Iframe Tag just adds another window on your Webpage and due to which whenever a browser loads a page with an iframe it loads the original Page and Iframe as a separate 2 different pages. So in some cases Iframe is not-supported and thus, displays an Error.
Sigma is a Greek letter that corresponds somewhat to our letter S and thus sigma is often used to stand for some concept that starts with S. There is an upper case (capital) sigma and a lower case (small) sigma. They are and. is most ofter used to indicate a sum.
sigma-t= [(Sea water density - 1) x 1000] Since seawater density is always slightly more than 1 g/cm3 and measured to the nearest .00001 of a gram, oceanographers do not want to worry about the decimal places or writing the 1 everytime.