A heat dome is descending upon as many as 21 states this weekend, begging the question: What exactly is a heat dome?

Moving into the weekend, a large swath of high temperatures is supposed to hit some or all of those states, mostly in the central part of the country. Temperatures could reach as high as 115 degree Fahrenheit in some areas, according to Weather.com. Weather experts are calling the dangerous wave of heat a "heat dome," a reference to the atmospheric dome of high pressure that will create conditions favorable to extreme high temperatures.

Heat alerts have been issued for more than a dozen states in the Midwest and Gulf Coast region, while heat advisories were extended to cover much of the Great Lakes region. Excessive heat warnings are in place for all of Iowa and large parts of Nebraska, North Dakota, South Dakota and Illinois. The heat dome is expected to reach and affect the east coast and mid-Atlantic regions by the end of the weekend. 

Though the term "heat dome" may be unfamiliar to many, it is not new. A heat dome can refer to any large formation of high pressure that results in a bubble of extreme high temperatures. Less than a year ago, in August 2015, a heat dome in Iran led to near record setting temperatures. In the city of Bandar Mahshahr, the heat index, which measures how hot it feels factoring in humidity, reached 165 degrees, nearing the highest ever recorded heat index, which was 178 degrees in Dhahran, Saudi Arabia in 2003.

Weather experts and government officials, including President Barack Obama, are recommending that residents in areas affected by the heat dome this weekend stay indoors if possible. Drinking extra water, checking in on neighbors, especially the elderly, and staying out of the sun are all encouraged. The heat wave could also make it dangerous for pets to be outside. 

The heat dome comes as 2016 looks to easily become the hottest year on record. NASA scientists announced this week that global temperatures so far this year were much higher than in the first half of 2015, which currently holds that record. Gavin Schmidt, the director of NASA’s Goddard Institute of Space Studies said that so far 2016 has "blown [2015] out of the water" and predicted that there was a 99 percent chance 2016's average global temperature would surpass that of 2015.