December weather - Madrid vs Seville

For simple comparisons between cities, I go to their entries in Wikipedia, where there is virtually always a climate-summary chart showing monthly averages. The average December high temperature in Seville is 12 degrees (F) higher than the average in Madrid. January is a tiny bit cooler in both cities, which means the second half of December could be trending down. A major reason for the temperature difference in the two cities is elevation: Seville is at sea level, whereas Madrid is at over 2000 feet. Barcelona and Valencia are also considerably warmer than Madrid in the winter.

To decide whether I'm willing to tolerate the range of weather I might experience in a particular location, I much prefer to look at the actual, historical, day-by-day weather statistics on the website timeanddate.com rather than averages. Averages hide extremes, and it is the extremes that can make you miserable (if you're temperature-sensitive). I focus on the weather data for the most recent five years.

Seville weather -- December 2023

Madrid weather -- December 2023

Weather varies a lot from year to year, so use the pull-down box at the right, just above the graph, to check other recent years.

Keep in mind that Spaniards eat dinner very late, typically after 9:30 or 10 PM. It will be quite cold in Madrid as you walk back to your hotel after dinner, if you eat on the local schedule. Even in the other cities I've mentioned, you'll need warm outer clothing at that time of night.

I was just in Seville through November 21 and in Madrid on November 22 and (without actually checking the data) 10 degrees cooler sounds about right to me.

In Seville, I was comfortable in a light, long sleeved layer plus a sweater. It felt like fall. The next day, In Madrid, I was glad to have those, plus my jacket. I saw people bundled in warmer coats.

Ok, it gives you the averages for what they're worth nowadays but this site is fun for comparing:
https://weatherspark.com/compare