A 6,000 km long band of clouds from the Kimberley to New Zealand

This Wednesday, a northwesterly band of clouds will cut through Australia like a giant belt, stretching from tropical northwest WA to the cold Southern Ocean southwest of New Zealand.

While the cloud band makes for a dramatic satellite image, there isn’t actually a lot of rain associated with this particular large-scale meteorological feature.

What is the northwest cloud line?

• NW cloud bands are large layers of clouds that flow over Australia from (you guessed it!) the northwest.
• Once fully developed, they extend across the country and often extend to the Tasman Sea and beyond.
• Northwesterly cloud bands form when moist, warm tropical air over the Indian Ocean moves southeast. As this warm, moist air moves further south, it rises above the cooler air in the mid-latitudes and forms clouds.

But not all cloud bands in the northwest are created equal.. Some cause a lot of rain, others less or none at all.

The northwest cloud line will be one of the driest clouds this Wednesday. If you add radar imagery to a raw satellite image, green precipitation blobs are few and far between.

Parts of Western Australia, southern Victoria and south-west New South Wales received rain from a cloud band overnight, but no weather station recorded it at more than a few millimeters.

Today is expected to be a similar story, with light only falling in areas where a line of clouds passes overhead.

Image: Areas of possible precipitation on Wednesday are shaped very much like a cloud band, although precipitation is expected to be light (shown in blue, meaning 10mm or less).

Why is it so dry?

The reason this particular northwest cloud band doesn’t produce much rain is because the cloud is located in the middle and upper layers of the atmosphere. This means that it does not contain as much moisture as lower level clouds.

In addition, the surface is quite warm and dry, so some of the rain evaporates before it hits the ground. There is a term for rain that never reaches the ground, and that is “virga.”

We’ve written about the virga before here on the Weatherzone news feed, and if you’re wondering what it looks like, here’s a photo from Canberra back in 2021.

Image: Virga is quite common in Canberra in the summer when storms form near the mountains, but the rain quickly evaporates in the much hotter air over the city’s outer western suburbs. Source: Tim Yowie Man