Overcast sky: Not just a regular cloudy day
We often hear or read the term ‘overcast sky’ and assume that the meteorologists are simply referring to another grey day with cloudy skies. We don’t give it much of a thought. However, this phenomenon doesn’t necessarily mean that there’s a chance of precipitation, i.e., rain or snow. In fact, it may or may not happen. Nevertheless, what it simply means is that most of the sky is covered in dull and grey-coloured clouds that cause low visibility. But guess what? There’s more to it than this.
How do meteorologists define an overcast sky?
Imagine the sky to be 90 per cent covered by clouds. This is what we can call an overcast. But is that all? No! Well, as it turns out, the type of clouds that are visible hardly matter. What’s important is the portion of the sky they cover.
Being experts, meteorologists have their own way of classifying an overcast sky. Not many of you know that oktas happen to be the scale (unit of measurement) to determine the amount of cloud cover in the sky. For this, meteorologists divide a pie chart into eight equal slices, where each slice represents one Oota. When eight oktas are filled up in the pie chart, it’s time for the weathermen to define a sky as overcast. In fact, this is when a spread of clouds is seen instead of distinct individual clouds, and the penetration of sunlight is at minimum. Also, remember that, an overcast sky is typically created by clouds that are high up in the atmosphere.
Oktas scale decoded
Cloud cover amounts are generally rounded up to the next highest Okta. For example: ‘2 and a bit’ oktas is rounded off to 3 oktas.
Let’s see what each okta means:
0 oktas: Complete absence of clouds
1 okta: A cloud amount of 1 eighth or less, but more than zero
7 oktas: A cloud amount of 7 eighths or more, but not full cloud cover
8 oktas: Full cloud cover with no breaks
9 oktas: Sky obscured by fog or other meteorological phenomena.
Cloudy vs. overcast
We all have at some time mistaken a cloudy sky as an overcast sky. But to tell you the truth, that are significant differences between the two. In fact, meteorologists classify cloud conditions into partly cloudy, mostly cloudy and overcast.
A partly cloudy sky refers to a cloud cover that is less than or equal to having 50 per cent or three to five Oktas. A ‘mostly cloudy sky can be classified as having 70 to 80 per cent cloud cover represented through five to seven Oktas. As already mentioned, ‘overcast sky’ is when the cloud cover is at its optimum (90 to 100 percent) represented by eight Oktas or above. Moreover, during partly cloudy conditions, clouds can be distinctly noticeable, while on mostly cloudy days, separation in clouds can be visible at times. However, on overcast days, the sky usually looks like one big cloud cover!