Ask Weather Dan: A question about highs and lows
One reader asks: A few years ago, I remember hearing a morningweather forecast that said we had already reached the high for the day at 4am and we would reach the low for the day around the afternoon/evening time. Up until that point I always assumed that the weather high came around the middle of the day and the low came around the evening/nighttime. What’s up with that?
Nicole asks:
A few years ago, I seem to remember hearing a morning TV weather forecast that said we had already reached the high for the day at 4am and we would reach the low for the day around the afternoon/evening time. Up until that point I always assumed that the weather high came around the middle of the day and the low came around the evening/nighttime. What’s up with that?
In short, yes, that’s the way things work, but they tend to happen in the afternoon and early morning rather than midday and evening.
In actuality, that’s the way things work most of the time. Sometimes, however, it’s a completely different situation.
Usually, it’s a pretty simple deal. The sun comes up, heats the air, and the temperature rises, peaking in the late afternoon. Once the sun goes down, the air loses heat, and we hit a minimum in the hours before sunrise. We call this a diurnal temperature cycle. You can see the change in this graph over 24 hours starting 1pm April 18 in Lavington, Australia. The vertical axis plots temperature in degrees Celsius, while the horizontal axis is time (0-24 hours). The graph starts at 13 hours (1pm local time).
You can see the temperature drop after 16 hours (4:30pm) local time, and then bottom out around 07 (7am). The temperature drop is a little earlier than we are used to in the afternoon simply because of the difference in seasons. Remember, it is autumn in the Southern Hemisphere, so despite the fact that Lavington is at a similar latitude to Richmond — Lavington is at 36 degrees south latitude; Richmond is at 37 degrees north latitude — Lavington is getting less sunlight than Richmond due to the Earth’s tilt.
However, if we see dramatic temperature changes during the course of a day, we may not hit the high or low temperature at the usual times. This typically happens when a strong cold front blows through an area, causing a sharp temperature drop, sometimes on the order of twenty degrees or more.
This exact situation happened on July 5, 2009. This is the 8am EDT surface analysis from the NOAA Hydrometeorological Prediction Center, which shows a cold front situated just south of the VA/NC border.
(Source: NOAA HPC; Click to enlarge)
Now let’s look at the temperature plot for July 5 in Richmond:
(Source: Weather Underground; Click to enlarge)
Richmond’s high temperature, 77 degrees, came at 12:54 am, and quickly fell into the 60s for most of the day. The low for the day, 64 degrees, came at 1:54 pm, completely the opposite of what we would normally expect.
There are other situations that can cause temperature to deviate from the normal diurnal pattern as well. On April 26 of last year, a heat burst caused by dissipating thunderstorms over the Eastern Shore led temperatures to spike more than 10 degrees at several locations in the early morning hours.
Salisbury’s temperature peaked at 87 at 2am, and then fell off sharply as the heat burst dissipated, only to rise again once the sun came up. While 87 wasn’t the daytime high for that day, it was quite close: Salisbury finally peaked at 92 that afternoon.
Ocean interactions can affect temperatures, as well. Because land and water absorb thermal radiation differently, the air temperature differences that result above these surfaces can create pressure differences. (Remember when we talked about pressure gradients and wind?) These differences create sea breezes, where cooler air flows onshore from the ocean, causing temperatures to peak earlier in the day. We can see this in this plot for the Norfolk airport from April 4th of this year.
(Source: Weather Underground; Click to enlarge)
There’s a peak in the temperature plot (top graph) just before 11am. At the same time, looking at the plot of wind direction (bottom graph), you’ll see that what was a wind coming from the southwest in the morning became an easterly wind after 11, and temperatures dropped sharply in response – almost 5 degrees over two hours. They increased again through the afternoon due to normal daytime heating, but the increased wind speeds (third graph) kept them from ever surpassing the 11am high.
The seabreeze situation isn’t as common in Richmond, mostly because, well, we’re nowhere close to the ocean. Summer thunderstorms can do a lot to suppress temperatures; we may see our high temperature earlier in the day especially in the case of afternoon storms. There are also times where the storms are associated with a passing cold front, so both the storms and the cold air mass act as a mechanism for depressing temperatures.
So, Nicole, while it is normally true that we hit our high and low temperatures around the same time every day, there are plenty of phenomena that can change this and give us temperature maxima and minima at different times of the day.
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Weather Dan
Dan Goff is now a two-time former Richmonder, having departed the River City yet again in favor of southwest Virginia, where he is working on degrees in geography and meteorology at Virginia Tech. Have a question about the weather or weather-related phenomena?
Notice: Comments that are not conducive to an interesting and thoughtful conversation may be removed at the editor’s discretion.
