This module adds a temperature field to Drupal 7; it uses the Field API. It has settings (via the temperature_ui module) that let users choose their preferred temperature unit and then it allows them to view the value of this field in their preferred value, if field settings allow such localization. It supports fahrenheit, celsius and kelvin. It also provides useful functions to format and convert temperature values. At this time is has a select list widget. I may add other widgets in the future if needed, such as a simple textfield.
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Here is a link to the sandbox project; please review the 7.x-1.x branch as this is where commits are being made.