WBI/Britt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WBI/Britt clear sky chart from 2005-11-06 to 2025-01-07.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	16% 11% 3 8% 5% 4 4 3 5% 6% 34%	104982
	Night	16% 13% 4 9% 5% 4 4 3 5% 6% 31%	104086
Transparency	Day	49% 6% 13% 22% 10% 1	108525
	Night	53% 6% 12% 19% 9% 1	108633
Seeing	Day	21% 14% 31% 23% 8% 2	15304
	Night	22% 13% 29% 25% 10% 1	24536

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 21% 10% 12% 5% 4 3 3 4 4 18%	2250
	11th to 20th	Day	11% 15% 6% 22% 7% 5% 5% 4 5% 5% 15%	2393
	21st to end	Day	19% 21% 8% 11% 4 4 4 3 4 4 18%	3124
February	1st to 10th	Day	12% 19% 5% 11% 5% 5% 5% 4 5% 6% 23%	1710
	11th to 20th	Day	13% 15% 6% 9% 7% 6% 4 2 4 5% 28%	1663
	21st to end	Day	15% 15% 6% 12% 6% 4 4 3 4 5% 25%	1475
March	1st to 10th	Day	20% 14% 4 8% 4 4 4 4 5% 5% 29%	3359
	11th to 20th	Day	11% 15% 6% 11% 6% 4 5% 3 5% 6% 28%	3480
	21st to end	Day	13% 13% 5% 8% 4 4 4 3 5% 6% 36%	3815
April	1st to 10th	Day	21% 11% 3 7% 4 4 4 4 5% 6% 30%	2216
	11th to 20th	Day	22% 9% 3 5% 4 3 3 3 5% 6% 37%	2433
	21st to end	Day	21% 10% 3 6% 5% 3 4 3 4 5% 37%	2470
May	1st to 10th	Day	18% 7% 2 7% 4 4 4 3 5% 7% 38%	4125
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 8% 43%	4398
	21st to end	Day	19% 6% 2 5% 4 3 4 2 5% 6% 44%	4837
June	1st to 10th	Day	16% 6% 3 7% 5% 4 5% 4 6% 7% 36%	2836
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	2850
	21st to end	Day	18% 8% 2 7% 4 5% 4 4 5% 8% 37%	2850
July	1st to 10th	Day	16% 6% 2 7% 4 5% 4 4 6% 7% 38%	4760
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 8% 48%	4760
	21st to end	Day	10% 6% 2 5% 4 3 3 3 5% 7% 52%	4603
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4104
	11th to 20th	Day	17% 6% 2 5% 4 4 4 4 6% 9% 40%	4044
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 8% 47%	4480
September	1st to 10th	Day	14% 7% 2 6% 5% 5% 4 4 6% 8% 40%	2280
	11th to 20th	Day	15% 5% 2 6% 4 4 3 4 5% 8% 45%	2117
	21st to end	Day	21% 7% 2 8% 4 4 4 3 4 6% 37%	2085
October	1st to 10th	Day	18% 7% 3 8% 5% 4 5% 3 5% 7% 36%	1900
	11th to 20th	Day	25% 10% 4 7% 5% 5% 4 3 5% 5% 26%	1900
	21st to end	Day	27% 12% 3 8% 4 4 4 3 4 6% 23%	2019
November	1st to 10th	Day	19% 14% 3 7% 5% 4 4 4 4 7% 28%	1638
	11th to 20th	Day	18% 17% 4 10% 6% 4 5% 3 5% 5% 24%	1600
	21st to end	Day	23% 21% 4 10% 5% 5% 4 3 3 5% 18%	1600
December	1st to 10th	Day	15% 22% 6% 11% 6% 4 5% 3 4 5% 18%	2600
	11th to 20th	Day	18% 20% 5% 12% 4 3 3 2 4 5% 23%	2732
	21st to end	Day	25% 20% 7% 11% 5% 4 3 2 3 4 16%	3476
January	1st to 10th	Night	16% 19% 9% 12% 6% 5% 3 3 4 5% 18%	4839
	11th to 20th	Night	13% 15% 6% 18% 6% 5% 4 3 6% 5% 19%	4558
	21st to end	Night	15% 22% 9% 13% 6% 5% 3 2 3 4 19%	4532
February	1st to 10th	Night	12% 16% 6% 12% 6% 5% 4 3 5% 5% 25%	2470
	11th to 20th	Night	12% 16% 6% 11% 5% 5% 4 3 5% 5% 27%	2403
