Blog Archives

What Are Meteotsunamis? (Credit: NOAA National Ocean Service)

1/31/2019

0 Comments

While the term meteotsunami might sound like the name of the latest natural disaster science fiction thriller, it is actually the name of a very real phenomenon that can occur in various bodies of water around the world. To put it simply, a meteotsunami is similar to a tsunami, however, instead of being caused by seismic activity they are caused by rapid changes in barometric pressure that cause water displacement.
The rapid change in air pressure needed to trigger a meteotsunami can be associated with fast moving weather phenomena such as severe thunderstorms or squalls moving over a body of water. As these storms move over a body of water, the rapid change in pressure causes a wave that moves towards the shore and becomes more amplified by a shallow continental shelf as well as by coastal features such as bays or inlets. These storms usually interact with the body of water for a short period of time, which could be anywhere from a few minutes to a few hours, and have been observed to reach heights of 6 feet or higher. The features of meteotsunamis are almost indistinguishable from that of a typical tsunami. The main way of differentiating between the two is looking for the presence of seismic activity during the occurrence of a tsunami-like event. If a tsunami-like event has occurred when there was no recorded seismic activity but a fast moving weather system such as a squall line had moved over the body of water near the area, it is likely that a meteotsunami, not a tsunami, had occurred.
Another meteorological phenomenon that meteotsunamis are often confused with are called seiches. Seiches are wind-driven standing waves with long periods of water-level oscillation. While they can be caused by the same types of weather events as meteotsunamis, and can often occur at the same time, seiches are primarily caused by the wind rather than pressure changes. Additionally, while seiches are standing oscillations of water level over a period of three or more hours, meteotsunamis are progressive waves that fall under shorter wave oscillation periods ranging from two minutes to two hours.

To learn more about various weather education topics, click here!
©2019 Meteorologist Stephanie Edwards

0 Comments

Fascinating Meteorology Terms: Virga (Credit: NOAA and “Meteorology Today” by C. Ahrens)

1/26/2019

0 Comments

(Photo credit: Birgit Bodén. Image taken in Boden Northern Sweden)

We might have all seen it before at one point, when driving along an interstate that crosses an area of plain, open moor, water, or farmland. That vast expanse of land, one where trees disappear and it opens up the sky for your road trips viewing pleasure. Usually when crossing this open canvas, you may occasionally see a rain filled cloud or thunderhead that seems to look like it has a curtain or shroud looming underneath it, indicating the release of rain to the ground below. But have you ever seen this curtain of rain not quite hitting the ground? Amazing enough, this type of phenomenon that you would witness has a very interesting name. To meteorologists; it’s known as “Virga”.

Virga is a Latin word for “twig” or “branch”. It’s most likely named to this type of precipitation because of its long thin-like arms that reach down underneath a cloud. Sometimes these clouds are referred to as the” jellyfish of the sky” as the virga under them resembles tentacles of a jellyfish. No matter how virga is witnessed, it gives to a very dramatic scene with its wispy appearance.

The reason why this curtain of precipitation never reaches the ground is because it evaporates. This occurs when precipitation falls from a cloud into a dry layer of air. This layer of air tends to have very low humidity and high temperatures. As precipitation falls from the cloud, it absorbs the energy from the higher temperatures and either evaporates (if water droplets) or directly changes from a solid to a gas, called sublimation (for ice crystals). This type of phenomenon is mostly seen in dry desert and steppe climates that are prone to dry weather, high temperatures, and low humidity. Another interesting fact about the process of virga: during evaporation and sublimation, the water vapors being released into the air actually increase the relative humidity. If there is enough precipitation in the cloud, as it falls, it continues to evaporate and release moisture into the air. Soon the air becomes moist enough for precipitation to reach further down into the layer of dry air and eventually hit the ground.

Virga is certainly a dramatic sight if you ever come around to seeing it in action. It also has a very intriguing name which puts it on the list of fascinating meteorology terms. Up next, you will learn about some fascinating meteorology terms for wind.

To continue learning about Meteorology, be sure to click here.

© 2019 Meteorologist Alex Maynard

0 Comments

Fascinating Meteorology Terms: Graupel (Credit NOAA)

1/23/2019

0 Comments

(Photo of graupel taken in the White Mountains of New Hampshire)

Often seen on days with many varieties of wintry precipitation, these soft, round pellets of snow may just appear on your windshield. It’s a very odd looking form of winter precipitation. With their cloudy, circular appearance, these white pellets take on a form resembling styrofoam or “Dippin Dots” ice cream. It goes by its most common name snow Pellets, but to meteorologists, it is known as graupel.

