Saffir-Simpson Hurricane Wind Scale for determining hurricane strength

Around the globe, there are many places that are hit by severe tropical storms. In some regions, they are called hurricanes, while in others they’re named cyclones or even typhones. It does not matter, as they’re all the same: a severe, large-scale storm, emerging from the tropics, and possibly very devastating.

However, we humans always need to know more than simply “there is a storm”. In fact, for us, it’s critical to know about storm strength – for example, in the case of insurance policies that cover storm damage, or for the sake of statistics. That’s why for hurricanes we have come up with a scale for measuring storm strength.

In this article, we’ll be taking a look at this scale, which is named the Saffir-Simpson Hurricane Wind Scale. It can be used for determining hurricane strength. In order to do so, we’ll first take a look at what a hurricane is. Why is it different than a regular storm? Why is it that certain parts of the world are hit often while others are hit very infrequently or never? Those questions will be answered.

Subsequently, we’ll be looking at the actual scale. We’ll look at category one hurricanes, category two, up to category five. Subsequently, we’re looking at related classifications – being tropical depressions and tropical storms, as well as the Beaufort wind scale.

Altogether, this blog article allows us to answer the following question: how can we determine hurricane strength?

Are you ready to take a look? Let’s go! 😎

The eye of hurricane Katrina, as seen from space. Photo by NASA, licensed to be in the public domain.

What is a hurricane?

Before we can introduce a scale for measuring hurricane strength, it’s important to take a look at what a hurricane actually is.

Meteorologically, doing so often means navigating to the American Meteorological Society, and specifically its glossary. It defines a hurricane as follows:

A tropical cyclone with 1-min average surface (10 m) winds in excess of 32 m s-1 (64 knots) in the Western Hemisphere (North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern and central North Pacific east of the date line).

AMS Glossary (n.d.)

Now, that’s the North American definition of a tropical cyclone. Meteorologically, that’s relatively uninteresting, because we don’t care about the region (no offence) – but rather, about the weather.

The westerlies (blue arrows) and trade winds (yellow and brown arrows). Tropical cyclones often begin in the regions of the trade winds, after which they enter regions where westerlies blow. Source: Wikipedia, created by KVDP, licensed to be in the public domain.

So let’s take a look at the glossary definition of a tropical cyclone, then (AMS Glossary, n.d.). This one’s more interesting. Although it’s quite a bit of text, answering what is a hurricane? is now possible. Here are some pointers which can be used to distinguish hurricanes (that is, tropical cyclones) from regular storms (meteorologically, also called cyclones, but they are regular areas of low pressure):

  • They occur in many regions of the world: North Atlantic, Caribbean Sea, Gulf of Mexico, North Pacific (near Mexico, Phillippines and China Sea), Hawaii, Bay of Bengal and Arabian Sea, Indian Ocean, Australia.
  • When they appear, they often do so in the areas of the trade winds (i.e. near the Equator), so that they move to the west. They also move a bit towards the poles, which means that they are captured by the westerlies, and subsequently move back to the east – often getting weaker and weaker.
  • Their appearance is due to a wide variety of atmospheric disturbances: monsoons, or a wave pattern in the tropics. Once formed, they can continue to grow because they can extract heat and moist from the ocean.
  • They are characterized by (1) deep convection (i.e. thunderstorms) and having a (2) warm core (because the thunderstorms make the storm relatively warm compared to its surroundings).
  • They can range from 100 to 1000 km in diameter.

Especially the warm core separates a tropical depression, storm or cyclone from a non-tropical (i.e. regular) storm. Generally speaking, tropical cyclones (and thus hurricanes) can become much stronger than regular storms and can thus create much more havoc. That’s why we’ll now look at a method for measuring hurricane strength: the Saffir-Simpson Hurricane Wind Scale.

Measuring hurricane strength: Saffir-Simpson Hurricane Wind Scale

In the late 1960s, people in the United States wanted to study how to build houses in hurricane-prone areas at lowest possible cost (Wikipedia, 2003). Doing so required measuring the effects of a hurricane given its strength.

Unfortunately, Herbert Saffir, the engineer who was tasked with studying the low-cost housing opportunities, had no method available for doing so. He did however know about another method that seemed to do what he wanted, but then for another phenomenon: the Richter scale was available for studying the effect of earthquakes.

For this reason, Saffir developed a scale that classified storms based on wind speed, indicating the expected damage it would bring. The scale went from 1 to 5 with category one storms being the lightest hurricanes, and 5 the most severe ones. Hurricanes of > category 3 are labeled major hurricanes.

From civil engineering into meteorology

After giving the scale to the National Hurricane Center (NHC), meteorologist Robert Simpson – who was the NHC director – adapted the scale by adding the effects of storm surges and consequential flooding. It was eventually released in 1973, and was adopted relatively quickly. Today, it’s used widely, and is in fact the only scale used prominently today when it comes to classifying hurricanes.

