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?

April 20, 2010; 9:38 AM • by Weather Dan

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.

April 20, 2010 9:38 AM 10