IN SEPTEMBER 1961 a small hurricane called Esther swirled into being above the warm waters of the mid-Atlantic. It bore down on America’s east coast, executed a graceful clockwise loop-the-loop off the shores of New York, then gusted up through Maine and into Quebec as little more than a squall.

Esther’s place in history was not assured through its destructive power, although it did kill seven people when it brought down an American navy plane that was on route to Monrovia, in Liberia. It was, rather, the surveillance of Esther that made the storm famous, for this was the first hurricane to be discovered from space. Tracking began after the third Television Infrared Observation Satellite (TIROS-3), an early meteorological satellite launched by the United States, spotted precursor thunderstorms in the eastern Atlantic, southwest of the Cape Verde Islands.

America’s suite of hurricane sensors has grown since 1961. The current Atlantic hurricane season, which began on June 1st, sees the country running a stack of instruments that reach from orbit to a kilometre beneath the ocean. TIROS-3’s successors keep a constant watch on storms’ tracks and sizes. Gulfstream jets fly over and around storms, dropping sensors into them to measure wind speeds. Propeller-driven planes fly right into storms, measuring their properties with radar and its modern, laser-based cousin, lidar. Unmanned drones fly in even deeper. And floats, buoys and aquatic drones survey storms from below.

All of the data these machines gather are transmitted directly to computer models which are used to forecast two things. The first is what track a hurricane will follow, and thus whether, where and when it will make landfall. The second is how much energy it will dump on North America if it does indeed cross the coast—a value known as its intensity.

An object of intense scrutiny

Of these two variables, intensity is by far the harder to predict, according to Paul Reasor of the National Oceanic and Atmospheric Administration (NOAA), an American government agency. Dr Reasor is the field director of this season’s research programme. He says that how and why storms intensify rapidly is the big unanswered question in hurricane forecasting. Finding an answer is important because rapidly intensifying storms have the greatest potential to cause damage and offer the least amount of time for preparation and evacuation on shore.

Conversely, a better understanding of intensification would also help ensure that evacuation orders are not issued needlessly. In 2011, for example, Hurricane Irene (pictured above) made landfall with far less intensity than predicted, after it ran into a pool of cold water that sapped its energy. Warnings that turn out to have been overblown may undermine public confidence in the forecasters. That could be risky when people really need to evacuate.

The reason a hurricane’s intensity is hard to predict, whereas forecasting its track is reasonably easy, is that the things which influence a storm’s course occur on a larger scale, and are thus simpler to measure, than those which affect its strength. The direction of the wind in the jet stream, a high-altitude circumpolar air current, plays a role in steering a storm, as do regions of high and low pressure around it. Satellites and high-altitude planes can measure both of these phenomena easily.

In contrast, many of the factors that contribute to intensity are tucked away in a storm’s heart, where lack of light (both visible and infra-red) means it is difficult to make measurements. Winds there can have wildly different speeds and directions at different altitudes, a phenomenon known as vertical wind shear. This pulls energy out of a storm, acting as a brake. Pools of dry air tucked deep inside a storm have a similar effect, and are tricky to spot even with radar. Measuring all of these properties means getting sensors deep inside a hurricane. And sensors are needed, too, in the ocean ahead of a storm, for another thing that affects intensification is the temperature of the sea in the storm’s path.

Advances in automated sensors, both those that fly and those that swim, are making it possible to gather more data from both of these places. This season, for example, will be the first in which a constellation of microsatellites called CYGNSS (Cyclone Global Navigation Satellite System) watches storms as they roll in towards the east coast. The eight-satellite swarm, which was launched in December, listens for radio signals that come from GPS satellites directly above it in space, and for the same signals when they have been reflected from the ocean’s surface beneath the hurricane being studied. Differences between the reflected signal and the original are a consequence of the state of that surface, and CYGNSS can use them to infer wind conditions there.

Satellite measurements like this are useful, but it also helps to get as close as possible to the hidden bottom kilometre of a storm. NOAA is doing this with drones called Coyotes, built by Raytheon, an aerospace company. Coyotes are released from tubes in the bellies of NOAA’s research planes, then piloted remotely in order to gather data from the region in a storm that is just above the ocean’s surface. The data the drones collect complement those from dropsondes, which are sensors that are pushed out of the same tubes and plunge down through a storm like bombs, transmitting as they go.

The research planes have also started using a device called a Doppler wind lidar to measure a hurricane’s moisture content more accurately. Radar, a standard instrument on these planes, works at radio frequencies, which means it is reflected only from large drops of water. Lidar’s use of light, which is also reflected by small—even microscopic—drops, paints a more accurate picture of the way moisture is distributed within a storm.

Engine room

What happens in the water beneath the storm is crucial, too. Hurricanes gain energy from warm water as they pass over it. But placing probes in front of a hurricane is a risky and expensive business. “You have to call up ships and get captains to say, ‘Sure, I’ll head out towards the hurricane’,” says Glen Gawarkiewicz, a research scientist at Woods Hole Oceanographic Institution, in Massachusetts. Unsurprisingly, many mariners are reluctant to do so.

A new instrument called ALAMO (Air-Launched Autonomous Micro Observer) solves the problem. It was designed by MRV Systems, a Californian company, and will be used at Woods Hole by Steve Jayne and his colleagues. It is intended to be launched out of the belly of an aircraft, in the way that dropsondes and Coyotes are. ALAMO parachutes into the ocean in front of a hurricane. Once there, it starts a cycle of descent and ascent, gathering a profile of the sea’s top kilometre as the storm passes over it. Dr Jayne is currently stationed at Keesler Air Force Base in Biloxi, Mississippi, preparing his instruments for this season’s battle. He says that they might drop two or three of the probes on a good flight. Each would then gather about 150 up-and-down profiles as the storm passes over, sending data on temperature and salinity back to Woods Hole and NOAA’s National Hurricane Centre via satellite. That ALAMO can be deployed this way is crucial to its success. Other sensors, such as Argo, are too big to fit in the launch tube, and must be pushed out of an open tailgate—an impossibility during hurricanes.

Every storm that rolls in from the Atlantic this summer will thus be trailed by planes, punctured and scanned by dropsondes and drones, scrutinised from space by satellites, and monitored from the depths by floats like ALAMO. All the data these probes collect will be pushed immediately into models that help the National Hurricane Centre predict where storms will go and how strong they will be. In the longer term, those data will also shed light on the atmospheric and oceanic physics that underpin intensification, making better forecasts possible in the future.

Correction: This piece has been modified to explain the role of MRV Systems in the design of ALAMO