--- ray/doc/man/man1/gendaymtx.1	2020/04/13 17:12:19	1.12
+++ ray/doc/man/man1/gendaymtx.1	2021/02/17 02:30:08	1.15
@@ -1,4 +1,4 @@
-.\" RCSid $Id: gendaymtx.1,v 1.12 2020/04/13 17:12:19 greg Exp $
+.\" RCSid $Id: gendaymtx.1,v 1.15 2021/02/17 02:30:08 greg Exp $
 .TH GENDAYMTX 1 01/19/13 RADIANCE
 .SH NAME
 gendaymtx - generate an annual Perez sky matrix from a weather tape
@@ -13,10 +13,12 @@ gendaymtx - generate an annual Perez sky matrix from a
 ][
 .B "\-d|\-s|\-n"
 ][
-.B "\-D sunfile"
+.B "\-u"
 ][
+.B "\-D sunfile"
+[
 .B "\-M sunmods"
-][
+]][
 .B "\-r deg"
 ][
 .B "\-m N"
@@ -93,8 +95,18 @@ will include its contribution in the four nearest sky 
 distributing energy according to centroid proximity.
 .PP
 The
+.I \-u
+option ignores input times when the sun is below the horizon.
+This is a convenient way to average daylight hours only with the
+.I \-A
+option or to ensure that matrix entries correspond to solar positions
+produced with the
+.I \-D
+option, described below.
+.PP
+The
 .I \-n
-option may be used if no matrix output is desired.
+option may be used if no matrix output is desired at all.
 This may be used to merely check the input, or in combination with the
 .I \-D
 option, below.
@@ -103,7 +115,7 @@ The
 .I \-D
 option may be used to specify an output file to contain a list of
 solar positions and intensities corresponding to time steps in the
-weather tape where the sun has any portion above the horizion.
+weather tape where the sun has any portion above the horizon.
 Sun radiance values may be zero if the direct amount is zero on the input.
 Sun modifiers and names will be indexed by the minute, numbered from
 midnight, January 1st.