Temperature vs Temperatures

Solution 1:

When you're talking about multiple readings of the thermometer: "With temperatures in the 90s this week, people should take precautions against heat exhaustion." 91° on Monday, 93° on Tuesday, 90° on Wednesday -- multiple readings, so temperatures plural.

(For those who speak Celsius, that's 32° - 37°.)

In your example, during the day of the cycling and baby-finding the temperature changed.

Solution 2:

News reports often use temperatures in place of temperature measurements, i.e. as a shorthand for official temperature measurements from an unspecified number of regional/national weather stations. The journalist clearly didn't have a report of the temperature at that specific place and time.

Solution 3:

A noun may have several distinct meanings in English language depending on the context. As a result, a noun can be plural or singular (and also countable or uncountable) depending on the intended meaning:

It is possible to use temperature as an uncountable, noun meaning 'degree/intesity of heat',

• The temperatures in Paris and Berlin reached record heights yesterday.

When we use it countably, we think more of individual figures. When "temperature" means "a temperature reading/value" (also true for "pressure"), then it is countable.

• The temperature dropped to minus one degrees.

Usage examples-

• When the temperature cools, people settle outside to watch the great show of daily life as if it were a television series. (New York Times, Jun 19, 2015)
• They hide themselves in the landing gear where they are exposed to the elements and have to endure plummeting temperatures. (The Guardian,Jun 19, 2015)
• Suppose you added equal amounts of heat to equal masses of iron and aluminum. How do you think their temperatures would change? (wiki)
• The newborn baby was discovered by passing cyclists on a day when the temperature surpassed 40 degrees Celsius. (OP)

Usage notes-

You can use temperature single or plural with preposition. ( temperature or temperatures of ). . The singular form is usually precededed by the definite article. (the temperature)