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 2026-05-02.

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% 4 8% 5% 4 4 3 5% 6% 34%	131701
	Night	15% 13% 5% 9% 5% 4 4 3 5% 6% 31%	130872
Transparency	Day	49% 6% 12% 22% 10% 1	135230
	Night	53% 6% 11% 19% 10% 1	135432
Seeing	Day	22% 14% 31% 22% 8% 2	17013
	Night	22% 13% 29% 25% 10% 1	26269

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	13% 18% 8% 11% 5% 4 4 3 5% 6% 24%	4094
	11th to 20th	Day	10% 19% 7% 20% 7% 6% 5% 4 4 5% 14%	4457
	21st to end	Day	11% 15% 7% 16% 6% 5% 5% 4 5% 5% 22%	5780
February	1st to 10th	Day	12% 19% 5% 11% 5% 5% 5% 4 5% 5% 24%	1890
	11th to 20th	Day	13% 16% 6% 9% 7% 5% 4 2 4 5% 27%	1843
	21st to end	Day	15% 16% 6% 12% 7% 4 4 3 4 5% 23%	1626
March	1st to 10th	Day	20% 13% 4 9% 4 4 4 3 4 5% 30%	4938
	11th to 20th	Day	14% 13% 5% 10% 5% 4 5% 3 6% 7% 27%	5090
	21st to end	Day	17% 14% 5% 9% 4 4 4 3 4 5% 32%	5586
April	1st to 10th	Day	20% 12% 3 7% 4 4 4 4 5% 6% 30%	2450
	11th to 20th	Day	22% 9% 3 5% 4 3 3 3 4 6% 38%	2693
	21st to end	Day	21% 10% 3 6% 4 3 4 3 4 5% 36%	2730
May	1st to 10th	Day	18% 7% 3 7% 4 4 4 3 5% 7% 38%	5918
	11th to 20th	Day	14% 8% 2 6% 4 4 4 3 5% 7% 42%	6318
	21st to end	Day	21% 8% 2 6% 4 3 3 2 5% 6% 38%	6949
June	1st to 10th	Day	16% 6% 3 6% 5% 4 5% 4 6% 7% 37%	2985
	11th to 20th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	3000
	21st to end	Day	18% 8% 2 7% 4 5% 4 4 5% 8% 36%	3000
July	1st to 10th	Day	14% 6% 2 7% 4 5% 4 4 6% 8% 39%	6819
	11th to 20th	Day	11% 4 2 6% 4 4 5% 3 5% 7% 50%	6820
	21st to end	Day	9% 5% 2 5% 3 3 4 3 5% 8% 53%	6617
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4234
	11th to 20th	Day	16% 6% 2 5% 4 4 4 4 6% 9% 40%	4174
	21st to end	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	4621
September	1st to 10th	Day	14% 7% 2 6% 5% 5% 4 4 6% 8% 40%	2400
	11th to 20th	Day	14% 5% 2 6% 4 4 3 4 5% 8% 47%	2228
	21st to end	Day	20% 7% 2 8% 4 4 4 3 4 6% 37%	2195
October	1st to 10th	Day	17% 7% 3 8% 5% 4 5% 3 5% 7% 37%	2000
	11th to 20th	Day	25% 9% 4 7% 5% 5% 4 3 5% 5% 27%	2000
	21st to end	Day	26% 12% 4 8% 4 4 4 3 4 5% 24%	2125
November	1st to 10th	Day	20% 14% 3 7% 5% 4 4 4 4 6% 28%	1722
	11th to 20th	Day	18% 17% 4 10% 6% 4 5% 3 5% 5% 24%	1680
	21st to end	Day	23% 21% 4 10% 5% 5% 4 3 3 4 19%	1680
December	1st to 10th	Day	15% 22% 6% 11% 6% 4 5% 3 4 5% 18%	2680
	11th to 20th	Day	18% 20% 5% 12% 4 3 3 2 4 5% 23%	2806
	21st to end	Day	25% 20% 7% 11% 5% 4 3 2 3 4 16%	3553
January	1st to 10th	Night	12% 17% 8% 12% 6% 6% 4 3 4 6% 22%	8831
	11th to 20th	Night	12% 20% 7% 16% 5% 5% 4 3 5% 4 19%	8437
	21st to end	Night	9% 17% 9% 15% 7% 6% 5% 3 5% 5% 21%	8401
February	1st to 10th	Night	12% 16% 6% 12% 6% 4 4 3 5% 5% 26%	2730
	11th to 20th	Night	12% 16% 6% 12% 5% 5% 5% 3 5% 5% 26%	2663
	21st to end	Night	14% 16% 4 11% 6% 5% 5% 4 4 5% 24%	2176
