Why do airplanes sound louder during the polar vortex? Here are the answers to your biggest deep freeze questions

Minnesotans are usually cold weather pros. But this week's polar vortex ushered in rare, double-digit subzero temperatures that have some doubting how best to weather the deep freeze. We asked Star Tribune readers to send us their most pressing questions:

It's not your imagination. If you've been startled by the rumbling of a plane overhead this week, it's because it's actually louder. Sound travels further in dense, cold air. And because background noise is lower during the winter, a plane overhead sounds louder, said Steve Orfield of Orfield Laboratories, which analyzes acoustics and other environmental factors.

"It's [on] brutally cold days we really hear the difference," said Dana Nelson, the manager of noise, environment and planning for the Metropolitan Airports Commission.

But she said the office doesn't field any increase in complaints from residents who live near the airport on frigid days since fewer people are outside. Plus, there's a positive side to the dense, cold air: it can improve a plane's acceleration. Planes climb faster from the runway, increasing altitude better in the subzero temps, she said.

Homeowners should keep faucets running if they have pipes that are in cold areas, such as unheated crawl spaces or adjacent to outside walls.

"Drip-drip is not going to stop a freeze," said Mike Larson, manager at Norblom Plumbing in St. Paul. "It needs to be a slow-trickle stream to help prevent the freeze."

Every house is different, but turn on the faucets that are closest to the cold pipes so the water will flow and prevent freezing. It also helps to open cabinet doors in kitchens or bathrooms to help furnace heat reach pipes. If there are pipes that have frozen before, consider using a space heater, Larson said. The problem affects both newer and older homes, he said.

"We got a barrage of calls this morning," Larson said.

Xcel Energy says to keep the doors of your refrigerator and freezer shut as much as possible to maintain the cold temperature. If an outage lasts eight hours or more, food in your refrigerator begins to spoil and may no longer be safe to eat, so pack your food into a cooler with ice. And on Wednesday, after outages, Xcel Energy asked customers to turn down thermostats to at least 63 to reduce consumption and avoid further outages.

While schools and businesses are shut down, tow trucks are out in full force to respond to stalled vehicles and flat tires. The AAA office in St. Louis Park had more than six times the normal number of calls seeking help Wednesday, spokeswoman Meredith Terpstra said, adding: "It's all hands on deck."

She said AAA recommends that motorists warm up their car every four to six hours, if possible (turn off your lights, radio and heaters to help the engine warm up faster) and after the car warms up, drive it around the block for a few minutes.

If your car doesn't start in the subzero temps and you don't need to drive it, you can wait for the thermometer to rise (40s are in this weekend's forecast); you'll have a better chance it will start then, said Gary DeRusha, the operations manager at Bobby & Steve's Auto World in Eden Prairie, where the staff turned the car wash into another spot to check car batteries on Wednesday.

"I mean, that's a 65-degree temperature swing," DeRusha said. "If you don't have to drive, don't … or head to Tampa, Florida."

Batteries last about three years, so if your battery is older, get it tested, he added. If you do have to drive, have a winter survival kit in your car that has blankets, a flashlight and spare batteries, bottled water and snacks, booster cables, tools, and sand or cat litter to help with traction.

Dr. William Roberts, a U sports medicine physician, explained to Runner's World that the burning sensation when you're breathing in cold air is due to the combination of heat and water occurring in cold air, which is very dry.

"For most people, this sensation goes away after a few breaths," he wrote, adding that cold air doesn't harm your lungs but can trigger a bronchospasm attack for someone with asthma.

Canceling school statewide in Minnesota because of the cold is extremely rare. Gov. Arne Carlson made front-page news on Jan. 18, 1994, when he canceled classes in all public K-12 schools after windchills in some areas plunged to 80-below under the measure used at the time (the formula has since been changed). Carlson called off classes again on Feb. 2, 1996 — the day the state recorded its all-time low temperature of 60-below near Tower, Minn., and the Twin Cities dropped to 32-below. Carlson canceled schools for a third time on Jan. 16, 1997. Some Minnesotans complained that local school officials, not the governor, should make that call.

On Jan. 6, 2014, Gov. Mark Dayton canceled classes statewide after windchills plunged to 50-below in some areas of the state. Walz said this week that he was allowing local school districts to make their own decisions about whether to hold classes.

"One of the things I'm concerned about is when you close a school, sometimes that is the place of warmth and food that is not available elsewhere," Walz said, adding that local officials often know best what's right for their students.

The deep freeze will likely kill many invasive emerald ash borer larvae, which have been chewing through the state's ash trees. (Research after the last polar vortex in 2014, when temperatures reached 20 below, showed that 60 to 70 percent of the ash borers died.) But the Japanese beetle larva overwinters in the ground, not in trees. That means that it will be largely unaffected by the cold, turning into the winged beetles that shred roses and apple trees. The only way Mother Nature could kill off the beetles is in a drought when they're unable to live without moisture.

Staff writer Glenn Howatt contributed to this report.