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The Where Is Webb page shows an empty temperature tab:

Enter image description here

Reading the About this Page, it is delayed by 1-2 days after the launch:

NOTE:
Temperature data will begin updating 1-2 days after launch.
Displays will read "---" until that point.
Screenshot

Why won't they show the temperatures right away?

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    $\begingroup$ Dude the temperatures cant be displayed until sunshield is open $\endgroup$
    – Stars Tracker
    Commented Dec 28, 2021 at 15:33
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    $\begingroup$ @StarsTracker Dude, if that is true, how about making it an answer? $\endgroup$
    – CGCampbell
    Commented Dec 28, 2021 at 15:41
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    $\begingroup$ For the silence downvoter, please let me know how I can improve my question. If you've downvoted because it's very obvious, please post an answer. $\endgroup$
    – 0stone0
    Commented Dec 28, 2021 at 16:10
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    $\begingroup$ Until the sunshade is deployed, there is no hot and cold side. It's like asking which size of your pizza the cheese is on, when you are still kneading the dough. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Commented Dec 28, 2021 at 21:38
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    $\begingroup$ FWIW it's up now. $\endgroup$
    – Ben Bolker
    Commented Dec 29, 2021 at 16:38

2 Answers 2

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According to the NASA blog on Dec 26:

Separately, overnight, the temperature sensors and strain gauges on the telescope were activated for the first time. Temperature and strain data are now available to engineers monitoring Webb’s thermal and structural systems.

Presumably the temperature data is currently (as of Dec 28) being calibrated as the sunshield unfurls, and will be streamed to their website when it is normalized.

edit: The temperature is now showing, accompanied by this message:

There are many more temperature sensors on the observatory that our engineers monitor throughout the deployment, commissioning and operations processes. The data points shown here give a good overall indication of the temperature trends on each side of the sunshield as we move through deployment and commissioning. They illustrate the great contrast between the hot and cold sides of the spacecraft and the incredible engineering and effectiveness of the sunshield. These temperature observations are reported daily from actual spacecraft telemetry data.

I think this supports my guess that there are a bunch of sensors and it took a few days to turn them into data they found acceptable for the website.

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The cold side should be at 50 K (−223.2 °C; −369.7 °F). If they use thermocouples for measurement, they may use only type T (copper–constantan Cu-CuNi) for temperatures below -200 °C. The voltage of type T is negative for temperatures below 0 °C.

If they digitize only negative voltages, they can't display temperatures above 0 °C. To display no erroneous temperatures, they replace the digits with "---".

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    $\begingroup$ This sounds ... like speculation? Lot's of "if they do this" implies you don't actually know? $\endgroup$
    – davidbak
    Commented Dec 28, 2021 at 22:37
  • $\begingroup$ Re "The voltage of type T is negative...": Implying it is not physically possible(?) (not just a measurement problem) $\endgroup$
    – Peter Mortensen
    Commented Dec 29, 2021 at 3:38

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