March	1st to 10th	Night	17% 12% 4 8% 4 4 4 3 5% 6% 33%	5121
	11th to 20th	Night	15% 12% 4 9% 5% 5% 4 4 6% 7% 29%	5089
	21st to end	Night	20% 11% 4 8% 4 4 4 3 5% 5% 33%	5588
April	1st to 10th	Night	21% 12% 3 7% 5% 5% 4 3 4 6% 29%	2169
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 4 6% 37%	1893
	21st to end	Night	20% 10% 3 9% 5% 4 4 3 5% 6% 32%	1890
May	1st to 10th	Night	16% 7% 3 7% 5% 4 5% 4 6% 6% 38%	4044
	11th to 20th	Night	11% 6% 3 6% 4 5% 4 4 6% 8% 43%	3575
	21st to end	Night	16% 8% 2 7% 6% 4 4 4 6% 6% 36%	3926
June	1st to 10th	Night	14% 8% 2 7% 5% 5% 5% 4 6% 7% 38%	1411
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 7% 9% 44%	1400
	21st to end	Night	13% 7% 3 6% 5% 4 4 4 6% 7% 41%	1400
July	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 6% 8% 43%	3160
	11th to 20th	Night	13% 4 2 5% 4 4 4 3 5% 7% 50%	3160
	21st to end	Night	10% 7% 2 6% 3 3 4 3 4 9% 50%	4231
August	1st to 10th	Night	10% 7% 2 6% 4 4 5% 4 6% 9% 43%	2915
	11th to 20th	Night	13% 7% 2 7% 4 3 4 3 5% 7% 44%	2875
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 6% 51%	3242
September	1st to 10th	Night	13% 4 2 6% 4 5% 4 4 5% 6% 46%	2000
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 5% 7% 50%	2170
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 8% 41%	2204
October	1st to 10th	Night	19% 6% 3 7% 5% 4 4 3 5% 6% 38%	2400
	11th to 20th	Night	26% 11% 3 7% 4 4 4 3 5% 6% 28%	2400
	21st to end	Night	26% 12% 3 8% 4 4 3 3 4 6% 27%	2710
November	1st to 10th	Night	21% 12% 3 7% 5% 4 3 3 4 7% 31%	2794
	11th to 20th	Night	18% 15% 4 9% 6% 5% 4 4 5% 5% 26%	2940
	21st to end	Night	23% 19% 5% 9% 4 4 4 4 5% 5% 17%	2940
December	1st to 10th	Night	16% 23% 5% 10% 5% 5% 4 4 4 5% 17%	4720
	11th to 20th	Night	19% 18% 5% 13% 6% 5% 3 3 4 5% 19%	5600
	21st to end	Night	21% 20% 7% 12% 5% 4 3 2 3 4 18%	7667

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	61% 5% 7% 20% 6% 1	4095
	11th to 20th	Day	73% 5% 7% 8% 7%	4487
	21st to end	Day	65% 4 7% 13% 10%	5780
February	1st to 10th	Day	62% 5% 11% 16% 6%	1890
	11th to 20th	Day	60% 4 11% 16% 9% 1	1852
	21st to end	Day	62% 4 11% 17% 7%	1626
March	1st to 10th	Day	56% 5% 13% 9% 14% 3	4941
	11th to 20th	Day	54% 5% 15% 17% 8% 1	5090
	21st to end	Day	54% 4 13% 18% 9% 2	5586
April	1st to 10th	Day	53% 6% 12% 18% 10% 1	2450
	11th to 20th	Day	48% 4 9% 23% 14% 1	2693
	21st to end	Day	50% 5% 10% 20% 13% 3	2730
May	1st to 10th	Day	44% 6% 11% 23% 12% 4	5927
	11th to 20th	Day	40% 7% 13% 22% 16% 2	6322
	21st to end	Day	47% 6% 11% 19% 14% 2	6949
June	1st to 10th	Day	44% 6% 12% 24% 13% 1	2985
	11th to 20th	Day	38% 7% 12% 26% 16% 1	3000
	21st to end	Day	46% 7% 14% 21% 11% 1	3000
July	1st to 10th	Day	42% 6% 17% 25% 9% 1	6746
	11th to 20th	Day	33% 6% 15% 38% 8%	6739
	21st to end	Day	30% 5% 15% 34% 15%	6429
August	1st to 10th	Day	37% 7% 16% 29% 10% 1	4221