This is why I don’t like to hear the “high and the low” and instead prefer to look at the graph of the day’s temperature.
Like this (it tells you the rain by hour, too!)
http://forecast.weather.gov/MapClick.php?CityName=Richmond&state=VA&site=AKQ&textField1=37.5536&textField2=-77.4606&FcstType=graphical
Though you explained why Lo`s and Hi`s can sometimes not always occure near sunrise or mid afternoon :
I did not see where you mentioned that at All N.W.S. Stations are based on a Midnight to Midnight { 12:01 a.m. to 11:59 p.m. } 24 hour period.
I`m a Cooperative Station here in Jackson, Tn.
For the Memphis, Tn. National Weather Service.
I`m station Bemis { Jackson 4 SE }
I`m set up to make All of my Observations at Midnight.
Sometimes the midnight temp could very well turn out to be the Hi for the given day that startes at 12:01 a.m.
Or the Lo may drop to its lowest degree at 11:59 p.m. making it the Lo for that day that ends at 11:59 p.m.
This happens often here in Jackson, Tn. in the Winter Season or like you said before and after a storm system passes.
I really enjoy going outdoors and making my obs. at midnight everynight.
After all midnight is the Official time change for each day.
Several other coop. stations are set up for a 7 a.m. observation.
Great question and answer.
William, that’s a great point. I showed an example of an early morning high, but that same concept works in both directions: an early morning high and a late-night low. While it makes sense to delineate a “day” as running from midnight to midnight, it can still artificially affect the temperatures recorded as the high and low for a sunrise-sunrise cycle.
I also didn’t touch on the idea of record low maxima and record high minima — the record coldest high temperature and the record warmest low temperature for a given day.
i am learning so much about the weather. i’m totally gettin’ my weather nerd on. i love the column. thanks weather dan. muah.
Yes Mr. Weather Man Dan : That is another good point : Record Warmest Lo`s and Coldest Hi`s : This is what I call them.
They are records also though the Record Hottest Hi and Coldest Lo gets the main attention.
I wish the N.W.S. would mention these as well.
I have All 4 Record Extremes Listed for my area on My Main Blog that you see when you click on my Name here.
I bet being over there on the Atlantic Coast : You have even more extremes than what I have.
Thanks for such a informative Blog.
Dan, is the midnight to midnight thing standard? So if the high/low for Tuesday is 70/40 and Wednesday is 80/60, that means (assuming standard diurnal pattern) Monday into Tuesday was a cold night and Tuesday into Wednesday was warmer? That part always confuses me when I’m trying to decide whether I can handle flip flops at night.
Hey Melissa : I`m sure that Dan wil agree with me that Midnight to Midnight is Standard with the N.W.S. and has been since they started back in 1870 or so.
I believe midnight has been the standard time for keeping days seperated since time has existed.
That is why I like making my observations at midnight.
Yesterday here in Bemis – Jackson, Tn. The Lo ended up happening at 11:59 p.m. rather than at sunrise.
it was 53 ° at sunrise but dropped to 49 ° by 11:59 p.m. thus the Calendar day Lo was 49 °
You really will not notice it say a year or more later when you look at the records unless you make a note of it.
Thanks for such a great topic and allowing me to make my comments about it.
William: The NWS office for Richmond is Wakefield, VA and they actually do keep track of all four extrema and will issue Record Event Reports if they are set. The other Virginia offices – Blacksburg and Sterling – do the same.
Richmond’s daily record extrema are located here: http://www.erh.noaa.gov/er/akq/climate/RIC_Climate_Records.pdf
For NWS and climatological purposes, yeah, midnight to midnight is the standard. The highs and lows are based on a calendar day and not any other definition of a 24-hour period.
You pretty much nailed it, Melissa. It means that temps fell into the 40s Monday night, but only into the 60s Tuesday night before bottoming out Wednesday morning. The caveat here is that there are many times where the pattern isn’t standard, and I’m not even taking into account TV forecasts, where they may not forecast the same way. I would imagine (but cannot verify) that the low temperature forecasts are issued for the entire nighttime period between two days – so if a TV station forecasts a low of 45 for Tuesday night, it means the period between sunset Tuesday night and sunrise Wednesday morning. Again, all TV stations are different.
But really, is it ever too cold for flip flops at night?
Oh, and Homeslice? You’re totes welcome.
Hey Weather Man Dan : I`m glad to know that Wakefield and the other N.W.S. Offices you mentioned there do mention all 4 Temp records.
Memphis, Tn`s Does Not However.
That is why I purchased them back in 1994 for My area and found them very interesting to know and keep up with.