A little tropical terminology

It’s only July 10th and we’re already dealing with our 6th named system. I said in my Atlantic Outlook, this would be a hyperactive year in the Atlantic Basin, and so far, that is surely true.

I’ve been using certain terms that apply to the tropical activity so far this season, and I know other outlets and your local weather outlet is also using certain terms. I want to quickly go through the most common terms all y’all will here this season. I’m sure y’all know some of them, but maybe not all of them. So here we go.

TROPICAL CYCLONE
This is the generic term for tropical depressions, tropical storms, and hurricanes.

A tropical cyclone can’t be attached to a warm or cold front. It must have a warm core (air is warmer in center of the cyclone than elsewhere), and persistent deep convection wrapped close to the center. This has their strongest winds closely wrapped around the center This deep convection and the warm SST under it and evaporation from the ocean are how tropical cyclones develop and strengthen.

These are the attributes commonly referred to as tropical characteristics

There are three types of tropical cyclones.

TROPICAL DEPRESSION
A tropical depression is an area of low pressure accompanied by thunderstorms with a closed center of circulation producing maximum sustained winds 23-38 miles per hour. A depression does not get a name, while a storm does.

TROPICAL STORM
A tropical storm is a very powerful tropical weather system. A tropical storm has deep low pressure, allowing for sustained surface wind speeds ranging from 39 to 73 mph. They also have very heavy rainfall leading to coastal and inland flooding. Tropical storms are less powerful than hurricanes, but they’re still very disruptive and dangerous.

HURRICANE
A hurricane is a tropical cyclone in which the extreme low pressure at the center allows for the maximum sustained surface wind speed to be at least 74 mph. Hurricanes come in five categories based on wind strength. Cat 1 has max sustained winds of 74-95 mph, Cat 2 has winds 96-110 mph, Cat 3 has winds of 111-129 mph, Cat 4 has winds of 130-156 mph, Cat 5 has winds that are 157 mph or higher.

A typhoon is the same thing in the western Pacific in East Asia. They’re called cyclones in South Asia and the Indian Ocean. The same storm is called willy-willies in Australia. They all develop in the same way, have the same characteristics

INVEST
Is an area of disturbed weather that is becoming much more organized, the National Hurricane Center will designate this area as an Invest (short for investigative area). Each invest is assigned a number from 90–99, such as “Invest 90L,” with numbers repeating after 99 is reached. The letter identifier tells us the body of water, such as L for Atlantic (A is reserved for the Arabian Sea) and E for Eastern Pacific.

TROPICAL WAVE
These are little low-pressure disturbances (troughs) and move east to west. They can also be called an easterly wave; since they move off the African Coast into the Atlantic, they can also be called an African easterly wave. On satellite they look like unorganized cluster of thunderstorms. There are a lot of tropical waves every season, a small percentage of which develop into tropical cyclones.

TROPICAL DISTURBANCE
This is a generic term for weather disturbances in the tropics and subtropics, as I said above it’s another name for a tropical wave. A tropical disturbance consists of unorganized but persistent convection and sometimes low pressure. It can become a tropical cyclone down the road.

TROPICAL LOW
Sometimes the term tropical low will be used when an invest becomes strong and fairly organized before it becomes a tropical cyclone. The term can also be used sometimes to describe low pressure that is no longer, officially a tropical cyclone.

POST-TROPICAL CYCLONE
This is when a tropical cyclone starts to lose its tropical characteristics. A post tropical cyclone can be either become extratropical or a remnant low. One important thing to remember becoming post tropical doesn’t mean the storm any less dangerous. In fact, Post Tropical Cyclones, like Sandy, can have winds well over Hurricane force.

EXTRATROPICAL CYCLONE
A non-tropical cyclone. Extratropical cyclones get their energy primarily from the imbalance between air masses of different temperatures. Tropical cyclones often become extratropical, undergoing extratropical transition, as they move out of the tropics or subtropics; and once in a while an extratropical cyclone can transition into a tropical one.

