Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: The taxonomy of refraction
    From: Brendan Kinch
    Date: 2011 Jan 10, 12:10 -0800

    I am sorry that I am a bit late with this information. I knew where to look but did not have the book to hand.

    In the Mariner�s Handbook there is information available on refraction that may be of interest, in particular, the paragraphs on �Sub-Refraction�. I quote from the book....
    �Sub-refraction or upward bending occurs when humidity increases and temperature decreases abnormally rapidly with height.
    Likely Conditions:
    Sub-refraction may occur when;
    a. Cool air flows over a relatively warmer sea. This is most likely in coastal waters and especially polar regions in the vicinity of very cold land masses or ice fields.
    b. In warm moist air over the sea when an increase in humidity with height may occur. In this case a temperature inversion will usually accompany the humidity inversion, but when the humidity factor is dominant a sub-refraction will result. These conditions may sometimes be found in the warm sector of a depression in temperate latitudes.

    Optical effect:
    The ranges at which objects are visible are decreased.
    Inferior mirage appears as a shimmering horizon, possibly having the appearance of water and may be seen over hot surfaces such as desert sand, rock or road surfaces when a hot sun is beating down with comparatively cool air above them. Objects such as an island, a coastline or a ship may appear to be floating in air above a shimmering horizon. The lower features of the object, for example the hull of a ship, may be either invisible or have an inverted image underneath. Inferior mirage is uncommon at sea and is more likely to be observed near the coast than offshore.� End Quote.

    There is also a diagram in the book which I cannot scan just now - but can supply later if anybody wishes.


    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site