The odds of you forgetting to pack something on your next outdoor expedition are about 1 in 2. The chances of you underestimating the weather are roughly 1 in 10. The odds of you getting the perfect outdoorsy selfie that scores 100 likes are about 1 in 50 (be realistic). Worry about these odds later, because your chances of scoring a permit in one of these parks' lotteries are lower than any other park in America.

What makes these four parks so picky? Millions of people stomp, claw, grope, scrape, and poke at them every year, and often direct their unintentional abuse at a single –- and probably the most famous –- feature. Heavy traffic jeopardizes the preservation of these premier parks’ unique ecosystems, and can also lead to safety concerns for the visitors (i.e. your kid getting bowled over by a group of eager German tourists climbing up Half Dome’s cables).

Lotteries offer a win-win solution: the National Park Service (NPS) gains funds while visitors get a less crowded experience. Lottery fees help weed out visitors who aren’t serious about experiencing a particular feature, so consider yourself among outdoor purists if you win.

Winning a permit is not exactly a walk in Golden Gate Park (which does not require a permit, by the way) –- you’ve got to be determined and crafty. Even if you optimize your chances, you’ve still got to be aware of game-changers like extreme weather events, Super Moons, summer solstice, rabies outbreaks, and last year’s government shutdown. If those don’t scare you, you’re ready to take on the four most competitive park permit lotteries in America:

1.) HALF DOME DAY HIKER PERMIT LOTTERY

Photo by iStock / jjwithers

Until 2008, nearly 1,200 visitors a day trekked up the cables of Half Dome; a part of the 16-mile hike. Now permits are capped at 300 per day, and stretch from May 21st to October 12th. Here’s the day permit breakdown: up to 225 in the pre-season lottery, up to 75 for multiday backpackers, and the remainder for the daily lottery. Backpackers can stipulate they’d like a day-hike permit to go with their wilderness permit, and might get more leniencies.

There were over 30,000 total applicants last year, so there’s stiff competition. You’ve got less than a 20 percent chance of scoring a Saturday permit. So ask yourself, do you absolutely have to hike Half Dome on Saturday? Your chances of getting a permit for Friday or Sunday are twice as good. If you must choose Saturday, however, it’s best to aim for August or September, when there are half as many lottery applications going into the pot (and the hot sun you’ll practically be touching is little more forgiving).

If your dates are more flexible, you’ve got a better shot. Your winning potential rises to 50 percent and can go up to 100 percent if you shoot for somewhere between Monday and Thursday. You, the “trip leader,” can have six other people in your group, but beware: larger groups have worse odds for the daily lottery. You can only submit an application once, but the other people in your group should all apply to increase your odds. Just make sure one of the alternative trip leaders listed will actually be there, and is not your 14-year-old kid brother.

Enter the Half Dome day hike permit lottery here. It costs only $4.50 to apply and an additional $8 is charged if you win. Talk about cheap thrills.

Photo by iStock / sarahneal

2.) GRAND CANYON RIVER PERMIT LOTTERY

Before the 2006 introduction of a lottery, there was a 25-year-long waitlist to get a permit for a self-guided trip from Lees Ferry to Diamond Creek on the Colorado River. People were basically willing to die waiting. This year, over 4,200 applications competed for only 460 launch opportunities. Thankfully the Grand Canyon lottery gods are merciful: the weighted lottery uses a point system to give the previous years’ losers a better shot.

Your best chance of scoring a trip is to exclude trip leaders and potential alternative trip leaders (PATLs) that have been down the Colorado in the past four years. This ensures you get five points (or “chances”) thrown into the pot. To further increase your odds, try buttering up anyone with “bonus points” to join you –- these are people who were on the waitlist for a long time before 2006. NPS is also considering doubling the points of lottery applications that list PATLs, so get authorized as a trip leader if you can.

Previously waitlisted members are allowed to lay claim to some of the 500 launch dates permitted each year, which can remove more than 100 dates from the lottery. Statistics from the 2013 lottery show that most dates were requested in May through June, followed closely by September through October. Winter launch dates (November through February) were much less in demand – merino wool, anyone? Winter weather can be risky, so go for a mid-summer trip’s dream instead.

Enter the Grand Canyon river permit lottery here. It costs $25 to apply and an additional $100 per person in a winning group. It’s pricey, but definitely worth it, even if you have to carry your poop with you.

3.) DENALI ROAD LOTTERY

Photo by iStock / Wheninusa

Most people have to view Denali National Park by an NPS shuttle, but you’re not most people, are you? Each September the park hosts a four-day event that allows a select few to drive their personal vehicle down Denali Park Road. That’s right: none of those coughing, sneezing, loud, and odor-deficient shuttle people to (literally) cramp your style.

It’s the stuff of dreams. The lottery attracts over 10,000 applicants a year, but only 400 per day are chosen. That's about a 1 in 7 shot. While you can select the order of dates you'd prefer, it won't affect your chances: you are assigned a date based on availability.

You can only apply once, but "there is not an established group size" as long as everyone has a seatbelt (now's the time to buy that 1980s mini-van you've been eyeing). More good news: winning permits are transferrable. Get all of your friends, family, neighbors, and whoever else you can rally to apply -- they can transfer the permit to you with a written letter.

Outside of locals, the road lottery has mostly flown under the rest of America’s radar –- do yourself a favor and don’t spread the word. Enter the Denali road lottery here –- shh! It’s only $10 to apply and an additional $25 if you win.

Photo by iStock / EdStock

4.) YELLOWSTONE SNOWMOBILE PERMIT LOTTERY

Yellowstone recently announced it would begin accepting applications for self-guided snowmobile trips for this coming winter. It’s a big deal for the park, which hasn’t granted these permits in over 10 years.

The lottery will award access to one group of up to five snowmobilers per day, through each of its four winter entrances. The West Yellowstone entrance is expected to be the most popular, so aim for one of the other three. You can select up to 8 different date and entrance combinations to increase your chances. Remaining permits will be sold on a first-come-first-serve basis.

Enter the Yellowstone snowmobile access lottery here. It’s only $6 to apply and an additional $40 if you win, plus Yellowstone’s standard gate fee. Make sure your snowmobile is on the current Best Available Technology list.

Still feeling lucky? Why not try a few of these honorable mentions, hot shot: permits to hike Mount Whitney, which connects to the John Muir Trail; the Four Rivers lottery in Salmon-Challis National Forest; and the McNeil River (guided) bear viewing access permit in Alaska.

Follow  Sierra on Facebook, Twitter, Pinterest, Instagram, and YouTube.