Tropical Storm vs. Tropical Depression: What’s the Difference?

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Summertime brings atmospheric disturbances that cause massive amounts of rain and wind to be inflicted on an area, creating billions of dollars in damage and killing countless people. This devastating meteorological activity includes tropical depressions and tropical storms. 

While many individuals use these terms interchangeably, they’re not the same. In this article, we’ll explain the difference between the two. 

What’s a tropical cyclone? 

“Tropical cyclone” is a catch-all phrase for hurricanes, typhoons, and cyclones. Meteorologists use the term to describe a rotating system of clouds and thunderstorms that originates over warm tropical waters and has closed, low-level air circulation. 

Tropical cyclones bring with them thunderstorms, instability, and intense wind shear. These are all the ingredients needed to whip up terrifying tornadoes that can cause further damage to an already devastated area. The only good thing is tornadoes birthed by tropical cyclones tend to be weak and short-lived. 

97% of tropical cyclone activity occurs between June 1st and November 30th. 

What’s a tropical depression? 

The first stage in the evolution of a tropical cyclone is a tropical depression, which is the weakest tropical cyclone type. These are low-pressure areas surrounded by circulating thunderstorms and maximum sustained winds of 38 miles per hour or less. 

Tropical depressions aren’t talked about as much as tropical storms. However, because their formation is one of the first signs that a hurricane might be on the horizon, they can act as a hurricane early warning system. Even though there are a lot of tropical depressions during storm season, only around 14 reach the magnitude of tropical storms. 

While tropical depressions aren’t as powerful as tropical storms or hurricanes, they can still do a lot of damage with the raging thunderstorms and heavy rains they bring to an area. Winds from a tropical depression can generate life-threatening rip currents along the coast. 

In 2019, Tropical Depression Imelda pelted Southeast Texas with 44 inches of rainfall, triggering massive flooding. 

What’s a tropical storm? 

Meteorologists upgrade tropical depressions to tropical storms when maximum sustained winds reach a speed of between 39 and 73 miles per hour. These types of meteorological events begin as low-pressure atmospheric disturbances. Wind moves into the low-pressure area from surrounding high-pressure regions. 

As warm seawater heats the air, it rises from the storm’s center. The air condenses, creating thunderstorms in the process. The system feeds off itself using warm ocean water as nourishment. 

Tropical storms carry heat from latitudes near the equator to more northern latitudes. Because they do, they’re an integral part of the world’s ecological balance

The most significant danger of tropical storms is the heavy rainfall they produce, which can cause destructive floods and mudslides. Many of these storms have downpours of over three feet over several days. 

Tropical storms generate enough wind, wave, and rain activity to damage boats, erode beaches, and flood city streets. 

Tropical storm vs. tropical depression: what’s the difference?

Tropical depressions have many of the same characteristics as tropical storms. These include humid air, torrential rain, heavy winds, and low pressure. The most significant difference is that conditions in a tropical depression aren’t as severe, with lower wind speeds and overall intensity. 

Another crucial difference is that weather scientists don’t give names to tropical depressions–only a number

For example, Tropical Depression Ten means the storm is the tenth tropical depression to form during a season. On the other hand, meteorologists assign a brand-new tropical storm the next available name on that season’s names list. 

A tropical storm keeps its name even if it downgrades to a depression. That’s why some tropical depressions appear to have names. 

What’s a hurricane? 

Once the winds of a tropical storm attain speeds of 74 miles an hour or more, it’s no longer considered a mere tropical storm. In the North Atlantic, central North Pacific, and eastern North Pacific, it’s called a “hurricane.” In the Northwest Pacific, it’ll go by “typhoon.” However, in the South Pacific and Indian Ocean, weather scientists use the generic term “tropical cyclone,” regardless of wind strength. 

If a hurricane’s winds reach a speed of 111 miles per hour or more, it’s considered a Category 3 storm. A meteorological event of this magnitude is a major hurricane. Hurricanes are usually downgraded to tropical depressions just before they peter out. 

Hurricanes can only form if they’re far enough away from the equator. The Coriolis effect is too weak near the equator to give developing storms the spin they need. 

The Coriolis effect is when a mass moving in a rotating system experiences a force acting perpendicular to the direction of motion and the axis of rotation. This effect deflects moving objects to the right in the Northern Hemisphere and the left in the Southern. This deflection is what causes hurricanes to spin. 

Hurricane categories 

Weather scientists place hurricanes in one of five categories based on their wind speed: 

  • Category 1: 74-95 mph 
  • Category 2: 96-110 mph 
  • Category 3: 111-129 mph 
  • Category 4: 130-156 mph 
  • Category 5: above 157 mph 

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What the National Hurricane Center (NHC) does

In North America, the entity responsible for monitoring hurricanes is the National Hurricane Center (NHC). Once a tropical depression comes into being, the agency gives it a number based on its order of formation during the hurricane season. For example, Tropical Depression One or Tropical Depression Two. 

The NHH assigns names to tropical storms using the official name list for that season developed by the World Meteorological Organization. A hurricane keeps the same name it had when it was a tropical storm. 

The list of names rotates every six years unless a storm is so destructive that the WMO votes to retire that name. For example, Harvey in 2017, Sandy in 2012, or Katrina in 2005. 

During the northeast Pacific and Atlantic hurricane seasons, the Hurricane Specialists Unit (HSU) of the NHC puts out routine tropical weather outlooks. Let’s say the unit anticipates hurricane conditions in the next 48 hours. In that case, it will issue watches and warnings using media outlets and National Oceanic and Atmospheric Administration (NOAA) Weather Radio. 

The agency is located on the campus of Florida International University in University Park, Florida. 

Hurricane hunters 

The NHC gathers information about tropical systems by having intrepid weather scientists known as “hurricane hunters” fly reconnaissance aircraft directly into the dark hearts of storms. A hurricane hunter’s mission is to collect data to help the NHC team know when a tropical depression evolves into a tropical storm and predict when a hurricane will make landfall. 

These courageous pilots need to remain calm even when flying under dangerous circumstances. The crew collects pressure, temperature, wind, and humidity data using onboard weather instruments and a “dropsonde.” 

This is an instrument package the crew parachutes right through the turbulent maelstrom of the storm. These flights take place twice each day until the meteorological disturbance reaches land or starts to disperse. 

Is climate change making tropical cyclones worse? 

The scientific community generally agrees that there’s a correlation between rising global temperatures and extreme weather events. Evidence from aircraft, satellites, climate model projections, and ground measurements increasingly show that this is becoming a disturbing reality. 

Seas progressively getting warmer are the perfect breeding ground for tropical cyclones, including hurricanes. Warmth in the ocean’s upper fuels these kinds of weather events, which need sea surface temperatures (SSTs) greater than 79 degrees Fahrenheit to be born. 

In the last 27 years, there have been 17 above-normal Atlantic hurricane seasons. That’s the longest period of above-normal seasons ever recorded. A season needs to have tropical cyclones with greater-than-average wind speed and duration to be categorized as above normal. 

It’s evident that while there might not be more hurricanes than ever before, the ones that form are getting more powerful, including more Category 4 and 5 storms. 

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