Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of air cargo specialist Atlas Air Worldwide (Nasdaq: AAWW) were in a nosedive today, falling as much as 15% in intraday trading after a Wall Street downgrade.

So what: A SunTrust Robinson Humphrey analyst downgraded Atlas Air from "buy" to "neutral," triggering today's sell-off. The call follows a second-quarter earnings report that topped expectations, but left investors feeling flat after the company lowered its full-year outlook.

Now what: The market action today suggests that investors are shooting first and asking questions later when it comes to Atlas. But should they? Management's lowered guidance would have the company pulling in $5 per share for all of 2011. If the company can manage that, then shares would currently be trading at just a bit more than seven times earnings. A downgrade like this can be a good reason for investors to revisit their assumptions, but it doesn't necessarily mean that long-term investors need to rush to hammer the sell button.

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