	21st to end	Night	15% 15% 4 11% 6% 5% 5% 4 4 5% 24%	1975
March	1st to 10th	Night	15% 13% 4 8% 4 4 4 3 5% 7% 34%	3480
	11th to 20th	Night	12% 13% 5% 9% 6% 4 4 3 5% 6% 34%	3479
	21st to end	Night	13% 10% 4 8% 5% 5% 4 3 5% 7% 37%	3817
April	1st to 10th	Night	22% 12% 3 7% 5% 5% 4 3 4 6% 30%	1963
	11th to 20th	Night	19% 9% 3 6% 4 4 4 3 4 6% 38%	1711
	21st to end	Night	20% 10% 4 9% 5% 4 4 3 5% 5% 31%	1710
May	1st to 10th	Night	15% 6% 3 7% 5% 4 5% 4 6% 6% 39%	2815
	11th to 20th	Night	12% 6% 3 7% 4 5% 4 4 6% 8% 41%	2495
	21st to end	Night	12% 7% 2 7% 6% 4 4 4 6% 7% 41%	2738
June	1st to 10th	Night	14% 8% 2 7% 5% 5% 5% 4 5% 7% 38%	1341
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 7% 8% 45%	1330
	21st to end	Night	14% 7% 3 6% 5% 4 3 4 6% 7% 41%	1330
July	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 7% 45%	2210
	11th to 20th	Night	14% 4 2 6% 4 4 4 3 5% 7% 47%	2210
	21st to end	Night	9% 6% 2 6% 3 3 4 3 4 8% 51%	2945
August	1st to 10th	Night	11% 7% 2 6% 4 4 5% 4 6% 9% 43%	2825
	11th to 20th	Night	13% 7% 2 7% 4 3 4 3 5% 7% 44%	2785
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 6% 51%	3142
September	1st to 10th	Night	13% 4 2 6% 4 5% 4 4 5% 6% 47%	1900
	11th to 20th	Night	13% 5% 2 6% 4 3 4 3 5% 7% 49%	2061
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 8% 41%	2094
October	1st to 10th	Night	20% 6% 3 7% 5% 4 4 3 5% 6% 36%	2280
	11th to 20th	Night	25% 11% 3 7% 4 4 4 3 5% 6% 28%	2280
	21st to end	Night	27% 12% 3 8% 4 4 3 3 4 6% 25%	2574
November	1st to 10th	Night	22% 12% 3 6% 4 4 3 3 4 7% 32%	2658
	11th to 20th	Night	18% 15% 4 9% 6% 5% 4 4 5% 5% 26%	2800
	21st to end	Night	23% 19% 5% 9% 4 4 4 4 5% 5% 17%	2800
December	1st to 10th	Night	17% 23% 5% 10% 6% 5% 4 4 5% 5% 17%	4580
	11th to 20th	Night	19% 18% 5% 13% 6% 5% 3 3 4 5% 19%	5454
	21st to end	Night	21% 20% 7% 12% 5% 4 3 2 3 4 18%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	70% 6% 9% 11% 3	2251
	11th to 20th	Day	69% 7% 10% 8% 4 1	2393
	21st to end	Day	70% 5% 8% 10% 5%	3124
February	1st to 10th	Day	62% 5% 12% 15% 6% 1	1710
	11th to 20th	Day	58% 4 11% 16% 9% 1	1672
	21st to end	Day	61% 4 11% 17% 8%	1475
March	1st to 10th	Day	55% 6% 16% 12% 10% 1	3362
	11th to 20th	Day	54% 5% 15% 18% 6% 1	3480
	21st to end	Day	47% 4 15% 20% 11% 2	3815
April	1st to 10th	Day	53% 6% 13% 18% 10% 1	2216
	11th to 20th	Day	48% 4 9% 24% 14% 1	2433
	21st to end	Day	50% 5% 10% 19% 13% 3	2470
May	1st to 10th	Day	43% 7% 11% 26% 12% 1	4137
	11th to 20th	Day	38% 8% 14% 28% 10% 1	4402
	21st to end	Day	40% 7% 13% 19% 19% 2	4837
June	1st to 10th	Day	45% 6% 12% 23% 13% 1	2836
	11th to 20th	Day	38% 7% 12% 25% 16% 1	2850
	21st to end	Day	45% 7% 14% 22% 11% 1	2850
July	1st to 10th	Day	43% 6% 17% 23% 10% 1	4686
	11th to 20th	Day	33% 6% 18% 33% 9%	4679
	21st to end	Day	32% 5% 13% 31% 19%	4457
August	1st to 10th	Day	37% 6% 16% 29% 10% 1	4091
