Already a couple different solid answers here, with slightly different methods, but I'm going to post it anyway. One difference with mine is that it keeps everything in a single table:
IMPORTANT NOTE: I did not correct your inequalities as @Voss did. But I believe the same thing he does, that you mixed them up.
% Read the table
tbl = readtable("T.xlsx",'VariableNamingRule','preserve');
% Calculate the median T from the columns, and add it as a column
tbl.medianT = median(tbl{:,["T0","T1","T2","T3","T4","T5","T6"]},2);
% Add an empty column for the sunniness
tbl = addvars(tbl,strings(height(tbl),1),'NewVariableNames','Sunniness');
% Fill in the sunniness for each median range
tbl{tbl.medianT <= -12, "Sunniness"} = "sunny area";
tbl{tbl.medianT > -12 & tbl.medianT <= -14,"Sunniness"} = "sun-shade area";
tbl{tbl.medianT > -14, "Sunniness"} = "shaded area";
disp(tbl)
