Posts Tagged ‘meaning of forecast terms’

Amy Freeze

Words that are used in the forecast are frequently questioned so I thought this would be a good chance to clear the air…¬† with a¬†few words about the weather words used in forecasting.¬†

Over the years, dozens of emails have come in asking “What’s the difference between Partly Sunny or Partly Cloudy?”¬† “Rain versus Showers?”¬†¬† “Breezy versus Windy?”¬†¬†¬†“Does 50% chance of rain mean that¬†it might not rain?” ¬† There is a difference in these weather terms and understanding them can make or break¬†the forecast¬†for viewers!

When people talk¬†about the weather casually, they¬†likely call the moisture that falls from the sky “rain.”¬† But it could¬†be drizzle, light rain, showers, scattered rain, scattered showers, thunderstorms, a heavy downpour,¬†or maybe even “mixed precipitation.”¬†¬†

When putting together the forecast, it’s very clear to the meteorologist what they are¬†predicting.¬† But if they can’t convey the particular details¬†of their forecast through words¬†on tv then¬†their work was in vain.¬†¬†The way¬†weather is described in words on television makes a difference!¬†¬† Forecasters who¬†talk about weather on tv should be direct, not evasive nor vague. The forecast needs not only to be right but also to be delivered correctly with words that can be defined. (Viewers also¬†need to be good listeners in order to receive the forecast correctly. If one or the other is not doing their part to communicate – it’s basically a 50 percent chance of anything!)

Partly Sunny or Partly Cloudy

Talking about the weather is not¬†about how you look at life – the cup half empty or the cup half full?¬†¬† It’s not an optimistic or a pessimistic view of how many clouds are out there…¬†describing details of cloud¬†is not about the forecaster’s mood!¬† There is¬†a way¬†to be clear when using¬†Partly Sunny versus Partly Cloudy.¬† For current conditions,¬†I use the terms¬†“Partly Sunny” during the day, and “Partly Cloudy” at night… obviously it will never be Partly Sunny when describing night conditions.¬† The way sky conditions are¬†described ¬†in the National Weather Service are defined according to specific parameters.

From the National Weather Service Operations Manual, Chapter C-11 under Zone Forecast Guidelines and Procedures:
DAY                         NIGHT/DAY                         SKY                                                                                                                                                                         Cloudy                      Cloudy                                  8/8 opaque clouds
Mostly Cloudy      Mostly Cloudy                   6/8 Р7/8 opaque clouds
Partly Sunny          Partly Cloudy                    3/8 Р5/8 opaque clouds
Mostly Sunny         Mostly Clear                      1/8 Р2/8 opaque clouds
Sunny                         Clear                                     0/8 opaque* clouds

* opaque means that an observer can not see through the clouds; the sun, moon , stars, and blue sky are hidden.

When talking about “Partly Sunny” and “Partly Cloudy” in future or forecast¬†terms, the definitions can be even more specific.¬† Instead of the sky¬†description for “right now,” the forecast looks ahead.¬† To describe¬†how long sky conditions will remain a certain way, certain definitions are used.¬† Partly Sunny means only part (less than half) of the time period will be sunny. In that same respect, Partly Cloudy means only part of the period will be cloudy.¬†From least cloud cover to most cloud cover when talking about the future sky conditions, the scale is: sunny, mostly sunny, partly cloudy, partly sunny, mostly cloudy, cloudy. Mostly sunny means more sun than clouds, partly sunny means more clouds than sun, and partly cloudy generally means an equal amount of clouds and sun.

Because of their “close call” definitions, “Partly Cloudy and Partly Sunny are essentially the same thing,” said David Wert, meteorologist-in-chief at the National Weather Service¬†Office (Blacksburg.) “Both are used for conditions when the average amount of opaque cloud cover ranges from 45 percent to 75 percent. We usually use ‘partly sunny’ for daytime conditions, and ‘partly cloudy’ for nighttime conditions under these situations. It wouldn’t make too much sense to use ‘partly sunny’ for a nighttime condition.”

Rain, Showers, and Precipitation

Rain is precipitation that forms from stratus clouds.  Rain is a condition that is more widespread than showers.  Rain is steady, and is less intense than showers.  Showers form from cumulus clouds, more isolated, short-lived.  Showers affect a smaller area and are often more intense than rain.

Rain can occur in different durations changing the description.  Brief rain Рshort, sudden showers or periods of rain. Intermittent Рon and off intervals, not continuous.  Occasional Рirregular, infrequent intervals of precipitation. Frequent Рpersistent short intervals, happening regularly and often.  Periods of precipitation Рrain or snow falling most of the time with breaks.

The area or distribution of the rain is also describable in a forecast.  Isolated Рshowers separated during a given period of time.  Few showers Рindicated in time, not over an area.  Local Рrestricted to a smaller area.  Patchy Рirregularly occurring in an area. Scattered Рnot widespread but of greater occurrence than isolated showers.

