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

The stock market ended modestly lower Monday, as weakening numbers from the services sector and extreme winter weather conditions from the Midwest to the Southeast put investors in a sour mood. The Institute for Supply Management's Non-Manufacturing Index fell from 53.9 in November to 53 in December; while any reading above 50 indicates expansion, slowing expansion generally doesn't sit well with Wall Street. The Dow Jones Industrial Average (DJINDICES:^DJI) fell 44 points, or 0.3%, to end at 16,425. 

The more immediate concern for many millions of Americans is the freakish frigidity being seen from Illinois to Tennessee and beyond. Caused by a so-called "polar vortex" -- a term for when wayward winds from the North Pole drift south -- many areas are experiencing near-record lows, and the governor of Illinois even proposed a statewide disaster. Home Depot (NYSE:HD), while it's been able to keep snow removal products at higher prices, still lost 1% today as investors fear the cons of the cold weather may outweigh the pros. Home Depot stock has benefited greatly from the resurgent real estate market, but one can hardly blame Dick and Jane for putting off their remodeling project until they can safely drive to the store. 

Unfazed by Arctic frosts, Pandora Media (NYSE:P) stock rocketed 14 % higher today, after a double-whammy of good news got investors excited. Its active listener data continued to impress in December, adding 13% from the year before as 76.2 million consumers actively listened to the streaming music service last month. Pandora, which has brilliantly teamed up with automakers to include its services in many new passenger vehicles, also announced it will start selling ads on the auto platform, which should prove to be a welcome new revenue stream for the company.

Another company that knows a thing or two about music in the car, Sirius XM (NASDAQ:SIRI), saw its stock soar 7.3% after a tender offer from Liberty Media, which already owns a controlling 52% stake in the satellite-radio business. The late-Friday offer from Liberty Media valued Sirius XM at roughly $3.68 a share an all-stock deal, and represented a 3% premium to the closing price that day. Today's 7% spike indicates Sirius XM shareholders are looking for a heftier offer -- and that they think they may receive one.

Fool contributor John Divine has no position in any stocks mentioned. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

The Motley Fool recommends Home Depot and Pandora Media and owns shares of Liberty Media. and Sirius XM Radio. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.