It was a little after 10 p.m. on a bitterly cold January evening in 2015 when ISIS launched one of its most devastating assaults yet. Paddling on rafts across the muddy brown Upper Zab River, a tributary of the mighty Tigris River, 160 fighters used the cover of a thick fog bank to sneak up on a dozing company of Iraqi Kurdish troops stationed near Mosul. By the time the unprepared soldiers had roused themselves and located the ISIS fighters among the mist, the jihadists had burrowed deep into their base and killed over 30 men.
Two weeks later, ISIS deployed similar tactics—a sneak attack in bad weather—50 miles down the frontline, at the disputed northern Iraqi city of Kirkuk. Under a steady downpour, with visibility cut to almost zero, three suicide bombers hurled themselves at a Kurdish security installation located in an abandoned hotel and several nearby checkpoints, intent on punching holes in the concrete and rebar defenses. They failed, but not before they’d killed at least four sentries, who had been unable to spot the bombers before it was too late.
Across war-torn swathes of Iraq and Syria, particularly in the hilly regions, weather has become a military ploy in the war with ISIS. Storms often provide rare windows in which the jihadists can win decisive breaks on the ground. For those facing off against ISIS along a several hundred-mile-long battlefront, it’s added a level of worrying unpredictability. “When you’re fighting in nice weather, there are few surprises,” Ebbas Mohammed, an officer in the Syrian Kurdish YPG militia told me soon after his unit had beaten back an ISIS night time strike to the south of Qamishli, in late 2014. “But when it’s raining, when you can’t see, anything can happen.”
ISIS first displayed this environmental awareness when its fighters surged out of the desert in June 2014 and seized Mosul, Iraq’s second largest city and the country’s northern hub. Operating in convoys of Toyota pick-up trucks, its fighters launched lightning assaults on unsuspecting towns and cities, where demoralized and poorly led security forces mostly melted away in fright. But with the black-flag-waving extremists threatening to push even deeper into Iraq, the U.S and its regional allies stepped in, targeting the group’s vehicles with air strikes. ISIS commanders, unable now to dispatch conspicuous convoys across the semi-arid flatlands, were left with no choice but to change tactics.
Recognizing that heavy rain and overcast skies limit ground-spotters’ ability to accurately send coordinates to circling coalition bombers in the sky, jihadists have taken to attacking with greatest frequency from early December to early March, when Iraq’s usual sun and high temperatures—which can reach 130 degrees Fahrenheit—temporarily give way to fierce downpours, occasional electrical storms, and overcast skies. The four most deadly attacks against Iraqi Kurds, including those at Gwer and Kirkuk, have since taken place in winter. Humanitarian coordinators in Iraq say that when the skies turn particularly bleak, there’s almost bound to be a battle. They call these conditions “ISIS weather.”
“The first time I went to the Bashiqa area [a jihadi-occupied town near Mosul] in 2014, there was a storm. It was awful, and you just knew there was going to be an attack,” says Tom Robinson, director of the Rise Foundation, which analyzes aid requirements among displaced Iraqis. “And yes, that night the weather gave them an opportunity to crawl right up to the Pesh positions.” Robinson and his colleagues, like others who work in close proximity to the frontline, are careful to factor the weather into their risk assessments when considering potential dangers in the field.
Anti-ISIS troops have, for the most part, realized their increased vulnerability during inclement weather and have changed their behavior as well. Rather than relax during lulls in the fighting, as is typical in sunnier months, many maintain an edgy alertness when there’s low visibility.
The approaching winter also presents serious obstacles for troops launching counter offensives against ISIS. Armored vehicles get stuck in boggy terrain when the rains come. Narrow routes turn into impassable mires. It’s become relatively common to see stalled or exceedingly slow-moving military convoys near the frontlines in December. It’s so punishing that the U.S Army, which has special forces on the ground in Iraq and eastern Syria, has a history of restricting operations during the nastiest conditions for fear of being unable to offer support if needed. “Patrols generally didn’t leave bases if the weather was so poor as to prevent medevac helicopters from flying,” said Wayne Hsieh, a military historian at the U.S Naval Academy and former Iraq-based State Department political officer.
Taking advantage of the weather as a military tactic isn't new of course. Hundreds of campaigns and battles were determined by severe conditions—from Napoleon’s march on Moscow in 1812, in which his massive army was badly weakened by the cold and ice before it reached the czar’s gates, to the Spanish Armada, which was largely sunk in a storm on its way to invade England in 1588. But in this era of advanced weaponry and hugely improved communications, many thought rain and sun had lost their capacity to affect wars. The violence in Syria and Iraq, where death tolls are soaring and new horrors appear daily, shows we remain as primeval as ever.