It's the first week of May. The tarmac at most major U.S. air cargo hubs just hit its first 80-plus-degree afternoon of the year. By the end of August, ground temperatures at JFK, Memphis, Louisville, and Atlanta will have averaged above 90 for stretches of two to three weeks at a time. And yet most pharmaceutical shippers I talk to haven't touched their dry ice protocols since they wrote them, regardless of season.
That gap, between the hardest 120 days of the year and the procedures most shippers run year-round, is where the bulk of summer cold chain failures live.
I'll take this in three parts. What actually changes in summer. Why the existing monitoring stack misses it. And what the shippers who have already lived through a few bad summers are doing differently.
What actually changes between May and September
Most cold chain content is timeless. GDP compliance applies in February the same as in August. The physics doesn't care about your fiscal calendar. But the operating environment does, and four things shift between May and September that quietly compound on top of each other.
Sublimation rates climb faster than people expect. The FAA's 2024 sublimation study (DOT/FAA/TC-24/24, the most rigorous public dataset on dry ice in air cargo conditions) showed measurable acceleration in sublimation as ambient temperature rose. The relationship isn't linear at the edges. Once you cross typical hold-room conditions into 80-plus-degree warehouse aisles or 100-plus-degree tarmac surfaces, the rate increase is steep enough that the same five pounds of dry ice that gave you 48 hours of headroom in February might give you closer to 32 in July. Most shippers haven't recalibrated their pack-out specifications for that.
Tarmac dwell time expands. Summer is thunderstorm season across most of the U.S. ramp. ATC delays, ground stops, and weather-driven re-routes pile up fastest from June through August. A pallet that would have moved through a hub in 90 minutes in March can sit on a hot ramp for four hours in July without anyone in the chain calling it a deviation. The shipment is still moving. It's just moving through a much harsher thermal environment than the original lane qualification assumed.
Last-mile vehicles run hotter. The cargo box of a typical refrigerated truck holds its setpoint reasonably well in summer. The cargo box of a non-refrigerated last-mile vehicle, which carries a meaningful share of pharma's same-day and next-day deliveries, doesn't. A van parked in a sunny lot for an hour while the driver makes another stop will see internal temperatures north of 110°F. The dry ice inside the validated shipper compensates, until it doesn't.
Receiving docks get backed up. Summer holidays, vacation coverage, and reduced weekend staffing all increase the time between "delivered to dock" and "received into temperature-controlled storage." A box that arrives at 7 AM with three pounds of dry ice left and sits on a hot dock until 11 AM with only a USB logger inside will show a perfectly fine temperature trace, then start warming after the logger has stopped recording.
None of these are new failure modes. They're the same failure modes the industry deals with year-round, just stacked closer together and operating against a thinner margin of safety. Five pounds of dry ice is the same five pounds in February or July. The clock on it runs faster in July.
Why the monitoring stack misses summer
Here's the part that makes this a monitoring problem and not just a logistics one.
Almost every active failure mode I just listed is invisible to passive temperature logging. A USB logger inside a validated shipper will keep reporting "in spec" right up until the dry ice runs out, because the box is holding its target temperature as long as solid CO₂ is present. The thermal cliff, the moment when the last of the dry ice sublimates and the contents start warming toward ambient, doesn't show up in the trace until it's already happening. By then the shipment is past the airport, past the linehaul, often past the receiving dock.
Active monitoring catches the temperature when it goes out of spec. It doesn't catch the dry ice running out, because dry ice is what keeps the temperature in spec. That's the part of the problem the industry has been pretending it solved.
The summer compounding effect makes this worse. In February a shipment with marginal pack-out can survive a four-hour tarmac hold because sublimation is slow at 40°F ambient. In July that same shipment with the same pack-out and the same hold doesn't have the headroom. The shipper used to get away with it nine times out of ten. In a hot summer, the failure rate climbs in a way that nobody priced in at the start of the year.
What the shippers who have lived through a bad summer are doing
I'll keep this section to what we actually see on the ground, because the rest is theoretical.
Dry ice headroom is the most common adjustment, and the dullest. Shippers who learned the hard way add 10 to 25 percent more dry ice for shipments moving in the summer months and accept the higher per-shipment cost. It's brute force. It works for shipments with a known thermal profile and a predictable lane. It doesn't help if the routing changes mid-flight or a hub experiences an unexpected delay, because the extra dry ice was sized to the original lane qualification.
Lane substitution is the next move. Shippers with multiple routing options will quietly shift summer volume away from hubs known to have weather delays, even when the substitute lane is more expensive. Memphis and Louisville are reliable workhorses, but they have specific weather windows where shipments park on the ramp longer than the contracted service level. A shipper with a thermally tight product will avoid those windows when they can, and accept the freight premium in exchange for predictability.
Increasing monitoring cadence is rarer than it should be. The honest reason is that increasing the cadence on a passive logger doesn't help, because passive loggers don't tell you anything until you read them. Increasing the cadence on an active platform helps, but only if there's somebody on the receiving end of the alert who can do something about it. Most shippers don't have that escalation path stood up. The data flows in. Nobody acts on it. The shipment fails anyway, just with better-documented evidence of the failure.
A smaller group has started thinking about the dry ice itself, not the temperature. They want to know how much CO₂ is left in the box, in real time, on the routes that matter most. Not the temperature trace. The remaining capacity. That's a different measurement and a different alerting model. It's also the only one that gives a useful answer before the temperature alarm fires. This is the group we work with, and the group the rest of the market eventually has to join, because predictive intervention isn't possible without it.
The honest read
Most cold chain content treats failure as an event. A specific shipment, a specific lane, a specific deviation. Summer makes it a season. The same lane that worked in March produces three excursions in July. The same SOP that passed an audit last fall fails one this August. The product hasn't changed. The packaging hasn't changed. What changed is the air the shipment is moving through, and the chance of any one stop running long.
That's why the shippers who have been through a bad summer treat it differently than the ones who haven't. The first group plans for summer the way airlines plan for thunderstorm season: extra capacity, conservative routing, and a working assumption that some things will go wrong. The second group plans for the average year, then deals with summer when it shows up.
Nobody solves summer with hardware alone. You solve it with a combination of harder pack-outs on the lanes that need them, better routing, real-time visibility into the thing that actually matters (dry ice mass, not just temperature), and somebody on staff who picks up the phone when the dashboard says trouble. Take any one of those four out and the failure mode comes back.
It's the first week of May. There are roughly 18 weeks of hot-quarter shipping ahead. The shippers who use them well will spend the next 30 days re-walking their summer SOPs, looking at their high-value lanes against last year's delay data, and standing up the alerting workflow that isn't in place yet. The ones who don't will be writing CAPAs in August about a shipment that left in July, and explaining to a customer why a USB logger that read in spec the whole way didn't actually mean the product arrived intact.
We'll keep writing about what we're seeing. If you're in pharma logistics, cold chain QA, or shipper operations and your lanes get harder between Memorial Day and Labor Day, we'd like to hear how you're handling it. The patterns under the trend are how the trend actually moves.