Dew Point Calculator: Arduino Humidity and Temperature

The dew point is one of the most useful weather parameters for everyday life, yet most hobbyist weather stations ignore it. It tells you exactly how muggy the air feels, when fog will form, and when condensation will damage electronics or crops. In this guide, we build an Arduino-based dew point calculator using temperature and humidity sensors — essential for anyone living in India’s humid coastal and plains regions.

What Is Dew Point and Why It Matters

The dew point temperature is the point at which air becomes saturated and water vapour begins to condense into liquid droplets. When the air temperature drops to the dew point, you get dew on surfaces, fog in the air, or clouds in the sky.

For practical purposes, dew point tells you the absolute moisture content of the air. Unlike relative humidity (which changes with temperature), dew point is an absolute measure. A dew point of 24°C feels tropical and uncomfortable regardless of whether it is 28°C or 40°C outside. In Indian cities like Mumbai, Chennai, and Kolkata, summer dew points regularly exceed 26°C — among the highest in the world.

Dew Point vs Relative Humidity

Many people confuse dew point with relative humidity, but they measure different things:

• Relative humidity (RH) — The percentage of maximum moisture the air can hold at its current temperature. A desert at 45°C with 20% RH and a hill station at 15°C with 80% RH can have similar actual moisture content.
• Dew point — The absolute moisture level. A dew point of 20°C means the same amount of moisture regardless of the air temperature.

Comfort scale based on dew point:

• Below 10°C: Dry and comfortable (typical of Ladakh, Rajasthan winter)
• 10-15°C: Pleasant (Bangalore, Pune winter)
• 16-20°C: Slightly humid (Delhi winter, Bangalore summer)
• 21-24°C: Humid, uncomfortable for outdoor work (Mumbai post-monsoon)
• Above 24°C: Oppressive, heat stroke risk increases (Mumbai/Chennai summer, pre-monsoon everywhere)

Sensors for Dew Point Calculation

To calculate dew point, you need both temperature and humidity. The sensor choices are:

• DHT22 — ±0.5°C temperature, ±2% humidity. The most popular choice for dew point calculation. Accuracy is sufficient for comfort monitoring and most applications.
• BME280 — ±1°C temperature, ±3% humidity, plus barometric pressure. Slightly less accurate humidity than DHT22 but adds pressure data for comprehensive weather monitoring.
• DHT11 — ±2°C temperature, ±5% humidity. Budget option for learning projects, but humidity accuracy is too poor for reliable dew point calculation.
• DHT20 — Next-generation I2C sensor with improved accuracy over DHT11. Good middle ground between DHT11 and DHT22.

Magnus Formula for Dew Point

The Magnus formula (also called the Magnus-Tetens approximation) calculates dew point from temperature (T in °C) and relative humidity (RH in %):

// Magnus Formula Constants
// a = 17.27, b = 237.7°C (for -45°C to 60°C range)
// gamma = (a * T) / (b + T) + ln(RH / 100)
// Dew Point = (b * gamma) / (a - gamma)

This formula is accurate to within ±0.4°C for temperatures between 0°C and 60°C — covering the entire range of Indian ambient conditions. For sub-zero temperatures (Himalayan regions), use the August-Roche-Magnus variant with a = 17.625 and b = 243.04.

Arduino Dew Point Calculator Code

Here is the complete Arduino sketch for a dew point calculator with DHT22:

#include <DHT.h>

#define DHTPIN 2
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  dht.begin();
}

float calcDewPoint(float t, float rh) {
  float a = 17.27;
  float b = 237.7;
  float gamma = (a * t) / (b + t) + log(rh / 100.0);
  return (b * gamma) / (a - gamma);
}

void loop() {
  float temp = dht.readTemperature();
  float hum = dht.readHumidity();

  if (isnan(temp) || isnan(hum)) {
    Serial.println("Sensor error!");
    delay(2000);
    return;
  }

  float dewPoint = calcDewPoint(temp, hum);
  float heatIndex = dht.computeHeatIndex(temp, hum, false);

  Serial.print("Temp: "); Serial.print(temp); Serial.print("°C  ");
  Serial.print("RH: "); Serial.print(hum); Serial.print("%  ");
  Serial.print("Dew Point: "); Serial.print(dewPoint); Serial.print("°C  ");
  Serial.print("Comfort: ");

  if (dewPoint < 10) Serial.println("Dry");
  else if (dewPoint < 16) Serial.println("Pleasant");
  else if (dewPoint < 21) Serial.println("Slightly Humid");
  else if (dewPoint < 24) Serial.println("Humid");
  else Serial.println("Oppressive!");

  delay(2000);
}

Applications in Agriculture and HVAC

Dew point monitoring has important practical applications in India:

• Agriculture — When dew point equals air temperature, dew forms on crops. This promotes fungal diseases like powdery mildew and late blight. Alerting farmers 2-3 hours before dew formation allows preventive fungicide application.
• HVAC and building management — If the wall surface temperature drops below the dew point, condensation forms inside walls, causing mould. Dew point monitoring prevents this in air-conditioned buildings, server rooms, and museums.
• Electronics storage — Components stored in humid environments without dew point monitoring can develop corrosion. A dew point threshold alarm at 18°C triggers dehumidifiers automatically.
• Paint and coating — Industrial painting standards require the surface temperature to be at least 3°C above the dew point. A portable dew point calculator helps ensure proper coating conditions on construction sites.

Building a Dew Point Alert System

Add a buzzer and relay to create an automatic alert system. When the dew point approaches within 2°C of the current temperature (indicating imminent fog or condensation), the system triggers an alarm and can activate a dehumidifier or heater via the relay.

For agricultural use, pair the sensor with a soil temperature probe (DS18B20). If the dew point is higher than the ground temperature, dew will form on low-lying crops. Send an SMS alert using a GSM module or push notification via ESP32 WiFi.

Dew Point Mapping Across Indian Regions

India’s dew point geography is fascinating. Mumbai averages 24-26°C dew points in June-September — among the highest of any major city globally. Delhi sees extreme seasonal swings from 2°C in December to 26°C in July. Bangalore maintains a comfortable 14-18°C year-round, which is why it feels pleasant despite similar temperatures to other cities.

By setting up multiple sensors across a farm or campus, you can map dew point microclimates. Low-lying areas near water bodies will have higher dew points than elevated, exposed areas — even just 50 metres apart. This data helps optimise crop placement and building orientation.