Humidity is, in general, a measure of water vapor in the air. Meteorologists typically describe water vapor in the atmosphere in two different ways: absolute humidity and relative humidity. Dew point is an absolute measure and helps us understand how muggy and humid it feels outside.

Imagine: You take a nice, long, hot shower. Stepping out, you feel it’s pretty muggy and warm in the bathroom, and the mirror is foggy. You wipe your hand across the mirror and suddenly, your hand is all wet. That’s because of the moisture in the bathroom!

This happens because the foggy mirror is actually tiny droplets of water that have formed on the mirror. When you take a hot shower, water vapor in the bathroom increases due to evaporation from the shower. When that water touches the cool mirror, it condenses back to water, in the form of tiny droplets.

But, what does this all mean? What is humidity? Why does it get humid outside? And how does humidity affect our weather?

First, we have to talk about water vapor. Not knowing what water vapor is would be a MIST opportunity! (Get it?!)

What is water vapor?

Water is a super special substance. It is basically everywhere on Earth – in our soils, our oceans, and our air.

Water exists in three states – liquid, solid, and gas. Liquid water helps hydrate us during a hot day or after a long sports game. Solid water, or ice, is colder and gives us a surface to ice-skate on. Water in its gas form is water vapor, or moisture.

There is water vapor in our atmosphere because of evaporation. Evaporation happens when water changes from a liquid to a gas. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain.

So, what is humidity? Well, there are actually two different types of humidity!

What is absolute humidity?

Absolute humidity is the actual amount of water vapor in the air. The higher the amount of water vapor, the higher the absolute humidity.

What is relative humidity?

Relative humidity also measures water vapor in the air, but it is compared to the maximum amount of water vapor that can exist in the air at its current temperature. It is written as a percent.

For example, warm air can hold more water vapor, or moisture, than cold air. So, with the same amount of absolute humidity, cold air will have a higher relative humidity and warm air will have a lower relative humidity.

The relative humidity is 100% when the air is completely full of water vapor.

What is dew point?

The dew point marks the temperature at which water vapor will turn into liquid water droplets. This process is called condensation. Condensation is what causes clouds to form, which can then lead to precipitation such as rain, snow, and hail. The dew point will always either be the same or lower than the actual outside temperature.

The higher the dew point rises, the greater the amount of moisture in the air. This affects how "comfortable" it will feel outside. So, just reading the relative humidity can be misleading.

Quiz time! Which combination do you think would feel more humid? A 30℉ day with a relative humidity of 100%? Or an 80℉ day with a relative humidity of 50%?

Ready to check your answer? It would feel much more humid on the 80℉ day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So, if you want to know just how "dry" or "humid" it will feel outside, look at the dew point instead of the relative humidity. The higher the dew point, the muggier it will feel.

Check out the chart below to see the general comfort levels using dew point that can be expected during the summer months!

How does humidity affect our weather?

Basically, the more water vapor in our atmosphere, the more possibility of precipitation.

Precipitation happens when the air rises and causes condensation of water vapor into liquid water droplets. This is the beginning of the formation of precipitation! These little droplets grow through a process called "collision-coalescence". During collision-coalescence, these droplets come together and form larger drops. Once the drops are a certain size, they become too heavy for the air and the drops fall as precipitation from the clouds.

In order to determine the heat index using the chart above, you need to know the air temperature and the relative humidity. For example, if the air temperature is 100 ° F and the relative humidity is 55%, the heat index will be 124 ° F. When the relative humidity is low, the apparent temperature can actually be lower than the air temperature. For example, if the air temperature is 100 ° F and the relative humidity is 15%, the heat index is 96 ° F.

It surprises many people to learn that the heat index values in the chart above are for shady locations. If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F. As shown in the table below, heat indices meeting or exceeding 103 ° F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.

Use this weather calculator if you prefer to enter numbers manually instead of reading a chart. If you're really mathematically inclined, there is an equation that gives a very close approximation to the heat index. However, this equation was obtained using a multiple regression analysis, and therefore, it has an error of ±1.3 ° F.

