Can I use Kelvin for temperature..

marint ~ It's not exactly what you want, but you can create a simple custom shortcut in Apple's Shortcuts app. The shortcut below shows just the current temperature in kelvins – but you can modify it to also display the forecast in kelvins...

...And here's how it's done:

Here are all the Weather actions:

Shortcuts User Guide for iPhone and iPad

Shortcuts User Guide for Mac (macOS Monterey)

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No, you cannot. No one here in this user-to-user forum can make it so that you can. You can, however, let Apple know your thoughts here:

Product Feedback - Apple