photo #5084263

Cirrus clouds can help forecast the weather. A marked change in the appearance of Cirrus clouds over time is what’s significant. A few isolated patches of this high ice-crystal cloud that change little don’t mean much. When the streaks of Cirrus are invading the sky and joining together, they can. Spreading Cirrus can indicate that a weather front is on its way and will arrive in a matter of hours. Typically, it is a warm front, and that might bring steady and prolonged precipitation. As the weather front arrives, the thickening and joining Cirrus change into the mid-level Altostratus or Altocumulus clouds, and eventually these thicken into the thick layer of rain cloud known as Nimbostratus.You might think, therefore, that this tangled, fibrous, spreading display of Cirrus spotted by app user ‘Bozmo’ over Perth, Western Australia was indicating a significant change in the weather to come. But not a drop of rain followed. Before you ask for your money back, remember the changes in the atmosphere that clouds indicate don’t necessarily result in precipitation. Around the time of this sky, the light winds over Perth were shifting – in a clockwise direction, from northwesterly to easterly and back again. This suggests that a trough of low pressure was passing over. It just wasn’t strong enough to cause anything significant like precipitation, merely a shift in light winds. And who cares about that? Well, Cirrus clouds do.