What is the best alternative to Thermo-hygrometer on iPhone?

Barometer & Altimeter & Thermo, S tier, #1 in barometer.

Is there a free alternative to Thermo-hygrometer?

Yes: Barometer & Altimeter & Thermo (S tier, #1) is free to download, with in-app purchases.

Yes. 3 apps rank above it in barometer: Barometer & Altimeter & Thermo (#1), Thermometer++ App (#2), WeatherX Forecast (#3).

Thermo-hygrometer is B tier, #4 of 6 in barometer. Here is everything that competes with it on iPhone, in tier order.

1
Barometer & Altimeter & ThermoShigher tier
Free + IAP · ★ 4.8 · 9,616 ratings · score 93
Measures atmospheric pressure and altitude, using GPS, device sensors and nearby weather station data.
2
Thermometer++ AppAhigher tier
Free download · ★ 4.7 · 7,288 ratings · score 83
Shows current outdoor temperature, humidity, pressure, and wind for your location or a location chosen on a map.
3
WeatherX ForecastAhigher tier
Free · ★ 4.3 · 2,357 ratings · score 83
Shows a 5-day barometric pressure forecast and history with push alerts, meant to be used alongside WeatherX earplugs.
5
Gauges: Speedometer & SensorsB
Free download · ★ 4.6 · 3,079 ratings · score 78
Live gauges for iPhone and iPad measuring speed, altitude, pressure, compass, sound, light and other sensors.
6
eWeather HD - Weather & AlertsC
$3.99 · ★ 4.7 · 8,819 ratings · score 69
Weather app with hourly and 15-day forecasts, radar, lightning tracking, severe alerts, tides and air quality.

What is the best alternative to Thermo-hygrometer on iPhone?: Barometer & Altimeter & Thermo, S tier, #1 in barometer.
Is there a free alternative to Thermo-hygrometer?: Yes: Barometer & Altimeter & Thermo (S tier, #1) is free to download, with in-app purchases.
Does anything rank above Thermo-hygrometer?: Yes. 3 apps rank above it in barometer: Barometer & Altimeter & Thermo (#1), Thermometer++ App (#2), WeatherX Forecast (#3).