They say this is because the red warning lights aren't visible in fog or low cloud.
BBC: London helicopter crash: What are the rules for pilots?
The Met Office issued a rare red warning warning of snow and blizzards on Thursday.
When swimming, watch out for the red warning flags or ask a lifeguard as the waves can be harsh.
Heavy snow is falling in Wales, where a rare Met Office red warning for severe disruption has been issued.
That date is generally taken three years from the point at which policyholders received their red warning letter from their provider.
"It also is kind of a warning sign or red flag that hey, we really need to tackle this problem, " he said.
With the stock market tumbling in the third quarter and other warning signals flashing red, there has been speculation that private equity portfolios would be hit hard and show big losses.
FORBES: Private Equity Surprise: Portfolios Are Holding Up Fine
The Met Office also had a rare red warning for snow and blizzards in part of the Brecon Beacons and Black Mountains while the rest of the country had an amber warning.
But the drug also leads to a change on electrocardiogram readings that is thought, in some cases, to be a flashing red warning that a drug could cause fatal arrhythmias, or heart rhythm disturbances.
With around four years to go at that point, it is doubtful that red warning signals were flashing over the 2014 hosts Brazil. (Some may argue that with the test event of a Confederations Cup in 2013 the organizers had less time).
FORBES: Why The IOC Should Be Very Worried About FIFA And Brazil 2014
The National Weather Service said a red flag warning was in effect until Friday evening.
Generally, a bearish momentum divergence is a red flag warning signal, which often presages a pullback or correction.
President Obama even made a light-hearted reference to Mr Netanyahu's warning of a "red line" with Iran's nuclear programme.
The National Weather Service in San Diego said a red flag warning was in effect until Friday evening.
The red Fire Warning light is bad news when it lights up, but you can silence the alarm bell by pressing it.
Jackson's ability to keep his private side private meant AEG executives could not see any red flags warning of Jackson's destruction, Putnam said.
Oriental Daily News says Hong Kong must upgrade its red travel warning for Egypt as travel agents and tourists are still going there for non-essential travel.
And, this whole medication-forgetting incident is a red flag warning.
He said the red snow warning remained in place with severe gale force winds and blizzard conditions expected to continue throughout the day and overnight, with only a slow improvement expected on Tuesday.
The blaze quickly moved through the area as Southwest Idaho remained under a red flag warning Sunday and Monday because of high temperatures, low humidity and high winds conditions conducive to explosive and destructive fires.
The lawsuit contends that AEG Live executives missed a series of red flags warning them that Jackson's life was at risk because of Dr. Murray, who was giving him nightly infusions of the surgical anesthetic propofol to treat his insomnia.
The warning signs would have triggered red flags and the suspect would have never been allowed to board that plane for America.
Last month the Commons Education Select Committee issued a damning report into the plans, which it described as a "red light" warning over the changes.
In 1996 Lockheed won competitions to build a replacement for the space shuttle, an infra-red satellite-warning system and a new rocket-launcher for the American air force.
Friday's warning was "a major red flag" and "quite a bit more expansive" than past alerts, said Kathleen Fairfax, vice president for global education at Arizona State University.
Last week a report from the Commons Education Select Committee issued a "red light" warning to the government, urging it to slow down and rethink its proposed changes to GCSEs and the exam system.
BBC: Planned switch from GCSEs to Baccalaureate in England 'abandoned' - BBC News
The campaign's Drink Smarter website shows a picture of a woman to demonstrate how the drinking mirror works, along with a warning that "deeper wrinkles, red cheeks and weight gain" are just some of the visible effects of regular heavy drinking.
CNN: 'Drinking Mirror' app aims to show how alcohol ages you
And health chiefs in Wales have said the single vaccine is a "red herring" and are warning parents against using it.
But he batted down any suggestion that Intel's Red Hat investment was a warning to Microsoft, which wants to win corporate hearts and wallets with its NT operating system running on Intel-based server machines.
"For now the warning light is orange but it risks turning red if the appropriate measures are not taken, " he said, recommending a national conference of all political parties to forge a common anti-terrorism strategy.
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