The conversion

99°F = 37.22°C

°C = (°F − 32) × 5/9, so (99 − 32) × 5/9 = 37.22.

As an outdoor air temperature, 99 °F sits in the hot band. The other scale is on the dial beside it in the app, so a reading like this needs no conversion at all.

Convert something else

68°F

The range either side

Neighbouring values, for when the number you actually wanted was a degree or two away.

Fahrenheit to Celsius either side of 99 °F.
FahrenheitCelsius
94 °F34.44 °C
95 °F35 °C
96 °F35.56 °C
97 °F36.11 °C
98 °F36.67 °C
99 °F37.22 °C
100 °F37.78 °C
101 °F38.33 °C
102 °F38.89 °C
103 °F39.44 °C
104 °F40 °C

The other direction

37.22 °C converts back to 99 °F, which is the same relationship read the other way. Going from Celsius to Fahrenheit, multiply by 9/5 first and then add 32. Adding first is the usual mistake.

The full converter handles any value, in either direction, including kelvin, and shows the formula as it works.