Heat index calculator

This heat index calculator helps you determine the heat index or ‘feels-like’ temperature. You can calculate the heat index using either air temperature and relative humidity or air temperature and dew point temperature.

The Heat Index is an important factor in assessing human comfort and the risk of heat-related illnesses. It’s particularly useful in situations where high temperatures and humidity levels can lead to dangerous heat conditions.

Heat index using air temperature and humidity

Heat index using air temperature and dew point temperature

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What is the heat index?

The heat index, also known as the “feels-like” temperature, is a measure of how hot it feels to the human body when relative humidity is factored in with the actual air temperature. It is a useful metric for assessing the perceived discomfort and health risks associated with high temperatures and humidity. The heat index is particularly relevant in assessing the impact of heat on the human body, as high humidity makes it more difficult for the body to cool itself through the evaporation of sweat.

What is humidity?

Humidity refers to the amount of water vapor or moisture present in the air. It is an essential component of the Earth’s atmosphere and plays a crucial role in weather and climate. Humidity is typically expressed as a percentage, known as relative humidity, and it indicates how close the air is to being saturated with moisture.

When the relative humidity is 100%, the air is holding the maximum amount of water vapor it can at a given temperature, and this often leads to the formation of clouds and precipitation. Lower relative humidity values indicate drier air, while higher values suggest that the air is closer to being fully saturated with moisture. Humidity levels can affect our comfort, as high humidity can make the air feel warmer and more uncomfortable, while low humidity can make the air feel dry and cooler.

There is also absolute humidity.

Absolute humidity refers to the actual amount of water vapor or moisture present in a unit volume of air, typically expressed in grams of water vapor per cubic meter (g/m³) or grains of water vapor per cubic foot. It is a measure of the mass of water vapor in a given volume of air and is not dependent on the air’s temperature or pressure.

Absolute humidity is different from relative humidity, which is a ratio of the current absolute humidity to the maximum amount of water vapor that the air can hold at a particular temperature and pressure. Absolute humidity provides a more precise and direct measurement of the moisture content in the air, while relative humidity is a percentage that describes how close the air is to being saturated with moisture.

Scientists and meteorologists use absolute humidity in various applications, such as studying weather patterns, indoor air quality assessment, and industrial processes where precise control of moisture content is necessary.

What is the dew point temperature?

The dew point temperature is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid water, forming dew. In other words, it’s the temperature at which the air can no longer hold all the moisture it contains, and some of that moisture begins to change from a gas (water vapor) into a liquid (water droplets).

When the air temperature falls to the dew point temperature, you’ll often see dew forming on surfaces like grass, leaves, and car windshields in the morning. If the temperature falls below the dew point, you can also get frost forming on surfaces.

Dew point temperature is a useful metric in weather forecasting because it provides information about the moisture content in the air. When the air temperature is close to the dew point, it can indicate that the air is nearly saturated with moisture, which might lead to the possibility of fog, dew, or other weather conditions. It’s also used in various fields like agriculture, aviation, and meteorology to assess humidity and potential weather-related issues.

What is the effect of heat index on the body?

The effects of the heat index on the body are primarily related to how our body regulates temperature and how efficiently it can cool down. When the heat index is high, the body may struggle to dissipate heat effectively, leading to various heat-related illnesses. These can include:

1. Heat cramps: Muscle pains and spasms that usually occur during or after physical exertion in hot conditions.
2. Heat exhaustion: Symptoms can include heavy sweating, weakness, dizziness, nausea, and a rapid pulse. It can be a precursor to heatstroke if not treated.
3. Heatstroke: A life-threatening condition where the body’s core temperature rises dangerously high. Symptoms can include confusion, loss of consciousness, hot and dry skin, and seizures. Immediate medical attention is crucial for heatstroke.

To categorize the heat index and its associated risks, the National Oceanic and Atmospheric Administration (NOAA) in the United States uses the following categories in the shade:

1. Caution (80-90°F) / (26.7-32.2°C): In this range, people should be aware of the potential for heat-related illnesses, especially if they engage in prolonged outdoor activities.
2. Extreme caution (91-103°F) / (32.8-39.4°C): In this range, heat-related illnesses become more likely. It’s essential to take precautions, such as staying hydrated and seeking shade.
3. Danger (104-124°F) / (40-51.1°C): This range poses a significant risk of heat-related illnesses. Outdoor activities should be limited, and individuals should take necessary precautions to stay cool.
4. Extreme danger (125°F and above) / (51.7°C and above): Heat-related illnesses are highly likely in this range, and there’s an increased risk of heatstroke. Extreme caution and protective measures are crucial.

The table below, as calculated by the National Oceanic and Atmospheric Administration (NOAA), shows the heat index for different values of humidity in the shade only. According to NOAA, full exposure to sunshine can increase the hear index by up to 15°F (8°C).

It’s important to note that these categories can vary depending on regional standards, and it’s essential to follow local advisories and take precautions when high heat index values are forecasted, such as staying hydrated, wearing appropriate clothing, and seeking shelter during extreme conditions.