Graupel is formed by a process called accretion. Accretion occurs within a cloud where ice particles, supercooled water droplets, and snow collide with each other as they get tossed by turbulent air. Turbulent air is the rising and sinking air due to temperature and pressure differences within a cloud. It can either be really strong, like within thunderstorms, or light, like within stratus or nimbus clouds that produce rain and snow. Once the ice and snow particles grow heavy by collision, they become too heavy to be suspended by the turbulent air and drop from the cloud as a form of winter precipitation. Unlike other types of precipitation, graupel sets itself apart in the accretion process. Within a cloud that produces graupel, temperatures need to be just below freezing (32 degrees Fahrenheit) with some portion being below 15 degrees Fahrenheit. This allows both supercooled water droplets and snow to exist in the cloud. Supercooled water droplets are droplets of water that are just below freezing but not cold enough to freeze completely. During the accretion process that forms graupel, the parent particle (snowflake) collides with supercooled water droplets as it gets tumbled by turbulence. When these supercooled droplets attach to the parent snowflake, the droplets immediately freeze on contact covering the snowflake with a thin layer of ice. This is a process called ice riming which gives graupel its circular shape.

Graupel tends to form in the same way that hail does. It is sometimes mistaken for hail, but they are two very different forms of precipitation. For example, they are both formed through the process of ice accretion and ice riming. But for hail, the parent particle isn’t a snowflake; it’s a particle of ice. Graupel gets its shape from a form of soft ice riming where the supercooled droplets don’t freeze into hard ice but freeze as soft ice. This is because the core temperature of a snowflake is much warmer that the core temperature of an ice particle. Since the particle of ice that starts a hailstone is much colder, the supercooled droplets freeze on the hailstone creating a harder surface. Size also plays a large role if differentiating between hail and graupel. Graupel is relatively small in form. It grows from about two to five millimeters in diameter compared to a hailstone which can grow from pea size to softball size. The type of cloud and time of year is also a factor in hail and graupel differences. The reason why hail can grow so large is because it is made within thunderstorm clouds called cumulonimbus. These types of clouds form in the summer and are highly turbulent due to larger changes in pressure and temperature from the surface up to the cloud top. The stronger turbulence in this type of cloud is able to suspend heavier particles such as softball size hail.

Graupel is a very interesting type of wintry precipitation. It’s very easy to spot and if you have already seen it, I’m sure it has raised your eyebrows. Next time it snows in your neck of the woods, keep a lookout for some graupel and share the knowledge of what you learned today with a friend. Up next: Sticking to the subject of precipitation, you will get the chance to learn about the meteorology term “Virga”.

To learn about more fascinating meteorology terms click here.

© 2019 Meteorologist Alex Maynard

0 Comments

What's in a Name? Defining Different Cloud Types

1/21/2019

2 Comments

How are Different Cloud Types Named and Defined?

   For anyone who has ever gazed up into the vast blue pool that encompasses the Earth, horizon to horizon, they will have noticed on more than one occasion the different cloud shapes dotting the sky. Some may note them as appearing like fluffy bunnies, others may see dragons, others denote them as simply describing the mood of the sky, while another may discern them to be bumpy or wispy. But what is it that is common amongst these descriptions? Reviewing them, all have to do with a physical appearance of the cloud. Describing the physical appearance of a cloud is actually one of two parts in how the different types are defined from one another. The second part of differentiating one type of cloud from another has to do with altitude in the atmosphere at which that cloud exists, and sometimes the ability of that cloud to precipitate. So what are those fluffy looking clouds called anyways, and what does their name indicate about the cloud?

   There are five main categories of cloud name roots that help describe a cloud. These roots are cirro-, alto-, cumulo-, nimbo-, and strato-. All are latin in origin and define the height, appearance, and the precipitation ability of a cloud. The roots are combined to name the specific cloud. Starting with cirro-, this root means “a lock of hair” or “curl-like fringe” and describes clouds that have a wispy, hair-like appearance. The root indicates that it is a high-level cloud, existing at about 6,000m and above in altitude. Situated so high in the atmosphere, they are composed of millions of ice crystals that give them their characteristic hairy, or wispy appearance. Some may even describe them as smudged.

   The root alto- means “middle”and describes clouds that exist at middle altitudes in the atmosphere, around 4,000m. As you might imagine, the roots cirro- and alto- cannot be combined to describe a cloud seeing as they both partially indicate the height at which a cloud exists.

   In contrast to the previous two roots, strato- does not indicate a cloud’s height whatsoever, and can be found at all levels in the atmosphere. Strato- means layer, such as something that is stratiform, or stratified. As such, stratus clouds are those with a layered look, like sheets covering the sky. Fog is actually a type of stratus cloud! The root can be combined with any one of the other roots to name a cloud. For example, a cirrostratus cloud indicates a cloud that is high level, composed of ice crystals, but in layered form. An altostratus cloud describes a mid-level cloud that appears in sheets or bands.

