What is UTC ? How do I tell at what time a satellite picture was taken ?

UTC stands for Universal Time Coordinated, what used to be called Greenwich Mean Time (GMT) and Zulu Time (Z). This is the time at the Prime Meridian (0° Longitude) given in hours and minutes on a 24 hour clock. For example, 1350 UTC is 13 hours and 50 minutes after midnight or 1:50 PM at the Prime Meridian.

The Greenwich Royal Observatory at Greenwich, England (at 0° Longitude) was where naval chronometers (clocks) were set, a critical instrument for calculating longitude. This is why GMT became the standard for world time. Meteorologists have used UTC or GMT times for over a century to ensure that observations taken around the globe are taken simultaneous
On most satellite pictures and radar images the time will be given. If it's not in local time then it will usually be given as UTC, GMT, or Z time.
To convert this to your local time it is necessary to subtract the appropriate number of hours for the Western Hemisphere or add the correct number of hours for the Eastern Hemisphere. And don't forget the extra hour adjustment for Daylight Savings Time or Winter Time over Standard Time for your zone.

(Credit: Neal Dorst, NOAA)

Local Time Zone Time Adjustment (hours) (- means subtract hours; + means to add hours):

Atlantic Daylight Time (ADT) -3
Atlantic Standard Time (AST)
Eastern Daylight Time (EDT) -4
Eastern Standard Time (EST)
Central Daylight Time (CDT) -5
Central Standard Time (CST)
Mountain Daylight Time (MDT) -6
Mountain Standard Time (MST)
Pacific Daylight Time (PDT) -7
Pacific Standard Time (PST)
Alaskan Daylight Time (ADT) -8
Alaskan Standard Time (ASA) -9
Hawaiian Standard Time (HAW) -10
New Zealand Standard Time (NZT)
International Date Line Time (IDLE) +12
Guam Standard Time (GST)
Eastern Australian Standard Time (EAST) +10
Japan Standard Time (JST) +9
China Coast Time (CCT) +8
West Australia Standard Time (WAST) +7
Russian Time Zone 5 (ZP5) +6
Russian Time Zone 4 (ZP4) +5
Russian Time Zone 3 (ZP3) +4
Bagdad Time (BT)
Russian Time Zone 2(ZP2) +3
Eastern European Time (EET)
Russian Time Zone 1(ZP1) +2
Central European Time (CET)
French Winter Time (FWT)
Middle European Time (MET)
Swedish Winter Time (SWT)
Middle European Winter Time (MEWT) +1
Western European Time (WET)
Greenwich Mean Time (GMT) 0

This week's question comes from Rob Gutro. Rob is a meteorologist who works at NASA Goddard in Public Affairs. He did forecasting for the Weather Channel Radio Network, was a technical writer for NOAA, provided broadcasts on NOAA Weather Radio in Baltimore, and was a radio personality for 20 years in various cities.