Update doc and fix make-images workflow

.github/workflows/make-images.yml

@@ -25,7 +25,7 @@ jobs:
 
     - name: Get current date
       id: date
-      run: echo "date=$(date +'%Y-%m-%d') >> $GITHUB_OUTPUT"
+      run: echo "date=$(date +'%Y-%m-%d')" >> $GITHUB_OUTPUT
 
     - name: Create Release
       id: create_release

README.md

@@ -4,14 +4,28 @@ This python code downloads daily worldwide sea surface temp datasets
 and visualizes them either as a world map on a given day, or as a time
 series per year.
 
-Latest files, auto-produced daily by a github action:
+It downloads the sea surface temp data from
+https://www.ncei.noaa.gov/data/sea-surface-temperature-optimum-interpolation/v2.1/access/avhrr.
+That contains daily HDF datasets containing .25° lat/lon cells over
+the entire Earth surface for each day since 1982, including temp and
+temperature anomaly compared to the historical mean for that date.
+
+This code processes those datasets into maps for any given day, or
+line charts with temp per day for the entire time period. The processing steps include:
+- limiting to +/- 60° latitude, to ignore the polar regions
+- removing cells containing > 50% ice cover
+- weighting each cell by its actual area (cells closer to the poles are smaller)
+
+Latest files, auto-produced daily by a github action. You can use these directly via these URLs:
+- https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-map.png
+- https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-anom-map.png
+- https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-all.png
+- https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-all-anom.png
+
 ![SST temp map](https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-map.png)
 ![SST anomaly map](https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-anom-map.png)
 ![SST temp graph](https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-all.png)
 ![SST anomaly graph](https://github.com/garyo/sea-surface-temp-viz/releases/latest/download/sst-all-anom.png)
 
-Examples:
-![SST temp map](doc/sst-temp-map.png "SST temp map")
-![SST anomaly map](doc/sst-var-map.png "SST anomaly map")
-![SST anomaly graph](doc/sst-anomalies.png "SST anomalies")
+For older examples, see the `doc` subdir.