Function to convert station pressure to SLP

weather/station_slp.js

@@ -0,0 +1,41 @@
+const states = global.get('homeassistant.homeAssistant.states')
+const temp_F = states['sensor.home_tempest_temperature'].state
+const elevation_ft = 693
+
+// Get the pressure from the payload
+let pressure_inHg = msg.payload
+
+if (isNaN(pressure_inHg)) {
+    node.error("Invalid pressure value in msg.payload: " + msg.payload)
+    msg.payload = null
+    return null
+}
+
+// Convert pressure from inHg to hPa
+let pressure_hPa = pressure_inHg * 33.8639
+
+// Convert temperature to °C
+let temp_C = (temp_F - 32) * (5 / 9)
+
+// Convert elevation from feet to meters
+let elevation_m = elevation_ft / 3.28084
+
+// Apply the barometric formula
+let slp = pressure_hPa * Math.pow(
+    1 - ((0.0065 * elevation_m) / (temp_C + (0.0065 * elevation_m) + 273.15)),
+    -5.257
+)
+
+// Optional: Convert back to inHg
+let slp_inHg = slp / 33.8639
+
+// Round both values
+slp = Math.round(slp * 100) / 100
+slp_inHg = Math.round(slp_inHg * 100) / 100
+
+// Return the SLP value
+msg.payload = slp_inHg
+
+node.status({fill:'Success',shape:'dot',text:'green'})
+
+return msg