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10^-3T² - 5.481717 x 10^-2R² + 1.22874 x 10^-3T²R + 8.5282 x 10^-4TR² - 1.99 x 10^-6T²R²

T - air temperature (F)
R - relative humidity (percentage)

Hot weather can lead to dehydration, heat exhaustion and heat stroke, but the dangers increase when you add humidity to the mix. When the temperature rises above 70F and the humidity registers more than 70 percent, you need to be on the alert.

Who’s most at risk?

People with high blood pressure, heart disease, lung disease or kidney disease are most vulnerable to the effects of humid conditions, as are those over age 50. Other risk factors that can affect your body’s ability to cool itself include being obese; having poor circulation; following a salt-restricted diet; drinking alcohol; having inefficient sweat glands; and taking diuretics, sedatives, tranquilizers or heart or blood pressure medication.

“Individuals who are at risk should discuss the potential impact of exercise and medication on their health with their physicians,” says Joseph A. Pellecchia, M.D., FACP, chief of staff and cardiologist at VA Medical Center in Huntington.

Exercising in humid weather

Since you generate heat during exercise, humidity can affect your heart rate when you work out, even at cooler temperatures. Your body cools itself by sweating—but only if sweat can evaporate. In humid weather, sweat evaporates more slowly, so your body temperature continues to rise.

What’s more, fluid loss from sweating decreases your blood volume. So while your heart is still working to cool you off, it must also work harder and faster to get that smaller amount of blood to your working muscles. That’s why it’s always important to replenish the fluids you lose by drinking plenty of water while you exercise.

Learn the warning signs

Recognize the signs of heat stress and take immediate action. Headache, fatigue, profuse sweating, muscle spasms or cramps, cold and clammy skin and swollen ankles and feet can mean you’re getting too hot. Move to a cool or air-conditioned area, drink fluids, shower in cool water and lie down.

Seek emergency medical help if symptoms don’t improve or if they progress to include nausea, dizziness, confusion, rapid pulse, high fever, fainting or warm, dry skin with no sweating.

Prevention is key

Pay attention to weather reports. Heat disorders can occur any time the temperature and humidity both rise above 70F and 70 percent respectively, or the heat index is greater than 80F. Drink plenty of fluids, avoid caffeine and alcohol (they’re dehydrating), limit activity to a cooler hour and wear light-colored clothing made of natural fabrics.

Home energy efficiency is often dominated by discussions about cooling and heating, which account for about 48% of energy costs in the average home. However, when your air has too much moisture, your home may feel warmer than the actual air temperature. Any discussion of energy efficiency, especially in a humid climate, must take moisture control into account.

Did you ever hear someone say the air in your house feels “close?” They’re likely talking about the “feels-like” temperature of the air because the indoor environment is holding a high amount of water vapor, and there’s less evaporation. These factors combine to create a sensation that the room is too warm and there’s less breathable air, despite the temperature reading on the thermostat.

A house with too much humid air in the “building envelope” can have issues with mold and mildew. Moist air can also provide a good environment for bacteria. All are known to adversely affect human health. Excess moisture in the air can take a toll on people, and on furniture, appliances, hardwood floors, carpeting, and, yes, the energy bill—chances are, you are setting your thermostat lower to maintain comfort as you offset the higher amount of humidity you feel in the air.

One solution to moisture control could be a whole-house or portable ENERGY STAR-rated dehumidifier that “wrings” moisture from the air. This is a big commitment that might not be right for everyone. Luckily there are other steps you can take to reduce the amount of moisture in your home.

