But winter can be nearly as bad for those trying to make it into the United States from Mexico. Temperatures can drop below freezing, snow and rainfall can soak migrants and shelter can be days away.
Most illegal immigrants apprehended this time of year are not prepared for the colder weather, said Dove Haber, a spokeswoman for the Border Patrol's Tucson sector, which covers most of the Arizona border except for an area around Yuma.
"I could say the same for the cold as for the heat: If you've been misled about the duration of your trip, if you're told that you're only going to be outside for three hours instead of three days, you're not going to be dressed warmly enough," Haber said. "They'll have basic jackets, but nothing compared with what they'd need, especially if they're crossing in mountainous areas."
The Border Patrol has recorded 27 deaths directly attributed to cold in the Tucson sector in the past four years.
In the fiscal year ending Sept. 30, 400 people died while entering the United States from Mexico in the sector. The primary cause of death was exposure to heat, while other causes include vehicle and train accidents, drownings, fatigue and banditry.
Volunteer groups that work all summer to warn migrants about crossing in the heat switch gears in the winter.
Humane Borders, which places water along desert trails frequented by illegal immigrants, sends people to search for those in need of help after dramatic weather changes such as heavy rains followed by sudden freezing.
The group's founder, the Rev. Robin Hoover, said that during peak migration times in February and March, some migrants who've endured a 28-degree night will walk to a road, flag down a volunteer and say "Get me out of here, call migra" - a slang term for the Border Patrol.
The rest of the year, Hoover said, illegal immigrants will hunker down in remote parts of the desert.
Not everybody who is caught this time of year is close to experiencing hypothermia, "but there are a large number of people who definitely show signs of advanced exposure," he said.
Severe cold snaps have affected hundreds of migrants crossing the Tohono O'odham Indian Reservation twice since 2000.
A winter storm that March forced 350 shivering people to knock on doors or flag down tribal police and seek shelter.
Frigid cold and rain in April 2001 led more than 400 migrants to turn themselves in to tribal police or Border Patrol agents, and five people were taken to Tucson for treatment of hypothermia.
So far this season, the weather hasn't turned cold enough to deter many illegal immigrants from entering Arizona, nor have there been any cold-related deaths or rescues, according to the Border Patrol.
But with temperatures dropping, each agent who goes on patrol takes precautions. Every agent has an emergency bag with blankets, signal flares, shovels, rations, first-aid kits, water and packets of electrolytes that can be mixed with water.
In addition, specially trained search, rescue and trauma agents are ready to apply hot packs to hypothermia victims who are being transported for medical assistance.
"Because we see such temperature extremes in this area, our agents are pretty accustomed to dealing with all such extremes (of heat or cold)," Haber said.