Rh Calculator

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Relative Humidity Calculator

Use this calculator to determine the relative humidity based on the dry bulb temperature and dew point temperature.

function calculateRelativeHumidity() { var dryBulbTempInput = document.getElementById("dryBulbTemp").value; var dewPointTempInput = document.getElementById("dewPointTemp").value; var resultDiv = document.getElementById("rhResult"); var T = parseFloat(dryBulbTempInput); var Td = parseFloat(dewPointTempInput); if (isNaN(T) || isNaN(Td)) { resultDiv.innerHTML = "Please enter valid numbers for both temperatures."; return; } if (Td > T) { resultDiv.innerHTML = "Dew Point Temperature cannot be higher than Dry Bulb Temperature."; return; } // Magnus formula constants for T in Celsius var A = 6.112; var B = 17.67; var C = 243.5; // Saturation vapor pressure at dry bulb temperature (Es) var Es = A * Math.exp((B * T) / (C + T)); // Actual vapor pressure at dew point temperature (E) var E = A * Math.exp((B * Td) / (C + Td)); // Relative Humidity (RH) var RH = (E / Es) * 100; resultDiv.innerHTML = "

Calculated Relative Humidity:

" + "" + RH.toFixed(2) + "%"; } .rh-calculator-container { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; background-color: #f9f9f9; padding: 25px; border-radius: 10px; box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1); max-width: 500px; margin: 30px auto; border: 1px solid #e0e0e0; } .rh-calculator-container h2 { color: #2c3e50; text-align: center; margin-bottom: 20px; font-size: 1.8em; } .rh-calculator-container p { color: #555; line-height: 1.6; margin-bottom: 15px; } .rh-input-group { margin-bottom: 18px; } .rh-input-group label { display: block; margin-bottom: 8px; color: #34495e; font-weight: bold; font-size: 1em; } .rh-input-group input[type="number"] { width: calc(100% – 22px); padding: 12px; border: 1px solid #ccc; border-radius: 6px; font-size: 1em; box-sizing: border-box; transition: border-color 0.3s ease; } .rh-input-group input[type="number"]:focus { border-color: #007bff; outline: none; box-shadow: 0 0 5px rgba(0, 123, 255, 0.3); } .rh-calculator-container button { background-color: #007bff; color: white; padding: 12px 25px; border: none; border-radius: 6px; cursor: pointer; font-size: 1.1em; font-weight: bold; display: block; width: 100%; margin-top: 25px; transition: background-color 0.3s ease, transform 0.2s ease; } .rh-calculator-container button:hover { background-color: #0056b3; transform: translateY(-2px); } .rh-calculator-container button:active { transform: translateY(0); } .rh-result { margin-top: 30px; padding: 15px; background-color: #e9f7ef; border: 1px solid #d4edda; border-radius: 8px; text-align: center; font-size: 1.1em; color: #155724; } .rh-result h3 { color: #28a745; margin-top: 0; margin-bottom: 10px; font-size: 1.4em; } .rh-result p { margin: 0; font-size: 1.6em; font-weight: bold; color: #007bff; } .rh-result p.error { color: #dc3545; font-weight: normal; font-size: 1em; }

Understanding Relative Humidity

Relative humidity (RH) is a crucial metric in meteorology, HVAC, agriculture, and even personal comfort. It quantifies the amount of water vapor present in the air relative to the maximum amount of water vapor the air can hold at a given temperature. Expressed as a percentage, 100% relative humidity means the air is completely saturated with water vapor, and any further cooling or addition of moisture will lead to condensation (e.g., dew, fog, rain).

Why is Relative Humidity Important?

  • Human Comfort: High RH can make hot temperatures feel even hotter because sweat evaporates more slowly from the skin, hindering the body's natural cooling process. Low RH can lead to dry skin, irritated nasal passages, and static electricity.
  • Health: Maintaining optimal RH levels (typically between 30-50%) can help prevent the growth of mold and dust mites (which thrive in high humidity) and reduce the spread of airborne viruses (which can survive longer in very low humidity).
  • Building Integrity: Excessively high humidity can lead to condensation within walls, promoting mold growth, wood rot, and damage to building materials. Very low humidity can cause wood to shrink and crack.
  • Industrial Processes: Many manufacturing processes, from textiles to electronics, require precise humidity control to ensure product quality and prevent damage.
  • Agriculture: RH plays a vital role in plant growth, affecting transpiration rates and the risk of fungal diseases.

How is Relative Humidity Calculated?

Relative humidity is typically calculated using two primary temperature measurements:

  1. Dry Bulb Temperature (DBT): This is the ambient air temperature measured by a standard thermometer. It's simply the temperature of the air.
  2. Dew Point Temperature (DPT): This is the temperature to which air must be cooled at constant pressure for water vapor to condense into liquid water (dew). When the air temperature cools to the dew point, the relative humidity becomes 100%.

The calculator above uses a common approximation based on the Magnus formula to determine relative humidity from these two temperatures. The core idea is to compare the actual vapor pressure in the air (derived from the dew point) to the maximum possible vapor pressure the air could hold at its current dry bulb temperature (saturation vapor pressure).

Using the Relative Humidity Calculator

To use the calculator:

  1. Enter Dry Bulb Temperature: Input the current air temperature in degrees Celsius.
  2. Enter Dew Point Temperature: Input the dew point temperature in degrees Celsius.
  3. Click "Calculate Relative Humidity": The calculator will instantly display the relative humidity percentage.

Ensure that your dew point temperature is not higher than your dry bulb temperature, as this would indicate an impossible physical condition (unless the air is supersaturated, which is rare and unstable).

Examples:

Let's look at a few scenarios:

  • Scenario 1: Comfortable Day
    • Dry Bulb Temperature: 25°C
    • Dew Point Temperature: 15°C
    • Calculated RH: Approximately 53.6% (A comfortable level for most people.)
  • Scenario 2: Humid Summer Day
    • Dry Bulb Temperature: 30°C
    • Dew Point Temperature: 25°C
    • Calculated RH: Approximately 74.7% (This would feel quite muggy and uncomfortable.)
  • Scenario 3: Dry Winter Day
    • Dry Bulb Temperature: 10°C
    • Dew Point Temperature: 0°C
    • Calculated RH: Approximately 49.8% (Still within a reasonable range, but could feel dry indoors if heated.)
  • Scenario 4: Air Nearing Saturation
    • Dry Bulb Temperature: 18°C
    • Dew Point Temperature: 17°C
    • Calculated RH: Approximately 94.1% (The air is nearly saturated; fog or condensation is likely.)

By understanding and calculating relative humidity, you can better assess environmental conditions for comfort, health, and various applications.

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