• Check your clothes dryer. An improperly vented clothes dryer can dump some or all that water from your wet clothes into the air. Make sure your venting system is sealed all the way from the back of your dryer to the exhaust port on your home’s exterior. And be sure to clean the vent regularly, according to the manufacturer’s recommendations.
• Vent your stove and bathroom fans outside. Exhaust fans from cooking surfaces or the bathroom that are vented into your attic only redistribute humidity within your home. All such fans should be vented to the outside, with the system checked for leakage.
• Seal air leaks. Finding the places where outside air is leaking in, and conditioned air is leaking out, and sealing those leaks is a cost-effective way to improve air comfort and cut energy costs. Check your doors and windows for the effectiveness of air seals and caulk or weatherstrip as needed.
• Insulate water pipes. Condensation can occur when there is a difference between the temperature of your water pipes and the humid air in your home. Insulating your water pipes keeps this condensation from occurring on the cold water pipes. This condensation contributes to humidity problems in the home. Insulating hot water pipes eliminates heat loss between the hot water heater and the tap, which means heating and using less water over time. Both will help keep your energy bill in line.
• Monitor drainage around your home. Rainwater and runoff from gutters and downspouts can easily flow toward your foundation and leak or leech into the structure. Directing the water away with landscaping and sealing your foundation can have a big impact on the humidity level in your home, and your energy bill.

We all know the saying, “it’s not the heat, it’s the humidity.” Well, now you know a few new ways to keep the air in your home a little drier, and hopefully a bit more comfortable.

Learn about heat-related illness and how to stay cool and safe in hot weather.

High temperatures kill hundreds of people every year. Heat-related deaths and illness are preventable, yet more than 700 people die from extreme heat every year in the United States.

Take measures to stay cool, stay hydrated, and stay informed. Getting too hot can make you sick. You can become ill from the heat if your body can’t compensate for it and properly cool you off. The main things affecting your body’s ability to cool itself during extremely hot weather are:

• High humidity. When the humidity is high, sweat won’t evaporate as quickly. This keeps your body from releasing heat as fast as it may need to.
• Personal factors. Age, obesity, fever, dehydration, heart disease, mental illness, poor circulation, sunburn, and prescription drug and alcohol use all can play a role in whether a person can cool off enough in very hot weather.

Those who are at highest risk include people 65 and older, children younger than two, and people with chronic diseases or mental illness.

Closely monitor people who depend on you for their care and ask these questions:

• Are they drinking enough water?
• Do they have access to air conditioning?
• Do they need help keeping cool?

People at greatest risk for heat-related illness can take the following protective actions to prevent illness or death:

• Stay in air-conditioned buildings as much as you can. Air-conditioning is the number one way to protect yourself against heat-related illness and death. If your home is not air-conditioned, reduce your risk for heat-related illness by spending time in public facilities that are air-conditioned and using air conditioning in vehicles. Contact your local health department or locate an air-conditioned shelter in your area.
• Do not rely on a fan as your main cooling device during an extreme heat event.
• Drink more water than usual, and don’t wait until you’re thirsty to drink.
• Check on friends and neighbors and have someone check on you.
• Limit use of the stove and oven—it will make you and your house hotter.

Even young and healthy people can get sick from the heat if they participate in strenuous physical activities during hot weather:

• Limit your outdoor activity, especially midday when the sun is hottest.
• Pace yourself. Start activities slowly and pick up the pace gradually.
• Drink more water than usual, and don’t wait until you’re thirsty to drink more. Muscle cramping may be an early sign of heat-related illness.
• Wear lightweight, loose-fitting clothing.

If you play a sport that practices during hot weather, protect yourself and look out for your teammates:

• Schedule workouts and practices earlier or later in the day when the temperature is cooler.
• Monitor a teammate’s condition and have someone do the same for you.
• Seek medical care right away if you or a teammate has symptoms of heat-related illness.

Everyone should take these steps to prevent heat-related illnesses, injuries, and death during hot weather:

• Stay in an air-conditioned indoor location as much as you can.
• Drink plenty of fluids even if you don’t feel thirsty.
• Schedule outdoor activities carefully.
  • Wear lightweight, loose-fitting clothing and sunscreen.
  • Pace yourself.
• Take cool showers or baths to cool down.
• Check on friends and neighbors and have someone check on you.
• Never leave children or pets in cars.
• Check the local news for health and safety updates.