In the early 2010s, the effect of storm surges and flooding was removed again from the scale in order to reflect pure wind speeds. In the years since, minor revisions have taken place (Wikipedia, 2003).

Here are the categories:

  • Category 1: very dangerous winds, some damage expected.
  • Category 2: extremely dangerous winds, extensive damage expected.
  • Category 3: devastating damage expected
  • Category 4: catastrophic damage expected
  • Category 5: catastrophic damage expected, but more severe than category 4.

We will now take a look at each individual category in more detail.

Category one hurricane

33-42 m/s, 64-82 knots, 119-153 km/h, 74-95 mph

When a category one storm, such as Hurricane Hanna displayed in the satellite image below, hits a region, the amount of damage is relatively low. This is especially true for permanent structures such as buildings. As non-fixed constructions can be damaged, category one hurricanes can be classified as having:

Very dangerous winds with some expected damage.

Category 1 storms further show the following characteristics:

  • Often, coastal flooding takes place already with category 1 storms. This can lead to damaged piers.
  • Power outages are frequent.
  • Life-threatening conditions can occur.
Hurricane Hanna (2020) making landfall on Padre Island, Texas on July 25, 2020. Source: NOAA, licensed to be in the public domain.

Category two hurricane

43-49 m/s, 83-95 knots, 154-177 km/h, 96-110 mph

A category 2 storm, like Hurricane Paulette displayed in the satellite picture below, can be quite damaging to building roofs. Poorly constructed constructions will also be damaged. For this reason, category 2 hurricanes can be characterized as having:

Extremely dangerous winds with expected damage.

Hurricane Paulette (2020) as a Category 2 hurricane on September 14, 2020. Source: NASA, licensed to be in the public domain.

Category three hurricane

50-58 m/s, 96-112 knots, 178-208 km/h, 111-129 mph

Now, when a hurricane further intensifies, it can grow into a storm of the third category. Hurricane Humberto, which can be seen in the satellite image below, is one such storm. Storms from this category can already bring significant damage to buildings and small residences, especially when they’re made of wood.

If the building has no solid foundation, such as a mobile home, it’s likely that it’s being destroyed. In addition, many trees will fall, and so will power lines. While it is very likely that flooding occurs near the coast due to water being forced inland, it’s also very much possible that flooding occurs further inland. For this reason, category 3 storms can be characterized as having:

The power to bring forward devastating damage.

Not fun, such storms.

Hurricane Humberto (2019) west of the island of Bermuda on September 18, 2019. Source: NASA, licensed to be in the public domain.

Category four hurricane

58-70 m/s, 113-136 knots, 209-251 km/h, 130-156 mph

Although increasingly rare, it’s possible that storms continue to grow – into a category 4 hurricane, in this case. Hurricane Laura from the 2020 hurricane season was one such storm, displayed below with a clearly visible eye.

Those storms can be characterized as having the capability of:

Bringing forward catastrophic damage.

Especially small residences that aren’t strong are very likely to be destroyed in category 4 hurricanes (Wikipedia, 2003). To other buildings, damage is often irreparable; structures without a solid foundation are flattened.

With respect to nature, it is often observed that most trees will have fallen, meaning that many areas can no longer be reached. Power loss of many weeks can be expected.

Hurricane Laura (2020) on August 26, 2020. Source: NOAA, licensed to be in the public domain.

Category five hurricane

>= 70 m/s, >= 137 knots, >= 252 km/h, >= 157 mph

A category 5 hurricane is an extremely devastating hurricane. In fact, category 5 is the highest category available in the Saffir-Simpson scale. When such a storm hits, it’s likely that many buildings are destroyed, as well as public infrastructure. It’s therefore also capable of:

Bringing forward catastrophic damage.

Only the heaviest buildings such as offices built with reinforcement can survive, but only if they are way inland (Wikipedia, 2003). Severe flooding is expected when a category 5 storm hits. In populated areas, it’s likely that large-scale evacuation plans are necessary.

Long-term power outages and water outages (of up to several months) are likely with such storms. Massive devastation occurs especially in populated areas. Storms can wipe entire cities off the map, and catastrophically damage others. You really don’t want such a storm to hit!

Hurricane Dorian, from the 2019 hurricane season, was a category 5 hurricane. Its clear eye is visible in the satellite image below. We see Florida on the left.

Hurricane Dorian (2019) at peak intensity on September 1, 2019. Source: NOAA, licensed to be in the public domain.

Related classifications

Now that we know what hurricanes are and how they can be classified, we can take a look at related classifications.

More specifically, we’ll be taking a look at tropical depressions and tropical storms as well as how they relate to tropical cyclones / hurricanes. Subsequently, we also take a look at the Beaufort wind scale, and see how it nicely complements the Saffir-Simpson scale.

Tropical depressions and tropical storms

Sometimes, it happens that low-pressure areas demonstrate the characteristics of a tropical cyclone discussed before.

However, they only do not yet produce wind speeds that allow them to be classified as a tropical cyclone, or a hurricane – for the US people among our readership.