	11th to 20th	Day	39% 7% 17% 26% 10%	4213
	21st to end	Day	34% 5% 14% 36% 10%	4621
September	1st to 10th	Day	41% 6% 16% 26% 10% 1	2400
	11th to 20th	Day	35% 6% 16% 31% 11%	2228
	21st to end	Day	48% 6% 14% 23% 8%	2195
October	1st to 10th	Day	40% 6% 13% 29% 11%	3260
	11th to 20th	Day	47% 7% 16% 24% 7%	3240
	21st to end	Day	60% 6% 14% 14% 6%	3414
November	1st to 10th	Day	56% 8% 12% 20% 5%	1722
	11th to 20th	Day	61% 8% 12% 13% 6%	1680
	21st to end	Day	71% 6% 10% 9% 4	1680
December	1st to 10th	Day	69% 7% 14% 8% 2	2680
	11th to 20th	Day	65% 4 8% 15% 7%	2806
	21st to end	Day	73% 4 6% 12% 5%	3553
January	1st to 10th	Night	63% 6% 9% 15% 7%	8831
	11th to 20th	Night	69% 5% 9% 9% 8%	8482
	21st to end	Night	66% 4 7% 13% 10% 1	8401
February	1st to 10th	Night	59% 5% 11% 18% 6% 1	2730
	11th to 20th	Night	59% 5% 12% 17% 6% 1	2665
	21st to end	Night	61% 5% 13% 14% 7% 1	2176
March	1st to 10th	Night	51% 5% 12% 15% 14% 4	5131
	11th to 20th	Night	53% 5% 16% 17% 7% 1	5090
	21st to end	Night	53% 5% 8% 19% 12% 2	5588
April	1st to 10th	Night	56% 4 9% 19% 11% 2	2169
	11th to 20th	Night	48% 4 8% 22% 16% 2	1893
	21st to end	Night	53% 5% 9% 17% 15% 2	1890
May	1st to 10th	Night	44% 6% 9% 23% 14% 4	4051
	11th to 20th	Night	37% 8% 10% 23% 20% 2	3576
	21st to end	Night	46% 7% 10% 23% 12% 3	3927
June	1st to 10th	Night	43% 6% 12% 22% 14% 3	1411
	11th to 20th	Night	35% 7% 13% 25% 19% 1	1400
	21st to end	Night	41% 6% 15% 22% 14% 1	1400
July	1st to 10th	Night	38% 5% 14% 29% 12% 2	3125
	11th to 20th	Night	34% 5% 12% 34% 13% 1	3118
	21st to end	Night	33% 5% 10% 34% 17% 1	4106
August	1st to 10th	Night	37% 6% 16% 26% 13% 1	2904
	11th to 20th	Night	39% 6% 18% 27% 10% 1	2902
	21st to end	Night	33% 5% 14% 35% 13% 1	3242
September	1st to 10th	Night	36% 8% 14% 27% 13% 2	2000
	11th to 20th	Night	34% 6% 15% 32% 12% 1	2170
	21st to end	Night	42% 7% 15% 26% 11%	2204
October	1st to 10th	Night	38% 6% 9% 30% 16%	3920
	11th to 20th	Night	45% 6% 15% 28% 5%	3899
	21st to end	Night	53% 6% 18% 16% 7%	4371
November	1st to 10th	Night	54% 7% 13% 20% 6%	2794
	11th to 20th	Night	60% 7% 12% 14% 5% 1	2940
	21st to end	Night	68% 9% 9% 10% 4	2940
December	1st to 10th	Night	69% 7% 12% 9% 3	4720
	11th to 20th	Night	68% 5% 8% 11% 8%	5599
	21st to end	Night	72% 4 9% 10% 5%	7667

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	24% 20% 32% 19% 5%	220
	11th to 20th	Day	22% 16% 36% 21% 5% 1	281
	21st to end	Day	26% 14% 35% 18% 7% 1	363
February	1st to 10th	Day	29% 24% 31% 13% 4	330
	11th to 20th	Day	22% 25% 38% 13% 2	330
	21st to end	Day	31% 25% 30% 13% 1	273
March	1st to 10th	Day	25% 20% 34% 18% 3	484
	11th to 20th	Day	24% 18% 35% 17% 4 1	540
	21st to end	Day	28% 19% 33% 16% 4 1	594
April	1st to 10th	Day	27% 17% 32% 16% 7%	540
	11th to 20th	Day	32% 19% 23% 15% 7% 4	532
	21st to end	Day	29% 16% 30% 17% 7%	540
