A weather cancellation or delay is closure, cancellation, or delay of an institution, operation, or event as a result of inclement weather. Certain institutions, such as schools, are likely to close when bad weather, such as snow, flooding, tropical cyclones, or extreme heat or cold impairs travel, causes power outages, or otherwise impedes public safety or makes opening the facility impossible or more difficult. Depending upon the local climate, the chances of a school or school system closing may vary. While some regions may close or delay schools when there is any question of safety, others located in areas where bad weather is a regular occurrence may remain open, as local people may be accustomed to travelling under such conditions.
Many countries and sub-national jurisdictions have mandates for a minimum number of school days in a year. To meet these requirements, many schools that face a likelihood of closure build a few extra school days into their calendar. If, by the end of the year, these days are unused, some schools give students days off. If all snow days are exhausted, and inclement weather requires more closures, schools usually make the days up later in the year. US State education departments have, for example by administrative decision late in the 2015 Texas school year, occasionally issued waivers to schools, so that they do not need make up days for weather-related cancellations.
There are various reasons for weather-related cancellations: such as icy streets, dangerous quantities of snow, etc.
Safety is prime when deciding whether to cancel or delay. Officials may close schools to prevent accidents and other problems caused by inclement weather.
Minor storms, when safety is of less concern, may cause few or no cancellations or delays. In severe inclement weather, however, only the most essential operations remain functional. Operations considered essential include health care, emergency services, and retail of basic necessities. In health care environments, employees may remain at the facility around the clock if travel is impaired or dangerous, as those facilities contain basic accommodations for sleeping and eating such as on-call rooms. While tourist attractions generally close, those housing live animals may need essential employees to provide proper animal care.
Television and radio services generally keep operating (unless they in turn are taken off the air by the weather), and travel as necessary. Elected officials travel as necessary to provide services to the public. Snow removal crews remain at work.
Some inclement weather makes road passage impossible or difficult. In developed nations, municipalities attempt to clear snow-covered roads—but this is not always possible, and often many cannot travel. In deeper snowfalls, personal vehicles may become trapped, and their removal may take several days. This influences decisions on closures beyond the end of snowfall. The ability of employees to reach work places is a factor.
Various types of severe weather can damage structures temporarily or render them permanently useless, cause power outages, or prevent heat or air conditioning from working.
A snow day in the United States and Canada is a day that school classes are cancelled or delayed by snow, heavy ice, or extremely low temperatures. Similar measures occur in response to flooding, tornado watches, and severe weather (storms, hurricanes, dense fog, heavy snow, etc.). The criterion for a snow day is primarily the inability of school buses to operate safely on their routes and danger to children who walk to school. Often, the school remains officially open even though buses do not run and classes are canceled.
More and more schools are closing due to extreme heat, "heat days". Heat days are like snow days. Some districts (San Diego Unified) provide schools with an operational procedure that is used as a guideline to address what actions schools should take at different levels of rising temperatures. 
Schools and businesses may also be canceled for reasons other than bad weather such as a pandemic, power failure, terrorist attack, or bomb threat, or in honor of a school staff member who has recently died. In some cases, only one school or business in a town may close, due to localized issues such as a water main break or a lack of heat or air conditioning.
Inclement weather that causes cancellation or delay is more likely in regions that are less able to handle the situation. Snow days are less common in more northern areas of the United States that are used to heavy winter snowfall, because municipalities are well equipped to clear roads and remove snow. In areas less accustomed to snow—such as Atlanta, Dallas, Memphis, or Chattanooga—even small snowfalls of an inch or two may render roads unsafe.
In most areas, schools include extra days in their calendar as "built-in" snow days which allow a school to get in the minimum number of hours or days for instruction (in Wisconsin for instance, 180 minimum teaching days are mandated by law). When the number of snow days taken is less than the number of built-in days, the days are given back by extending Memorial Day weekend, or making the school year end earlier. Once the number of snow days taken exceeds the number of built-in days, the snow days must be made up. In other states, like Maine, all snow days must be made up. For example, schools may open school on some federal holidays, shorten spring break, or make the school year end later. However, some schools are more forgiving, and do not mandate make-up days. In the event of heavy snow, tools have arisen to help students calculate the chance of a weather-related cancellation, most commonly snow days.