In those cases, we often speak about a tropical depression as well as a tropical storm.

We know that tropical cyclones can grow out of a wide variety of atmospheric disturbances (AMS Glossary, n.d.). When this happens, those disturbances (which are often areas with lower pressure at relatively minor scale) will grow into more extensive disturbances – often, areas of low pressure a.k.a. depressions.

This map shows the tracks of all Tropical cyclones which formed worldwide from 1985 to 2005. The points show the locations of the storms at six-hourly intervals and use the color scheme shown to the right from the Saffir-Simpson Hurricane Scale. Source: Wikipedia. Licensed to be in the public domain.

If the depression shows tropical characteristics – that is, it has grown over tropical water and in line how tropical cyclones form, but has insufficient wind speed yet – we call the disturbance a tropical depression. For US based storms, this happens when wind speeds are lower than 33 knots (38 mph, 61 kmh, 17 m/s; National Weather Service, n.d.).

If depressions grow, they can reach the stadium of tropical storm. This is the last stop before the storm is called hurricane, more specifically a category one hurricane. For US based storms, tropical storms require wind speeds to be higher than the ones of tropical depressions and lower than the ones of category 1 hurricanes, being 33-42 m/s, 64-82 knots, 119-153 km/h, 74-95 mph.

Hence, we can now clearly see the path towards a hurricane: from a tropical disturbance into tropical depression; then into tropical storm, and finally into a category 1 hurricane. Some of them then grow into more severe storms, up to category 5.

Relating Saffir-Simpson to Beaufort wind scale

In another article, we covered the Beaufort wind scale. This scale can be used to determine whether winds are strong and ranges from 0 to 12:

  • 0: Calm
  • 1: Light air
  • 2: Light breeze
  • 3: Gentle breeze
  • 4: Moderate breeze
  • 5: Fresh breeze
  • 6: Strong breeze
  • 7: Moderate wind
  • 8: Gale
  • 9: Strong gale
  • 10: Storm
  • 11: Violent storm
  • 12: Hurricane force

The Beaufort Scale nicely complements the Saffir-Simpson scale for hurricanes because the former allows people to classify regular winds, not hurricanes. As we can see, only 12 Beaufort is a hurricane-force wind speed. Hence, if we want to study wind, we can use both scales in sequence: first, we can determine wind strength using the Beaufort scale, and subsequently use the Saffir-Simpson scale to guide us when the storm has actually gained hurricane strength.

Check out the article about the Beaufort wind scale for more details about this scale.

Summary

In this blog, we looked at the Saffir-Simpson scale for classifying hurricanes with respect to their expected damage. Put more briefly, it is a scale for measuring the strength of a hurricane.

For doing so, we first studied what a hurricane is. We saw that it often emerges in the tropics and emerges from a wide range of tropical disturbances. Hence, they are a global phenomenon, however, they pretty much emerge in the tropics only. Moving westward with the trade winds, they divert a bit towards the poles and often pick up an easterly path. Many US hurricanes also determine weather in Europe weeks later, to give just one example.

After taking a look at hurricanes conceptually, we looked at the Saffir-Simpson scale. We saw that it was designed to measure hurricane strength and especially how much damage they cause. This was done in the late 1960s in order to support low-cost housing effort in the United States. Eventually sponsored by the National Hurricane Center, it has been widely adopted and is in major use today. It is revised every now and then.

We saw that hurricanes range from categories 1 to 5, with 5 being the most severe and catastrophically damaging storms. Tropical disturbances, tropical depressions and tropical storms nicely compliment hurricanes in the sense that they already show tropical characteristics, but do not yet demonstrate the necessary wind speed for a storm to be called hurricane. We also saw that the Beaufort scale for measuring wind strength complements the Saffir-Simpson scale, and that they can be used jointly in your weather communications.

I hope that you have learnt a lot from today’s article. I did when researching this scale! Please, if you have any questions, remarks or other comments, feel free to leave a message in the comments section below 💬 I’m looking forward to hearing from you 🙂

Thank you for reading MisterWeather today and enjoy the weather! 😎

References

Hurricane. (n.d.). AMS Glossary. https://glossary.ametsoc.org/wiki/Hurricane

Tropical cyclone. (n.d.). AMS Glossary. https://glossary.ametsoc.org/wiki/Tropical_cyclone

Saffir–Simpson scale. (2003, June 30). Wikipedia, the free encyclopedia. Retrieved September 24, 2020, from https://en.wikipedia.org/wiki/Saffir%E2%80%93Simpson_scale

Tropical definitions. (n.d.). National Weather Service. https://www.weather.gov/mob/tropical_definitions

When is wind strong? – A look at the Beaufort scale – MisterWeather. (2020, September 17). MisterWeather. https://mister-weather.com/2020/09/17/when-is-wind-strong-a-look-at-the-beaufort-scale/

Leave a Reply

Your email address will not be published. Required fields are marked *