	11th to 20th	Day	40% 7% 17% 26% 10%	4083
	21st to end	Day	34% 5% 14% 36% 11%	4480
September	1st to 10th	Day	41% 6% 15% 27% 11% 1	2280
	11th to 20th	Day	36% 6% 16% 30% 11% 1	2117
	21st to end	Day	48% 6% 15% 23% 8%	2085
October	1st to 10th	Day	41% 6% 13% 29% 12%	3160
	11th to 20th	Day	47% 7% 16% 24% 6%	3140
	21st to end	Day	60% 6% 14% 14% 5%	3308
November	1st to 10th	Day	55% 9% 12% 19% 5%	1638
	11th to 20th	Day	62% 8% 11% 13% 6%	1600
	21st to end	Day	71% 6% 10% 9% 4	1600
December	1st to 10th	Day	69% 7% 14% 8% 2	2600
	11th to 20th	Day	65% 4 8% 16% 7%	2732
	21st to end	Day	73% 4 6% 12% 5%	3476
January	1st to 10th	Night	69% 8% 9% 9% 5%	4839
	11th to 20th	Night	67% 6% 13% 10% 4 1	4558
	21st to end	Night	71% 5% 6% 12% 6%	4532
February	1st to 10th	Night	60% 6% 11% 16% 6% 1	2470
	11th to 20th	Night	58% 5% 12% 17% 7% 1	2405
	21st to end	Night	60% 5% 13% 14% 8% 1	1975
March	1st to 10th	Night	49% 5% 14% 17% 13% 1	3490
	11th to 20th	Night	51% 5% 15% 18% 9% 2	3480
	21st to end	Night	46% 7% 11% 23% 11% 2	3817
April	1st to 10th	Night	56% 4 9% 19% 11% 2	1963
	11th to 20th	Night	49% 4 7% 22% 16% 2	1711
	21st to end	Night	54% 5% 9% 15% 15% 2	1710
May	1st to 10th	Night	43% 8% 10% 25% 12% 2	2822
	11th to 20th	Night	37% 9% 10% 25% 17% 1	2496
	21st to end	Night	40% 8% 11% 23% 14% 4	2739
June	1st to 10th	Night	45% 6% 11% 22% 14% 3	1341
	11th to 20th	Night	35% 7% 13% 24% 20% 1	1330
	21st to end	Night	41% 6% 15% 22% 14% 2	1330
July	1st to 10th	Night	37% 5% 16% 25% 15% 3	2175
	11th to 20th	Night	36% 6% 13% 33% 10% 1	2168
	21st to end	Night	33% 5% 10% 29% 21% 1	2852
August	1st to 10th	Night	38% 6% 16% 26% 14% 1	2814
	11th to 20th	Night	38% 6% 18% 27% 10% 1	2812
	21st to end	Night	33% 5% 14% 34% 13% 1	3142
September	1st to 10th	Night	36% 8% 14% 27% 14% 2	1900
	11th to 20th	Night	35% 6% 15% 32% 12% 1	2061
	21st to end	Night	42% 7% 15% 26% 10%	2094
October	1st to 10th	Night	38% 6% 9% 30% 16%	3800
	11th to 20th	Night	45% 6% 15% 28% 5%	3779
	21st to end	Night	53% 6% 18% 16% 6%	4235
November	1st to 10th	Night	53% 7% 13% 20% 6%	2658
	11th to 20th	Night	60% 8% 12% 14% 5% 1	2800
	21st to end	Night	68% 9% 9% 10% 3	2800
December	1st to 10th	Night	69% 7% 12% 9% 3	4580
	11th to 20th	Night	68% 5% 8% 11% 7%	5453
	21st to end	Night	72% 4 9% 10% 5%	7502

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	23% 21% 31% 19% 5% 1	192
	11th to 20th	Day	22% 13% 36% 23% 6% 1	229
	21st to end	Day	28% 13% 34% 17% 7% 1	297
February	1st to 10th	Day	29% 24% 31% 12% 3	270
	11th to 20th	Day	20% 25% 41% 13% 1	270
	21st to end	Day	33% 27% 28% 11% 1	225
March	1st to 10th	Day	22% 20% 35% 20% 4	409
	11th to 20th	Day	22% 17% 36% 20% 4 1	460
	21st to end	Day	26% 18% 33% 17% 5% 1	506
April	1st to 10th	Day	26% 15% 33% 18% 8%	460
	11th to 20th	Day	32% 19% 23% 16% 7% 4	452
	21st to end	Day	28% 17% 30% 17% 8%	460
