Hurricanes are one of the most destructive natural disasters that occur regularly. As these powerful storm systems develop and move across the oceans, there is an important question about how their intensity changes with height. Specifically, do hurricanes intensify as they gain altitude? In this article, we will examine the scientific evidence and measurements to determine how hurricane intensity is impacted by increasing height above the ocean surface.
What causes hurricanes to form?
Hurricanes are tropical cyclones that originate over warm ocean waters with surface temperatures above 26.5°C (79.7°F). When warm, moist air begins to rise from the ocean surface, it leaves an area of lower pressure below. This causes surrounding air to rush in, increasing the rotation of the rising air. As the rotating winds continue to draw in warm ocean air, evaporation increases, providing more water vapor to power the storm.
The rising, rotating mass of air forms thunderstorms and begins spinning faster due to the Coriolis effect from the rotation of the Earth. As the spinning increases, an organized system develops with closed circulation vortex pulling the storm together. When maximum sustained winds reach at least 74 mph (119 km/h), the tropical system is designated a hurricane.
How do hurricanes intensify?
Hurricanes intensify through a combination of factors:
- An abundant source of warm ocean water to evaporate and provide moisture/energy
- Low vertical wind shear so the vortex circulation isn’t disrupted
- High relative humidity in the middle troposphere to promote thunderstorm development
- A sufficiently coriolis force from the Earth’s rotation to structurally organize the vortex
With these favorable conditions, tropical cyclones can undergo rapid intensification, with winds increasing by at least 30 knots (35 mph) in 24 hours or less. This occurs when the inner core of the storm becomes more vertically aligned, allowing for warmer air to rise more efficiently through the center of the storm.
Hurricane structure
Hurricanes have a distinct vertical structure, with characteristics that change at different altitudes:
- Eye – Calm, clear area at the center with sinking air
- Eye wall – Ring of strongest thunderstorms and winds surrounding the eye
- Rainbands – Outer bands of thunderstorms where most of the rainfall occurs
The most intense winds are found within the eye wall, where rapidly rising warm air flows inward and upward. Outside of the eye wall, wind speeds steadily decrease. The overall shape of the system takes on a vortex rotating around the eye and spanning outward across an area of ocean up to 600 miles across.
How do wind speeds change with height?
Within a mature hurricane, wind speeds generally increase with height through the lower half of the troposphere as the flows converge and accelerate upwards. However, at higher altitudes, wind speeds slowly decrease again as the vortex expands laterally and friction dissipates.
Measurements of actual hurricanes have found the peak winds speeds typically occur between 5,000 – 10,000 ft in altitude. The exact height of maximum wind speed depends on the size and intensity of the storm. Larger, more intense hurricanes have higher peaks.
For example, measurements in Hurricane Hugo (1989) found:
- Surface winds: 136 mph
- Winds at 10,000 feet: 173 mph
- Winds at 20,000 feet: 144 mph
How does central pressure change with height?
The central pressure at the eye of a hurricane is lowest at the ocean surface. It increases steadily with height through the vertical structure of the storm. This occurs because the circulating winds converging around the eye move air upward through the eye wall.
This upward motion removes air from the interior region while the external pressures compress inward. This creates an extremely low pressure zone at the surface eye, progressively increasing with altitude.
For example, Hurricane Wilma (2005) had an estimated central pressure of:
- 882 mbar at the ocean surface
- 950 mbar at 5,000 feet altitude
- 975 mbar at 10,000 feet altitude
Measuring hurricane intensity
Several parameters are used to classify hurricane strength:
- Maximum sustained winds – Speed of the highest 1-minute wind measured at 10 meters (~33 feet) above surface. Used to define category on Saffir-Simpson scale.
- Minimum central pressure – The lowest atmospheric pressure at the center of the hurricane. Lower pressures correlate with higher winds.
- Wind field size – Spatial extent of hurricane-force and tropical-storm-force winds. Larger wind fields indicate a larger, more intense storm.
Of these, maximum sustained wind speed is the primary metric used by the U.S. National Hurricane Center for real-time intensity and forecasts. Measurement of wind speed has improved in recent decades through the use of GPS dropsondes deployed in hurricanes. These instrument packages fall through the eye wall, directly measuring pressure and winds while transmitting data back to forecasters.
