Is this a run on or complete sentence?

The desert is very hot during the day, the temperature in the desert may drop below freezing at night.

Technically, it's called a comma splice -- but yes, it's a run-on, too.

http://grammar.ccc.commnet.edu/grammar/runons.htm

This sentence is a run-on sentence because it consists of two independent clauses that are not properly connected or separated. To determine if a sentence is a run-on or complete sentence, you can follow these steps:

1. Identify the subject and verb in each clause:
- First clause: Subject - "The desert" / Verb - "is"
- Second clause: Subject - "the temperature" / Verb - "may drop"

2. Determine if the two clauses can stand alone as separate sentences:
- The desert is very hot during the day. (Could be a complete sentence)
- The temperature in the desert may drop below freezing at night. (Could be a complete sentence)

Since both clauses are complete sentences, they should be either separated into two sentences or properly connected using coordinating conjunctions, such as "and," "but," or "so."