weather words symbol

Jeffrey Nesmith

Mostly cloudy or partly cloudy? Partly cloudy or partly sunny? And what exactly does "variable cloudiness" mean? Almost all of us are familiar with the above terms used in National Weather Service (NWS) public forecasts, but surprisingly few people know the criteria to which the NWS adheres when applying them. In this installment of "Weather Words" we will examine some of the terminology that the NWS uses when forecasting cloud cover and sky condition over the central interior of California.

In the absence of inclement weather, a quick "eye to the sky" is what most people use to assess current weather conditions. Because of this, the NWS usually includes a forecast of the sky condition that is anticipated during a given forecast period. An exception is when cloud cover can easily be inferred from a precipitation forecast. For example, a 100 percent chance of rain necessarily implies the presence of clouds. In such instances, references to cloud cover are often omitted from the forecast to minimize redundancy.

So what exactly are the definitions of partly cloudy, mostly cloudy, and all of the other terms used in NWS sky forecasts? To answer this one must first understand that when forecasting sky cover, the NWS divides the sky into eight equal parts called oktas. The number of oktas (i.e., the percentage of the sky) that is expected to be covered by opaque clouds is then included in the forecast using the conversion table found in figure 1.

Daytime Only Term Nightime or Daytime Term Sky Cover (oktas)
Cloudy Cloudy 8/8 opaque clouds
Mostly Cloudy, Considerable Cloudiness Mostly Cloudy, Considerable Cloudiness 6/8 to 7/8 opaque clouds
Partly Sunny Partly Cloudy 3/8 to 5/8 opaque clouds
Mostly Sunny Mostly clear 1/8 to 2/8 opaque clouds
Sunny Clear 0/8 opaque clouds
Figure 1

Hence, different expressions can be used to depict similar sky conditions; e.g., partly cloudy and partly sunny can be used interchangeably to describe 3/8 to 5/8 opaque sky cover. Since some terms are more fitting for use during daylight hours, (e.g., "partly sunny" and "mostly sunny"), forecasters usually favor their use during the daytime over their nighttime equivalent.

Although the table in figure 1 is convenient, situations often arise where the amount of opaque cloud cover is not expected to remain within a given oktal range during a forecast period; cloudiness may steadily increase, decrease, or undergo rapid changes through the day. Therefore, NWS forecasters occasionally use other terms to inform the public of anticipated sky conditions. For example, if the amount of opaque cloud cover is expected to vary considerably in a forecast period, sky forecasts may be worded "variable clouds" or "variable cloudiness" to convey the intermittent nature of the cloud cover. Or, if opaque cloud cover will steadily increase through the day, forecasts may be worded "increasing cloudiness" to herald the advance of clouds across the sky.

So, the next time you are at a party and someone inquires about the difference between partly cloudy and mostly cloudy, just respond, "Well, it's all a matter of oktas..."