SUBTROPICAL CYCLONE
Subtropical Cyclones are a mixture of the characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. Which is why they are often called a hybrid. So, while they’re not a true extratropical cyclone; they’re not fully a tropical cyclone either. Subtropical cyclones, although non-frontal like tropical cyclones, are typically associated with cold upper troughs rather than warm upper ridges, their warm cores are shallower than those of tropical cyclones, and their strongest winds are not as closely wrapped around the center.

REMNANT LOW
A tropical low which was once a tropical cyclone.

EYE
As tropical cyclones become stronger, eyes form, which are in the center of circulation and have relatively less wind and precipitation. Eyes are not seen often in tropical storms or even sometimes in weak hurricanes.

EYEWALL
Circular band of strong winds and heavy rain surrounding the eye. The highest wind speeds are usually in the eyewall.

STORM SURGE
storm surge is rapidly rising water that is pushed onshore and moves inland by the force of the wind accompanying a tropical cyclone. storm surge is often the greatest threat to life and property from a tropical cyclone.

STORM TIDE
The water level rise resulting from the astronomical tide combined with the storm surge.

Dew Point and Relative Humidity

For this post, I will discuss a little on dew point and relative humidity.

 Water is present as a liquid and as a gas in the atmosphere. When water is a gas, it’s called water vapor. The naked eye can’t see water vapor in the air, because water molecules are very small. When water is a liquid, you can see it in the form of clouds. It can also fall from the sky as precipitation.

The amount of water vapor in the air is called humidity.

There are two kinds of humidity, Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. Relative humidity also measures water vapor but relative to the temperature of the air. For the purpose of this post I will only briefly discuss relative humidity due to its relationship to dew point.

Relative Humidity:

Relative humidity is a percentage of the amount of water vapor in the air of the total amount that could be held at its current temperature. The warmer the air the more water vapor it can hold.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal. If the amount of water vapor in the air stays the same, but the temperature goes down, the relative humidity will increase. This is because the colder air cannot hold as much water vapor. If the temperature gets cold enough, the air gets to the point that it is holding the most water vapor it can hold. The relative humidity for this temperature would be 100 percent. This is also known as the dew point temperature.

Dew Point:

All Y’all hear the term dew point all the time on your local weather forecast during the summer. But what the heck is dew point really? And why should you care?

Dew point is the temperature to which a parcel of air must be cooled in order to reach saturation by the water vapor already present in the air. A parcel is like a helium balloon as it rises up into the air.  The higher the dew point, the more moisture there is in the air.

To put all of this in simpler terms, is that the dew point is the temperature at which the moisture in the air becomes liquid water. It is a complicated calculation to determine the dew point, but fortunately y’all don’t need to do the math. Here’s a chart that will show you the approximate dew point.

Dew point temperature is a marvelous indicator of how comfortable you'll feel outside. The higher the dew point rises, the greater the amount of moisture in the air, the higher the dew point, the harder it is for the sweat on your skin to evaporate, and the hotter and muggier the air will feel.

Going back to relative humidity, many times it can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80-degree day with 50% relative humidity than on the 30-degree day with a 100% relative humidity. This is because of the higher dew point.

When the air temperature gets down to the dew point, some of the water vapor turns back to liquid water (condensation). That fog you see on the mirror after you take a shower; is the result of dew, which is just liquid water that has condensed out of the air.

Human body temperature is dependent on the air as it absorbs and removes moisture from our skin to cool us down.  If the relative humidity is high, the amount of water evaporating from our skin is limited so we feel warm and smothered.

A dew point temperature less than 50 degrees is considered dry, 50-68 degrees is comfortable, 69-76 degrees is uncomfortable, while greater than 77 degrees is considered extremely uncomfortable.

The basic meaning of other heat related terms.

HEAT WAVE:

A period of abnormally and uncomfortably hot weather. To have an official heatwave the air temperature must at least 90 for at least three days, A heatwave can last from several days to several weeks.

HEAT EXHAUSTION:

The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT STROKE:

Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.