In the United Kingdom, snow days are a relatively uncommon event, especially in southern regions. Scottish, Northern Irish, Welsh, and Northern English schools may experience some closures during the winter months, often due to travel disruption. In Southern England, however, significant snowfall is a less frequent event, rarely lasting for more than a few days across low-lying areas.
Instead of canceling an entire school day, some schools may delay opening by, for example, one or two hours, or announce a particular opening time. This can be advantageous in places where schools are not charged a "snow day" by delaying their opening. Many school authorities cancel the whole morning kindergarten under these circumstances. This is particularly common during snowfalls of 2 inches (5.1 cm) or less in areas not accustomed to winter snowfall, such as the Baltimore metropolitan area and points south.
In the event of fog, some schools may delay the opening of school three hours but extend the day an extra hour.
Some schools in Sydney and New South Wales may, at their discretion, close when temperatures reach above 40 degrees Celsius (104 Fahrenheit).
Some school districts in the United States close at their discretion for high temperatures and air quality.
Workplaces are less likely to close during mildly inclement weather, but the more severe the storm, the more likely a workplace is to close.
Some employers who use the most essential types of employees, such as health care facilities, have some or all of their employees stay and sleep on the premises while off duty if bad weather that hampers commuting is anticipated. Many supermarkets, convenience stores, and gas stations try to remain open to meet public need and the opportunity for increased business. Less critical businesses, such as clothing or antique shops, may close in moderate or severe weather.
In most severe storms, police and fire departments use specialized vehicles to respond to emergencies. Other workers involved in handling issues pertaining to the inclement weather, such as snow plow operators, report to work, and reporters and local elected officials stay on duty to serve the public.
In severe weather, airlines, rail service providers, bus operators, and other public transport may cancel or reduce services. Route impassability, airport closure, employees' safety, and public safety may result in such action. However, some modes of transport are more prone to severe weather than others, and different forms of bad weather have different impacts.
Even when the service does operate, unplanned delays are possible, as operators take extra precautions or routes are congested. The level of service provided may be diminished due to a lower demand for service, fewer operators being available, or fewer passable routes. Yet sometimes demand for public transport may increase as more commuters choose not to drive their own vehicles.
In particularly hot weather, rail vehicles may be delayed by the expansion of the steel tracks.
Public transport may continue to operate on main arteries, though they may still experience delays. Buses that operate on secondary roads, especially those that are narrow or difficult to negotiate, may either be completely cancelled or diverted to a main road.
Although bad weather generally has little effect on an underground subway system in a major metropolitan area, electricity outages and flooding may disrupt services. For example, on August 8, 2007, following a tornado in Brooklyn in New York City, the New York City Subway flooded, causing all trains to grind to a halt during rush hour. Increased demand may result from employers and schools clearing out in anticipation of a tropical cyclone, hurricane, or other severe weather system as people turn to the subway when other modes of transport are threatened.
- "Statement of Commissioner Michael Williams regarding school district issues related to flooding". Texas. 2015-05-26. Retrieved 2015-07-12.
- "Houston ISD won't schedule flood makeup day". Houston, Texas: Houston Chronicle. 2015-05-26. Retrieved 2015-07-12.
- "Administrative Policy".
- Larsen, Dave (2009-01-27). "School districts are using up calamity days". Dayton Daily News. Dayton, Ohio. Retrieved 2009-02-05.
Ohio school districts can use five calamity days before they must start adding extra days to the school calendar.
- Willis, Donna (2009-01-30). "Districts Consider Calamity Options". WCMH-TV. Columbus, Ohio: Media General. Archived from the original on 2011-06-15. Retrieved 2009-02-05.
- Ferris, Joleen (2009-01-28). "Decision for city schools to stay open prompts calls from irate parents". WKTV. Utica, New York: Smith Media. Archived from the original on 2009-01-30. Retrieved 2009-02-05.
- Wolff, Christine; Tanya Albert (1999-03-09). "Snow may stretch out school year". The Cincinnati Enquirer. Cincinnati, Ohio. Retrieved 2009-02-05.
- O'Connor, Anahad; Bowley, Graham (2007-08-08). "Tornado Hits Brooklyn; Subway Back in Service". The New York Times.
- cancellations.com. Can be used to find cancellations or delays in your area or if a particular institution has canceled its plans.
- schoolsout.com. Lists school closings in 18 U.S. states and the District of Columbia
- Snowday.co.uk. Can be used to find out if schools or colleges are closed in bad weather. [UK only]
- Snow After. Accurate snow day calculator using information from National Weather Service.