   Moving on, the root nimbo- actually come from the Latin word “nimbus” which means “cloud” in Latin. This specifies a nimbus that precipitates, and can be combined with a few of the different roots, except for alto- and cirro-, seeing as these clouds exist above the freezing point in the atmosphere and cannot precipitate since they are composed of ice. Nimbus clouds are low-level clouds, existing from about 2,000m and below, that often precipitate, be it rain, snow, hail, or sleet.

   Finally there is cumulo-, meaning “heap or pile.” Cumulus clouds are interesting in that they occur at all levels of the atmosphere and can be combined with any of the roots listed above. At a low level these clouds are the ones people often make out to be fluffy bunnies or cotton balls thanks to their heaped and bulging appearance. Cirrocumulus appear to be round but raggedy white spots that repeat throughout the sky, while altocumulus are a bit softer and larger in appearance. Stratocumulus tend to occur in waves or lines, and often look as though they are connected, hence where the stratiform appearance comes into play. Finally, the largest and most dramatic of the cumulo- family is the cumulonimbus, a massive cloud that can reach the tallest heights of cloud existence in the atmosphere, up to the troposphere, and sometimes beyond into the stratosphere if they have what is called an “overshooting top.” These clouds are those that produce isolated thunderstorms and are often responsible for large hail storms. Reaching all heights within the atmosphere, these clouds tower, and appear ominous with their dark, bulging bases, and icy tops.

   When taking a look into the sky, no longer will you simply see cotton balls and bunnies, but perhaps the many individual types, from a cirrostratus, to a cumulonimbus, to a simple cumulus that so makes those fluffy bunnies! All are distinct in their nature and build, and are constructed from a variety of different atmospheric conditions, but there is no denying that each finds common ground in their unique beauty and ability to spark the imagination.

© Weather Forecaster Alexis Clouser
To learn more about weather and weather phenomena access more educational articles here: www.globalweatherclimatecenter.com/weather-education

Sources:
https://www.weather.gov/media/lmk/soo/cloudchart.pdf
https://www.etymonline.com/word/cumulus#etymonline_v_462c

2 Comments

How does fog form anyhow?

1/9/2019

0 Comments

DISCUSSION: During a typical Summer or Winter morning (or sometimes during the evening to overnight hours), there are often scenarios in which a given region can have a substantial amount of warmer and moistened air flowing in overnight. In such scenarios, it is quite common to find an atmospheric phenomenon such as fog as a result of the increased presence of warmer, moister air into the given region. This process acts to increase the regional dew point temperature which will allow the regional atmospheric environment to get closer to the actual air temperature. When the dew point does indeed reach the actual air temperature, this is the process most commonly known as and referred to as atmospheric saturation. When the atmosphere becomes saturated, this is when fog is most likely to form as the air parcels in a region reach the maximum holding capacity for water molecules that they can at a given air temperature.

It goes without saying that fog can be an incredibly dangerous precedent for any and all forms of ground and air travel. The primary reason for why fog is such a major hazard to general travel is because any type of fog can seriously impact people’s ability to have longer distance visibility when driving or for any pilot’s ability to see sufficiently well when attempting to fly a plane. For example, limited visibility can prevent a pilot or driver from being able to have enough depth perception when it comes to perceiving possible threats or obstacles in the path of a given vehicle or aircraft which can often make a typical trip quite troublesome even on an average day. Therefore, it certainly makes sense for why advisories such as dense fog advisories exist across the National Weather Service network in order to help protect average citizens from placing themselves in the path of danger when it comes to navigating around areas of dense fog when they do indeed occur.

The bottom line here is the message that whenever a major fog event unfolds, it is imperative for anyone and everyone in the path of a current or upcoming fog event to respect the natural power that the atmosphere holds. In addition, whenever there is a threat for an incoming fog threat, is it always best to plan your travel accordingly and avoid doing unnecessary driving through very dense fog since that can sometimes be a life-threatening issue if visibility gets down to 100 feet or less.

To learn more about other topics in global weather education, click here!

© 2019 Meteorologist Jordan Rabinowitz

0 Comments

Weather Education

What Are Meteotsunamis? (Credit: NOAA National Ocean Service)

Fascinating Meteorology Terms: Virga (Credit: NOAA and “Meteorology Today” by C. Ahrens)

Fascinating Meteorology Terms: Graupel (Credit NOAA)

What's in a Name? Defining Different Cloud Types

How does fog form anyhow?

Archives