Factors affecting intensification with height
Several factors influence the degree to which hurricane wind speeds increase vertically through the troposphere:
Storm motion – Faster storm motion adds momentum, increasing upper level winds in the direction of motion. Slower moving storms have less augmentation of winds at altitude.
Upper-level environment – Weak wind shear and upper-level outflow allow stronger vertical development. Strong shear disrupts intensification.
Ocean heat content – More available heat energy from warm ocean water provides fuel for intensification. Hurricanes stall when they move over colder water.
Internal dynamics – Development of the inner core and eye wall region allows for more focused rising of warm air and intensification.
Observational evidence of wind speed increase
Field studies analyzing wind measurements at different heights inside hurricanes reveal faster speeds at upper levels:
- Global Hawk drone flights in hurricanes find average increases of 25-35 knots from surface to 10 km altitude.
- Dropsondes show 35-45% increases in wind speeds from ocean surface to mid-levels of troposphere.
- Model simulations and radar data verify the overall pattern of increasing intensity with height.
Additionally, the higher the category of hurricane, the greater the increase in winds with height:
| Hurricane Category | Surface Winds (mph) | Winds at 10,000 ft (mph) |
|---|---|---|
| Category 1 | 74-95 | 105-115 |
| Category 2 | 96-110 | 120-130 |
| Category 3 | 111-129 | 135-155 |
| Category 4 | 130-156 | 160-195 |
| Category 5 | 157 or higher | 195 or higher |
This shows the more powerful the hurricane, the faster the winds accelerate vertically upwards through the eye wall, increasing the peak speeds attained at mid-levels. Category 5 systems can have winds over 100 mph faster at 10,000 ft than the surface.
Impacts of stronger upper-level winds
The faster winds at upper levels have several important impacts on hurricanes:
- Cause more damage to taller structures like high-rise buildings and radio towers
- Increase wave height and storm surge piling water onshore
- Intensify the updrafts and thunderstorm activity in the hurricane
- Generate greater turbulence and wind shear for aircraft avoidance
The stronger winds at altitude also drive the tropical cyclone circulation. The rising air requires fast upper-level outflow to remove exhaust out of the top of the system. This helps spin up the overall hurricane vortex to deeper levels.
Why don’t ground measurements show the highest winds?
Despite having faster winds at mid-levels, the most intense hurricanes don’t have the fastest winds right at the ground. Several factors contribute to lower winds nearer the surface:
- Friction with the ocean surface causes wind drag
- Low-level disruption from rain-cooled downdrafts
- Weak inflow angle near core compared to upper levels
- Eyewall tilt makes radius of max winds higher up
Because of these effects, measurements of extreme winds in the most powerful hurricanes are almost always above the surface rather than right at the ground. This supports the pattern of general wind speed increase with height.
How do dropsondes sample vertical wind profiles?
Hurricane hunter aircraft deploy GPS dropsondes to take direct measurements within hurricanes:
- Cylindrical instrument package weighing 1.4 pounds, 15 inches long
- Falls at ~12 m/s while transmitting pressure, humidity, temp, winds
- Provides data every 0.5 seconds during descent through storm layers
- Transmits to aircraft and satellites for relay to forecasters
- Measures eye wall winds, storm environment, and structure
Hundreds of dropsondes are released each hurricane season, providing high resolution data on the intensification of winds and decrease in pressure towards the center of storms. This has helped quantify wind speed increases with height inside hurricanes.
Conclusions
In summary, scientific evidence clearly shows that wind speeds in hurricanes increase substantially with height through the mid-levels of the troposphere:
- Field studies measure 25-50% increases in wind from the surface to 5-10 km altitude
- The most extreme winds are found about mid-way up the eye wall
- Higher category storms have greater vertical intensification
- Faster upper winds drive overall hurricane structure and intensity
- Peak winds aloft can exceed surface winds by over 100 mph in Category 5 hurricanes
So while hurricanes may originate from ocean heat, their most dangerous winds are found well above the seas as the rising vortex accelerates upwards. Understanding this vertical structure has helped improve intensity forecasts and highlights the risk extreme winds pose to aviation and tall structures. Tracking future changes in this altitude amplification will be important for evaluating hurricane behavior in a warming climate.