May	1st to 10th	Day	23% 11% 36% 21% 7% 2	507
	11th to 20th	Day	18% 14% 33% 22% 9% 3	527
	21st to end	Day	20% 12% 31% 24% 11% 2	573
June	1st to 10th	Day	17% 14% 35% 26% 7% 1	715
	11th to 20th	Day	15% 12% 36% 25% 9% 2	730
	21st to end	Day	22% 10% 35% 24% 7% 2	730
July	1st to 10th	Day	11% 8% 36% 32% 12% 2	721
	11th to 20th	Day	12% 9% 33% 31% 12% 3	718
	21st to end	Day	11% 8% 32% 32% 13% 4	596
August	1st to 10th	Day	16% 6% 27% 33% 13% 6%	526
	11th to 20th	Day	14% 5% 28% 30% 14% 9%	519
	21st to end	Day	15% 13% 26% 29% 12% 5%	583
September	1st to 10th	Day	16% 13% 27% 28% 13% 4	530
	11th to 20th	Day	15% 7% 23% 31% 16% 8%	530
	21st to end	Day	23% 9% 24% 25% 15% 3	530
October	1st to 10th	Day	23% 13% 28% 24% 11% 2	530
	11th to 20th	Day	28% 16% 34% 18% 4 1	530
	21st to end	Day	34% 17% 28% 18% 2 1	508
November	1st to 10th	Day	31% 20% 34% 13% 3	261
	11th to 20th	Day	22% 27% 35% 13% 3	220
	21st to end	Day	37% 21% 24% 15% 3	220
December	1st to 10th	Day	27% 17% 34% 17% 4	220
	11th to 20th	Day	33% 22% 31% 10% 3	220
	21st to end	Day	30% 22% 31% 14% 3	242
January	1st to 10th	Night	25% 15% 32% 23% 4 1	1050
	11th to 20th	Night	28% 16% 30% 20% 5%	1031
	21st to end	Night	19% 12% 33% 27% 8% 1	924
February	1st to 10th	Night	20% 20% 34% 23% 4	840
	11th to 20th	Night	19% 16% 35% 23% 5%	823
	21st to end	Night	22% 18% 33% 24% 3	692
March	1st to 10th	Night	20% 17% 38% 21% 5%	840
	11th to 20th	Night	23% 17% 32% 21% 6% 1	840
	21st to end	Night	21% 13% 34% 25% 6% 1	924
April	1st to 10th	Night	25% 15% 33% 21% 5%	840
	11th to 20th	Night	25% 12% 28% 25% 10% 1	649
	21st to end	Night	24% 12% 27% 26% 10%	630
May	1st to 10th	Night	17% 6% 29% 36% 11% 1	598
	11th to 20th	Night	16% 7% 30% 33% 14% 1	394
	21st to end	Night	12% 9% 30% 34% 13% 1	433
June	1st to 10th	Night	17% 9% 27% 36% 12% 1	400
	11th to 20th	Night	16% 8% 25% 36% 15% 1	400
	21st to end	Night	16% 6% 24% 35% 17% 2	400
July	1st to 10th	Night	11% 6% 23% 42% 17% 1	390
	11th to 20th	Night	11% 4 25% 37% 22% 1	388
	21st to end	Night	11% 6% 24% 37% 21% 1	585
August	1st to 10th	Night	11% 4 24% 38% 21% 3	597
	11th to 20th	Night	13% 5% 23% 31% 24% 4	594
	21st to end	Night	16% 6% 24% 31% 21% 2	666
September	1st to 10th	Night	14% 9% 27% 29% 20% 3	800
	11th to 20th	Night	14% 9% 21% 30% 21% 5%	800
	21st to end	Night	21% 9% 23% 29% 17% 3	800
October	1st to 10th	Night	21% 10% 30% 25% 13% 2	800
	11th to 20th	Night	30% 13% 28% 22% 6% 1	800
	21st to end	Night	31% 16% 29% 20% 4	880
November	1st to 10th	Night	29% 13% 29% 22% 6%	820
	11th to 20th	Night	25% 18% 31% 20% 6%	840
	21st to end	Night	34% 16% 29% 17% 4	840
December	1st to 10th	Night	31% 16% 33% 16% 4	840
	11th to 20th	Night	29% 18% 33% 16% 3 1	966
	21st to end	Night	32% 16% 30% 18% 3	1155

Page generated at 2026-04-29 02:21:46. Version ac07

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