Whenever I see that a tropical storm is threatening to convert into a tropical cyclone – that’s meteorology-speak for hurricanes in the Atlantic, typhoons in the Pacific or cyclones in the Indian Ocean – I consult my checklist. These are the factors that can nip that tropical storm in the bud or escalate it into a full blown hurricane.
Four Factors that Can Strengthen Tropical Cyclones
- Sea surface temperatures warmer than 79 degrees Fahrenheit (26 degrees Celsius)
- Low vertical wind shear
- Warm moist air
- Ocean area along the projected storm track
Four Factors that Can Weaken Tropical Cyclones
- Cooler Sea surface temperatures less than 79 degrees Fahrenheit (26 degrees Celsius)
- High vertical wind shear
- Dry air
- Land masses along the projected storm track
Figure 1: In a tropical cyclone, air rotates inward to the center (or “eye”), then rises to higher altitudes. As warm, moist air rises, the air cools and condenses to rain, releasing heat. This cycle of evaporation and condensation powers the storm. Adapted from a figure courtesy of NOAA.
How do these Factors Play a Role?
Here are a some figures and a brief overview of how these factors work in concert to influence hurricanes. For a little fuller description you can go to this two page summary that includes figure 1. It gives a quick glimpse into why these factors play a role in large part because warm moist air rising to upper levels helps fuel a hurricane. For a more technical description check out the Quick Study feature in Physics Today by Kerry Emanuel (Massachusetts Institute of Technology).
I first consult the NOAA sea surface temperature charts because warmer ocean waters evaporate and add moisture to the surface air at a greater pace than cold ocean water. Similar to when we see more tendrils of steam rise from a pot of water on the stove as it warms.
Figure 2: Sea Surface Temperature Map for August 23, 2012. I added a few labels to supplement the original tiny font in this NOAA figure. The left most end of the color bar scale is -2.0 degrees Celsius (28.4 degrees Fahrenheit) and the right most number is 34.3 degrees Celsius (93.7 degrees Fahrenheit). The color greater than the 26 degrees Celsius (critical for tropical cyclones) is also indicated in a few places.
Next it is worth checking on conditions for vertical wind shear – a change in wind speed or direction with altitude. If the shear is relatively weak, than warm moist air can rise and condense into rain releasing heat in a concentrated region helping to fuel the cycle shown in figure 1. If there is higher wind shear, the region where the heat is released is distributed over a larger area dissipating the energy compared to the more concentrated area in the low wind shear case. We can think of it as upper level winds blowing the upper level heat away.
Third look for dry air nearby to the tropical storm or hurricane that can be pulled in and deprive the hurricane of much needed moisture. If there are dry patches nearby or along the projected storm track this can give some solace. If moist air abounds, pay closer attention to the forecast. NOAA has made great progress in having more accurate hurricane storm tracking, leading to smaller land areas that need evacuation alerts. Hurricane intensity projections are still an active area of research.
Finally, given the important role that moist air plays, land area compared to ocean area really matters. Land, even with some lakes, hardly can compare to a vast ocean surface area ready to deliver moisture to a storm along the ocean portion of the storm track. That is if the water is warm enough. For example, even during hurricane season, vigorous winds from a prior storm can churn the waters bringing colder deeper waters to the surface putting the brakes on hurricane development.
As land dwellers we can breathe sigh of relief when the season slides toward fall and the ocean waters start to cool and the winds start blowing harder as the temperature difference between the tropics and the polar regions increases. Until next summer when I pull out my hurricane watch check list again.