hello
I interpreted your request as : pick one month (from 1 to 12) then define whatever latitude query , and do the interpolation
this boils down to make a 1D interpolation on the column that matches which month you're sking for (I didn't expect someone to ask for month = 3. 1548 for example, in my understanding the month "number" is always an integer)
data = readmatrix('rad data.xlsx',"NumHeaderLines",1);
lat = data(:,1); % latitude data
rad_data = data(:,2:end); % radiation data is an array which columns are the 12 months averages
% define which month
month = 3;
rad_data_this_month = rad_data(:,month);
% interpolate this array vs latitude query
lat_query = 86; % must be within -90 / +90 range
rad_data_int = interp1(lat,rad_data_this_month,lat_query)
