Plot multiple maps with transparency using raster data

I have a three-Dimension dataset var_slice, the first and second dimensions are latitude and longitude, the third is time. I want to plot all the images for all times together by setting the transparency. So I can get a final figure in which regions have multiple occurence have darker color. How can I overlay all the images? I put the code to plot the map for a singe time below and attached the data. Any help would be appreciated.

  % load the 3-D data
    load var_slice.mat 
    load lat.mat
    load lon.mat
    
    % plot
    pcolor(lon,lat,var_slice(:,:,1))
    shading interp;
    hold on
    borders('continental us','color','k','linestyle','-','linewidth',1.2)
    % axis tight
    hold off
    ylim([25,50])
    xlim([-130,-95])

Answer 1

[Walter Roberson]

  % load the 3-D data
    load var_slice.mat 
    load lat.mat
    load lon.mat
    
    % plot
    for K = 1 : size(var_slice,3)
        s = pcolor(lon,lat,var_slice(:,:,1));
        s.FaceAlpha = 0.05;
        s.EdgeColor = 'none';
        hold on
    end

    %shading interp;
    borders('continental us','color','k','linestyle','-','linewidth',1.2)

Unrecognized function or variable 'borders'.

    % axis tight
    hold off
    ylim([25,50])
    xlim([-130,-95])

[Qian]

Thank you so much for your help. In the for loop the index should be K: s = pcolor(lon,lat,var_slice(:,:,K)). It works! The borders can be added using https://www.mathworks.com/matlabcentral/fileexchange/50390-borders. Thanks again.