Working on the Forecast

Amy Freeze workding on Forecast

Chance, Possible, and Probability

What does it mean if a forecast calls for a 50% chance of rain? ¬†This is always confusing and it’s the main reason that I will rarely if ever use a percent chance in a forecast.¬† Instead I will use terms like slight chance, showers possible, or rain likely to give the viewer better guidance on what to expect. (see the chart below for guidelines)

However, percentages are often used and when you understand them then you will have a better idea of what kind of wet weather risk there is for the day.¬† When 50% chance is used, it is not describing a flip of the coin probability. Instead, it means there is a 50% chance that a given location within a forecast area will receive measurable rain. Technically, this ‚Äúprobability of precipitation‚ÄĚ is defined as the likelihood (expressed as a percent) of a measurable amount (at least 0.01 inch) of liquid precipitation during a specified time period at any given point in the forecast area¬†¬† A 20% chance of showers.¬† ¬†This means that of the last one hundred times that these conditions existed in this area, it rained twenty times. ¬†A 20% chance that part of the area will receive rain.¬†

According to The National Weather Service, Probability of Precipitation, or POP, is defined as the likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. ¬†The equation used to arrive at POP contains a value for “Percent of Areal Coverage,” so the result of the equation produces a number that also expresses the percent of areal coverage.Here’s a bit more about how % chances calculated:¬† Computer models also forecast the probability of precipitation (also known as “POP”) for 6 and 12-hour periods, which some forecasters will blindly pass along to the viewer without explanation and can be very confusing to viewers. ¬†While the math behind the POP is complex, the computer model takes into account the amount of moisture at different levels of the atmosphere, whether air is expected to be rising or sinking, and other meteorological factors.

The National Weather Service says, “The probability of precipitation is the likelihood of measurable precipitation (0.01 inches or greater) for a specified forecast time period, and occurring at any point for which the forecast is valid.¬† The probability gives the odds of any one place in the area covered by the forecast getting wet, whether it’s from rain or snow.¬† The 0.01 inches or greater comes from the fact that any less rain or water from melted snow or ice can’t be measured. If the bottom of the rain gauge is wet, but the water isn’t deep enough to measure, that’s called a “trace’ and really doesn’t count.”When the NWS forecasters assign a precipitation probability, it ¬†shows the forecaster’s confidence in the forecast for how much of the forecast area is likely to have precipitation, and low long the precipitation is expected to last.¬† ¬†The important point is that the odds are for the rain or snow to fall on any place in the area covered by the forecast. This means, that the probability could be low as 30% but you have hard rain for a few hours. In this case, you’re just one of the unlucky people who happened to be in the small part of the region(the 30% chance) ¬†that was expected to get¬†wet that day.¬† And, a couple of miles away the people who needed the rain can’t complain about the forecast not working out. Rain did fall on at least one place in the area.

Regardless of its accuracy, a weather forecast fails if the user does not understand the forecaster’s words!! ¬†The proper interpretation of a 30 percent chance of rain (assuming the forecast verifies perfectly) is that you will have rain on your head three out of ten times that you hear such a forecast.¬† A couple of scenarios could play out:¬† ¬†the weather forecaster may believe rain will cover 100 percent of the area if the rain arrives, but his/her confidence that it will arrive is only 30 percent. Alternatively, the forecaster might have great confidence that rain will occur, but he/she believes it will be scattered showers affecting only 30 percent of the area. Regardless of the forecaster’s rationale, the meaning for you is always the same: The chance of rain on your head is 30 percent.¬†

The Percentage of Probabiliy (POP) can be described in words by forecasters by following these guidelines:

10% probability: Slight chance, isolated or none.

20% probability: Slight chance, isolated

30-50% probability: Chance,scattered

60-70% probability:Likely, numerous

If the probability is 80% or above the forecast will be categorical, such as “Rain this afternoon.”

Amy Freeze at Bears

Amy Freeze at Soldier Field's Coldest Kickoff Ever

What About the Wind?

When describing the wind, the description is of¬†the prevailing direction from which the wind is blowing, with speeds in miles per hour. Here’s a list of wind speeds and the description for them.

 

Sustained Wind Speed                 Descriptive Term

0-5 mph Light /                                                     Light & variable wind
5-10 mph / 10-15 mph / 10-20 mph            Light, Breezy at Times
15-25 mph                                                                Breezy (mild weather)
15-25 mph                                                                Brisk/Blustery (cold weather)
20-30 mph                                                               Windy
30-40 mph                                                               Very windy
40-73 mph                                                              Strong, dangerous, damaging
 74 mph or greater                                                Hurricane force

What did She Say?!

I’ll admit there are times forecasters on tv will use terms that are misleading, confusing, and often down right impossible!¬†¬†It’s hard to be perfect on live tv but the goal is to be perfectly clear!¬†¬† Listen close and chances are good you’ll get the information you need… Thought I’d leave you with a laugh… there is an amusing list of mis-described weather¬† on a website. ¬† It includes misuses like cold air pushing in¬†¬†…what is it pushing?¬† Or a trough lifts ….lifting what?¬† Risk of showers …a risk implies danger!¬† Other confusing jargon weather¬†reporters use include:¬† Arrived on schedule …who scheduled it?¬† Normal …are you normal?¬† precipitation… well, what kind is it?¬† Cell ….only prisoners get this one.¬†¬†Entrainment …they serve mints in trains?¬† Other vague generalizations like these:¬†¬† Weather is expected ….by whom?¬† Odd terminology for weather:¬†¬† Thunderstorm activity… sounds like a party!¬†¬† The barometer is falling ….catch it.¬†¬† Temperature department… isn’t that near men’s apparel?¬† Rain event… is it free to the public?