May	1st to 10th	Day	24% 10% 35% 22% 8% 2	463
	11th to 20th	Day	18% 14% 33% 23% 9% 3	487
	21st to end	Day	19% 12% 31% 24% 12% 2	529
June	1st to 10th	Day	17% 14% 34% 27% 7% 1	666
	11th to 20th	Day	15% 12% 36% 26% 10% 2	680
	21st to end	Day	21% 10% 35% 25% 7% 2	680
July	1st to 10th	Day	11% 8% 35% 32% 12% 2	671
	11th to 20th	Day	12% 9% 32% 32% 12% 3	668
	21st to end	Day	11% 7% 31% 33% 14% 4	551
August	1st to 10th	Day	17% 6% 28% 31% 13% 6%	486
	11th to 20th	Day	14% 4 28% 30% 15% 9%	479
	21st to end	Day	16% 13% 25% 29% 12% 5%	539
September	1st to 10th	Day	16% 13% 27% 28% 12% 4	490
	11th to 20th	Day	16% 8% 24% 31% 15% 7%	490
	21st to end	Day	24% 10% 23% 24% 16% 3	490
October	1st to 10th	Day	24% 14% 27% 22% 11% 2	490
	11th to 20th	Day	28% 17% 32% 19% 4 1	490
	21st to end	Day	35% 18% 28% 16% 2 1	468
November	1st to 10th	Day	29% 20% 34% 14% 3	237
	11th to 20th	Day	23% 27% 35% 14% 3 1	200
	21st to end	Day	36% 22% 24% 15% 3 1	200
December	1st to 10th	Day	28% 17% 35% 17% 5% 1	200
	11th to 20th	Day	32% 22% 32% 11% 3 1	200
	21st to end	Day	30% 23% 30% 14% 3	220
January	1st to 10th	Night	26% 15% 31% 22% 5% 1	981
	11th to 20th	Night	28% 16% 30% 20% 6%	933
	21st to end	Night	20% 11% 33% 26% 8% 1	836
February	1st to 10th	Night	21% 20% 33% 22% 4	760
	11th to 20th	Night	20% 15% 36% 23% 5%	743
	21st to end	Night	22% 18% 33% 24% 3	628
March	1st to 10th	Night	20% 16% 37% 22% 5%	760
	11th to 20th	Night	22% 16% 33% 22% 6% 1	760
	21st to end	Night	19% 13% 35% 26% 6% 1	836
April	1st to 10th	Night	25% 14% 33% 22% 6%	760
	11th to 20th	Night	25% 12% 27% 25% 10% 1	587
	21st to end	Night	24% 12% 27% 25% 11%	570
May	1st to 10th	Night	17% 6% 28% 36% 11% 1	565
	11th to 20th	Night	16% 7% 29% 33% 14% 1	374
	21st to end	Night	11% 9% 30% 35% 13% 1	411
June	1st to 10th	Night	17% 8% 27% 35% 12% 1	380
	11th to 20th	Night	17% 7% 24% 36% 15% 1	380
	21st to end	Night	17% 7% 22% 35% 18% 2	380
July	1st to 10th	Night	11% 6% 23% 42% 17% 1	370
	11th to 20th	Night	11% 4 26% 36% 23% 1	368
	21st to end	Night	11% 6% 23% 36% 21% 1	555
August	1st to 10th	Night	11% 4 24% 37% 20% 3	567
	11th to 20th	Night	13% 5% 23% 31% 25% 4	564
	21st to end	Night	16% 6% 24% 30% 22% 2	632
September	1st to 10th	Night	14% 8% 27% 29% 19% 3	760
	11th to 20th	Night	15% 9% 22% 29% 21% 4	760
	21st to end	Night	20% 9% 23% 29% 16% 3	760
October	1st to 10th	Night	22% 10% 30% 24% 12% 2	760
	11th to 20th	Night	30% 13% 29% 21% 6% 1	760
	21st to end	Night	32% 17% 29% 18% 4	836
November	1st to 10th	Night	30% 13% 28% 22% 7%	780
	11th to 20th	Night	25% 18% 30% 20% 6%	800
	21st to end	Night	33% 16% 29% 18% 4	800
December	1st to 10th	Night	32% 16% 33% 16% 4	800
	11th to 20th	Night	29% 18% 33% 16% 3 1	920
	21st to end	Night	32% 16% 30% 18% 4	1100

Page generated at 2025-01-04 03:24:01. Version ac07

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