The dew point depends on how much water vapor is present in the air. If the air is very dry with few water molecules, the dew point is low, and surfaces must be much colder than the air for condensation to occur. If the air is very humid with a high concentration of water molecules, the dew point is high, and condensation can occur on surfaces only slightly cooler than the air.
What is dew point?
In meteorology, dew point refers to the temperature point at which the gaseous water in the air is just saturated and condenses into liquid water under a fixed air pressure. At this temperature, condensed liquid water becomes fog if it floats in the air, and becomes dew if it sticks to a solid surface.
In everyday life, we often observe that when the air temperature drops at night, a portion of the moisture in the air condenses to form dew or frost. This happens because the lowering temperature causes the previously unsaturated water vapor in the air to gradually become saturated, leading to the condensation of water from the air. Dew point is a crucial meteorological parameter used to predict the formation of dew, frost, fog, nighttime low temperatures, and even rainfall and tornadoes. It provides valuable insights into the moisture content of the air.
What will affect it?
Dew point vs humidity
Humidity is an important factor affecting the dew point and includes both absolute humidity and relative humidity.
Absolute humidity refers to the actual mass of water vapor contained in a certain volume of air, typically expressed as the number of grams of water vapor per cubic meter of air.
Relative humidity, on the other hand, is the percentage of water vapor pressure in the air compared to the saturation water vapor pressure at the same temperature. It is commonly expressed as a percentage.
High relative humidity indicates that the dew point is close to the current air temperature. A relative humidity of 100% means that the dew point is equal to the current temperature, and the air is saturated with moisture. When the moisture content remains constant and the temperature increases, relative humidity decreases, but the dew point remains unchanged.
| Temperature (°C) | Relative Humidity (%) | Dew Point Temperature (°C) |
|---|---|---|
| 30 | 10 | 1.0 |
| 30 | 20 | 5.4 |
| 30 | 30 | 9.1 |
| 30 | 40 | 12.3 |
| 30 | 50 | 15.0 |
| 30 | 60 | 17.3 |
| 30 | 70 | 19.4 |
| 30 | 80 | 21.3 |
| 30 | 90 | 23.1 |
| 25 | 10 | -3.3 |
| 25 | 20 | 1.2 |
| 25 | 30 | 5.0 |
| 25 | 40 | 8.3 |
| 25 | 50 | 11.0 |
| 25 | 60 | 13.3 |
| 25 | 70 | 15.4 |
| 25 | 80 | 17.3 |
| 25 | 90 | 19.1 |
Dew point is originally a temperature value, but it is used to represent humidity because when the air is already saturated with moisture, the air temperature is the same as the dew point temperature. When the air is not saturated, the air temperature is always higher than the dew point temperature. Therefore, the difference between the air temperature and the dew point temperature can indicate the degree of moisture in the air relative to saturation. At 100% relative humidity, the surrounding environment’s temperature is the same as the dew point temperature. The smaller the dew point temperature is compared to the environmental temperature, the less likely condensation is to occur, indicating drier air.
General aviation pilots use this data to calculate the likelihood of carburetor icing and fog formation and estimate the height of cumuliform cloud bases.
Increasing atmospheric pressure will raise the dew point. This means that if pressure increases, you must reduce the mass of water vapor per unit volume of air to prevent condensation. For example, consider New York City (elevation 33 feet or 10 meters) and Denver (elevation 5,280 feet or 1,610 meters). Because Denver’s elevation is higher than New York’s, its atmospheric pressure is generally lower. This means that if the dew point and temperature are the same in both cities, the air in Denver contains more water vapor.
Video from @WeLoveWeatherTV
Dew point vs temperature
Apart from humidity, temperature is the second influencing factor. At higher temperatures, the air can hold more moisture. As the temperature increases, the kinetic energy of water molecules increases, making it easier for water molecules to evaporate from the surface of liquid water into the air. This leads to an increase in the vapor pressure of water in the air, so at a constant relative humidity, the dew point also rises.
In winter, when we move from the cold outdoors to a warm indoor environment while wearing glasses, you may notice that a white fog quickly forms on the lenses. This occurs because the temperature rises, but the water vapor pressure also increases. When the surface temperature of the glasses is lower than the dew point temperature of the air, water vapor condenses on the cooler surface of the glasses, forming small water droplets, much like dew. Once the air is saturated, any surface with a temperature equal to or lower than the air temperature will undergo condensation.
What is a comfortable dew point?
In hot summer weather, the human body cools down through the evaporation of sweat. The faster sweat evaporates, the quicker the body temperature decreases. The rate of sweat evaporation is influenced by the moisture content in the environment and the capacity of the environment to hold water vapor. If the air is already saturated with moisture (humid), sweat won’t evaporate efficiently. In such conditions, the body’s temperature regulation relies on sweating, even if the rate of sweating exceeds the rate of evaporation. Therefore, in humid summers, even with normal sweating, people can still feel uncomfortably warm.
When the air around the body is heated by body heat, it rises and is replaced by other air. Moving air away from the body, either naturally through a breeze or with a fan, promotes faster sweat evaporation, effectively cooling the body. The more sweat that doesn’t evaporate, the greater the discomfort.
So, the dew point can affect people’s comfort, and both high and low dew points can lead to discomfort. For most people, dew points between 50-60°F are generally considered comfortable. When the dew point goes above 61°F, people may feel hot and sticky, and the perceived temperature rises. On the other hand, dew points in the 30s or lower can make the skin feel very dry. If dry air is a concern, using a humidifier can be helpful.
Why is dew point important?
Dew point is a fundamental indicator used to measure the state of the atmosphere. Meteorologists calculate the dew point by measuring and computing the moisture content and temperature of the air. It holds significant importance in weather forecasting and warnings of meteorological disasters. When the dew point in the air approaches or falls below 0 degrees Celsius, it can lead to icy weather conditions, adversely affecting transportation, agriculture, aviation, and other sectors. Therefore, timely predictions are crucial for disaster prevention and mitigation.
Beyond its meteorological impact, dew point is also essential in engineering, agriculture, and environmental fields. In engineering, monitoring moisture content in the air can predict the corrosion and damage levels of equipment and materials. In agriculture, it helps farmers assess the growth and development of crops in the field, aiding in the proper scheduling of irrigation and fertilization. In the environmental realm, it can be used to assess pollution levels and humidity in the air, providing data support for environmental protection and improvement efforts.
How to measure dew point?
Monitoring dew point is an important way to understand the moisture content in the air, particularly in fields such as meteorology, cooling, air conditioning, and climate control. Here are some methods that can help you:
Using Calculation Formulas
You can estimate the dew point temperature using dew point formulas. One common calculation formula is the Magnus-Tetens formula:
Td = (237.3 * (ln(RH/100) + (17.27 * Tc) / (237.3 + Tc))) / (17.27 - ln(RH/100) - (17.27 * Tc) / (237.3 + Tc))
Among these, Td represents the dew point temperature (in degrees Celsius), RH is the relative humidity (in percentage), and Tc is the current temperature (in degrees Celsius). You can implement this formula using programming languages or spreadsheet software.
Using Sensors
Dew point sensor is a device that can directly measure the dew point temperature. You can purchase these sensors and integrate them into your project to monitor this data in real-time.
Referencing Open Source Data
Some meteorological data providers and organizations offer free meteorological data, including temperature, humidity, dew point, and more. You can search for and use this data. For example, the National Weather Service in the United States provides free meteorological data.
Self-Observation
If necessary, you can conduct self-observations using basic instruments such as temperature sensors, humidity sensors, and pressure sensors, and then calculate the dew point based on the observed results.
