feat: Add weather phenomenon decoding to METAR parser

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

metar.html

@@ -89,6 +89,66 @@
             return `${code}: Visibility ${vis} statute miles`;
         }
 
+        function decodeWeather(code) {
+            const originalCode = code;
+            const intensityMap = {
+                '-': 'Light ',
+                '+': 'Heavy '
+            };
+            const descriptorMap = {
+                'MI': 'Shallow ', 'PR': 'Partial ', 'BC': 'Patches of ', 'DR': 'Low drifting ',
+                'BL': 'Blowing ', 'SH': 'Showers of ', 'TS': 'Thunderstorm with ', 'FZ': 'Freezing '
+            };
+            const weatherMap = {
+                'DZ': 'drizzle', 'RA': 'rain', 'SN': 'snow', 'SG': 'snow grains',
+                'IC': 'ice crystals', 'PL': 'ice pellets', 'GR': 'hail', 'GS': 'small hail and/or snow pellets',
+                'UP': 'unknown precipitation', 'BR': 'mist', 'FG': 'fog', 'FU': 'smoke',
+                'VA': 'volcanic ash', 'DU': 'widespread dust', 'SA': 'sand', 'HZ': 'haze',
+                'PY': 'spray', 'PO': 'well-developed dust/sand whirls', 'SQ': 'squalls',
+                'FC': 'funnel cloud(s)', 'SS': 'sandstorm', 'DS': 'duststorm'
+            };
+            const precipitationTypes = ['DZ', 'RA', 'SN', 'SG', 'IC', 'PL', 'GR', 'GS', 'UP'];
+
+            let decoding = '';
+            let codeToParse = code;
+
+            if (codeToParse.startsWith('-') || codeToParse.startsWith('+')) {
+                decoding += intensityMap[codeToParse[0]];
+                codeToParse = codeToParse.substring(1);
+            }
+
+            let vicinity = '';
+            if (codeToParse.startsWith('VC')) {
+                vicinity = ' in the vicinity';
+                codeToParse = codeToParse.substring(2);
+            }
+
+            const chunks = codeToParse.match(/.{1,2}/g) || [];
+            let decodedChunks = [];
+            let hasPrecipitation = false;
+
+            chunks.forEach(chunk => {
+                if (descriptorMap[chunk]) {
+                    decodedChunks.push(descriptorMap[chunk]);
+                } else if (weatherMap[chunk]) {
+                    decodedChunks.push(weatherMap[chunk]);
+                } else {
+                    decodedChunks.push(`unknown (${chunk})`);
+                }
+                if (precipitationTypes.includes(chunk)) {
+                    hasPrecipitation = true;
+                }
+            });
+            
+            if ((codeToParse.includes('TS') || codeToParse.includes('SH')) && !hasPrecipitation) {
+                decodedChunks.push('rain');
+            }
+            
+            decoding += decodedChunks.join('');
+
+            return `${originalCode}: ${decoding.trim()}${vicinity}`;
+        }
+
         function decodeClouds(code) {
             const coverMap = {
                 'SKC': 'Sky clear',