a switch turning from really cold to really warm. That switch was the jet stream as it moved across the region bringing in warm or cold air. But that story is for another post.
To get to the graphics in the last two plots takes some patience with the Weather Service site. The easiest way (but not the only way) is to follow these directions:
1. Start at this web site: http://www.nws.noaa.gov/climate/
2. Click on the region of the state your are interested in on the U.S. map. This will take some messing with to get where you want. When the right region comes up, select the station you want. I selected Missoula, MT (mso), so got this site: http://www.nws.noaa.gov/climate/index.php?wfo=mso.
3. Then click on the tab (upper part of the page) titled, “Local Data/Records”.
4. This brings up another page. Under “Climate Graphs” select “Graphical Climate Summaries for…” where the “…” is the list of stations in the region you selected.