{"id":1600,"date":"2014-11-21T08:42:23","date_gmt":"2014-11-21T08:42:23","guid":{"rendered":"http:\/\/\/wpweb\/?page_id=1600"},"modified":"2020-10-15T08:53:46","modified_gmt":"2020-10-15T08:53:46","slug":"pedra","status":"publish","type":"page","link":"https:\/\/www.meteo.cat\/wpweb\/divulgacio\/equipaments-meteorologics\/observadors-meteorologics\/manual-de-lobservador\/meteors-relacionats-amb-la-precipitacio\/pedra\/","title":{"rendered":"Pedra"},"content":{"rendered":"\n<h2>Definici\u00f3<\/h2>\n\n\n\n<p>Grans de gla\u00e7 esf\u00e8rics de m\u00e9s de 5 mm de di\u00e0metre, conjunt de grans de gla\u00e7 soldats entre s\u00ed, o trossets de gla\u00e7 irregulars, sovint formats per capes alternativament opaques i transparents. Cauen durant tempestes fortes i gaireb\u00e9 sempre acompanyats d\u2019intensa activitat el\u00e8ctrica. Es considera pedra gran, o grossa, quan el seu di\u00e0metre major \u00e9s superior als 2 cm.<\/p>\n\n\n\n<p>Creixen formant clovelles alternativament clares i opaques.<\/p>\n\n\n\n<p>Quan la seva grand\u00e0ria impedeix els forts corrents verticals mantenir-los en suspensi\u00f3, o els moviments de convecci\u00f3 disminueixen la seva velocitat, els grans de pedra precipiten cap a la superf\u00edcie.<br>Les capes opaques corresponen a fases de r\u00e0pida congelaci\u00f3 de petites gotes subfoses interceptades pel gr\u00e0nul de gla\u00e7. La rapidesa de la congelaci\u00f3 atrapa diminutes bombolles d\u2019aire que donen aquest color blanc al gla\u00e7. Per oposici\u00f3, les capes vidriades i transparents es produeixen per la lenta congelaci\u00f3 d\u2019aigua l\u00edquida que embolcalla el gr\u00e0nul de gel en el seu moviment d\u2019ascens cap a zones amb temperatura negativa.<\/p>\n\n\n\n<p>Cada parell d&#8217;anells blanc-transparent \u00e9s una volta d\u2019ascens i descens completa dins el n\u00favol.<\/p>\n\n\n\n<h2>Consells per a l\u2019observaci\u00f3<\/h2>\n\n\n\n<p>Quan la pedra cau sobre un terreny dur, el cobreix sense trencar-se.<br><\/p>\n\n\n\n<p>No existeix cap m\u00e8tode per con\u00e8ixer si un episodi produir\u00e0 o no pedra. \u00c9s clar que es tracta d\u2019intensos processos de convecci\u00f3. Per aix\u00f2 \u00e9s de gran inter\u00e8s el c\u00e0lcul del gradient t\u00e8rmic atmosf\u00e8ric.<\/p>\n\n\n\n<h2>Fotografies<\/h2>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"300\" height=\"225\" src=\"https:\/\/static-m.meteo.cat\/wordpressweb\/wp-content\/uploads\/2014\/11\/21084223\/BIG_427495615022712_00-300x225.jpg\" alt=\"Pedra\" class=\"wp-image-2936\"\/><figcaption>Meteors relacionats amb la precipitaci\u00f3 Pedra. Autor: Albert Fajula<\/figcaption><\/figure><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Definici\u00f3 Grans de gla\u00e7 esf\u00e8rics de m\u00e9s de 5 mm de di\u00e0metre, conjunt de grans de gla\u00e7 soldats entre s\u00ed, o trossets de gla\u00e7 irregulars, sovint formats per capes alternativament opaques i transparents. Cauen durant tempestes fortes i gaireb\u00e9 sempre acompanyats d\u2019intensa activitat el\u00e8ctrica. Es considera pedra gran, o grossa, quan el seu di\u00e0metre major [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":1261,"menu_order":0,"comment_status":"closed","ping_status":"open","template":"template-sidebar.php","meta":{"_mi_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0},"_links":{"self":[{"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/pages\/1600"}],"collection":[{"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/comments?post=1600"}],"version-history":[{"count":5,"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/pages\/1600\/revisions"}],"predecessor-version":[{"id":28876,"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/pages\/1600\/revisions\/28876"}],"up":[{"embeddable":true,"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/pages\/1261"}],"wp:attachment":[{"href":"https:\/\/www.meteo.cat\/wpweb\/wp-json\/wp\/v2\/media?parent=1600"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}