From Wikipedia:

Humidity is the amount of water vapor at any given time and relative humidity is the amount of water vapor in the air compared to the greatest amount it can hold at that air temperature

From National Weather Service:

WATER VAPOR:

Water is a unique substance. It can exist as a liquid, solid (ice), and gas (water vapor). A primary way water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Winds in the atmosphere then transport the water vapor from one place to another. A major source of water vapor in Kentucky is the Gulf of Mexico. Most of the water vapor in the atmosphere is contained within the first 10,000 feet or so above the earth's surface. Water vapor also is called moisture.

ABSOLUTE HUMIDITY:

Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount of water vapor, the higher the absolute humidity. For example, a maximum of about 30 grams of water vapor can exist in a cubic meter volume of air with a temperature in the middle 80s. SPECIFIC HUMIDITY refers to the weight (amount) of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of air). Absolute and specific humidity are quite similar in concept.

RELATIVE HUMIDITY:

Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

DEWPOINT:

Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare. While dewpoint gives one a quick idea of moisture content in the air, relative humidity does not since the humidity is relative to the air temperature. In other words, relative humidity cannot be determined from knowing the dewpoint alone, the actual air temperature must also be known. If the air is totally saturated at a particular level (e.g., the surface), then the dewpoint temperature is the same as the actual air temperature, and the relative humidity is 100 percent.

RELATIONSHIP OF DEWPOINT AND RELATIVE HUMIDITY TO CLOUDS AND PRECIPITATION:

If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air). However, for precipitation to occur, the air must be rising at a sufficient rate to enhance condensation of water vapor into liquid water droplets or ice crystals (depending on air temperature) and to promote growth of water droplets, supercooled droplets, and/or ice crystals in clouds. Droplets grow through a process called "collision-coalescence" whereby droplets of varying sizes collide and fuse together (coalesce). Ice crystal processes (including deposition and aggregation) also are important for particle growth. In thunderstorms, hail also can develop. Once the suspended precipitation particles grow to sufficient size, the air can no longer support their weight and precipitation falls from the clouds. In humid climates, thunderstorms often cause heavier rain than general wintertime rainfall since moisture content in the air typically is higher in the spring and summer, and since air usually rises at a much more rapid rate within developing thunderstorms than in general winter systems. "Cloud microphysics" is the study of droplet and ice crystal production and growth within clouds and their relationship to precipitation.

What Are The Benefits Of A Boat Dehumidifier?

Boats Are Possibly The Most Susceptible Places In The World For Mold Growth
For One Simple Reason:
Yachts Spend Time In Water And In Humid Conditions

All Boats Suffer Humidity And Spray From Lakes Or Ocean Water.

Any organic or cellulose based surface (wood, upholstery, etc.) that remains damp for more than 24-48 hours is potential mold and mildew growth.

Left untreated, this can cause up to a total loss of a boat.

Mold Needs Three Things To Grow:

1. ) A source of water, such as a hatch or port leak, condensation, and especially moist humid air
2. ) Darkness, for direct sunlight can eliminate mold
3. ) Oxygen, mold prefers less ventillated air

Important Dehumidifier Information

Why do you need a dehumidifier?

This seems like a natural question to anyone who has gone years without using a dehumidifier.

If you’ve ever lived in a humid area or been stuck in a moisture-ridden, closed-in room during the winter months, then you know how much of a relief it can be to run a dehumidifier in the room.

• But dehumidifiers do MORE than merely make the indoor environment more comfortable.
• Dehumidifiers have numerous additional benefits, some of which may even benefit your health.

Here Are The Key Things You Need To Know

Fighting Mold

As most people are aware, mildew and mold are much more likely to grow in dimly lit, closed-in areas where there happens to be a lot of moisture.

• By using a dehumidifier in any mold-prone areas, you can virtually eliminate this problem
• Rooms with sub-50% humidity rarely develop mold or mildew problems at all
• This is good for aesthetic and cleanliness reasons
• It’s also beneficial to your health to not have mold spores floating around

Reduce Allergy Symptoms

• Dehumidifying a room doesn’t just reduce mold growth, a significant allergen
• It also cuts down on dust mites and other potential irritants in the air
• If someone on your boat has allergies, run a dehumidifier
• There’s a good chance that their allergy symptoms will improve shortly

Enhance Respiratory Health

Of course, having air that’s clear of mold, mildew, and dust mites is good for the lungs. Studies have shown that dehumidifiers can have even more benefits for the respiratory system.

Dehumidifiers can help people recover from respiratory illnesses such as pneumonia, and they can also ease the symptoms of asthma.

Improve Cooling

• During the warm months of the year, excess humidity in the air can make the climate seem doubly warm, not to mention sticky and all-around uncomfortable.
• Running a dehumidifier in your boat can cut down on these effects.
• When used in combination with a good air-conditioning system, a dehumidifier can create a tremendously comfortable yacht environment, even during the hottest, muggiest months.

Diminish Pests

It’s no secret that insects and other pests love dark, dank areas of your boat.
By running a dehumidifier in your boat, you may not fully keep pests from invading, but you’ll at least deter them by a significant degree.
Run your dehumidifier, and they may naturally go to your dehumidifier-less neighbor’s boat instead.

Simplify Air Quality Management – Easy To Use

Finally, dehumidifiers are great because they’re easy to use.

The RiteAire Marine™ dehumidifier is very adjustable, enabling you to precisely regulate the amount of humidity that you have in your boat.

You Set It and Forget It.

The RAM solution will drastically lower the humidity from the air to 30% in just one pass through its coils.

• This RAM product is a hard worker for your yacht
• It's an engineered solution that is built to run continuously to reduce your yacht humidity

30% Humidity Solution

You Set The Humidistat Control And Forget It

• • The system automatically reduces yacht humidity to the 40 to 50% range

• This 40-50 % humidity range is not achievable by a yacht’s air conditioners
• RAM removes up to 70 pounds of water a day out of a boat
• RAM’s smart humidistat controls humidity to a specific level
• Once RAM reaches the desired level it automatically turns off and on
• It maintains the programmed humidity point
• It’s a smart humidity control solution for the entire boat

Trained Technicians Install The Dehumidifier System In The Boat, So You Never Notice It

• • All you should notice is the humidistat on the wall

• RAM installs an oversized condensation line to prevent the condensation line plugging up for many years to come
• We also add an alarm system to shut it off to safeguard the yacht from a flooding problem

One RAM Unit Installed Will Dehumidify Up To A 70-Foot Yacht Very Successfully

• In one pass, RAM reduces humidity by over 30%
• RAM will be running 50-80% of the time to maintain a healthy level of humidity
• Factors that will affect running times are:
  • Size and volume of Yacht
  • Outside humidity and temperature
  • Number of people onboard
  • Interior yacht temperature maintained

Example: 68' Sportfish In The Bahamas During The Summer

• With daily showers, and six people on board with doors frequently opening
• RAM will probably be running all the time
• At night when everyone is sleeping, the unit will be running a lot less