Bishop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bishop Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	13% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	138042
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 43%	134982
Transparency	Day	40% 7% 11% 20% 18% 5%	141817
	Night	39% 6% 10% 20% 20% 6%	139278
Seeing	Day	16% 11% 24% 24% 17% 8%	17018
	Night	16% 9% 23% 29% 20% 3	26995

By Month

Percentage Cloud Cover SE (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	20% 11% 3 5% 3 3 3 3 3 4 42%	5258
	11th to 20th	Day	11% 7% 4 5% 3 2 3 2 3 4 56%	5280
	21st to end	Day	22% 10% 3 7% 4 3 4 2 4 6% 34%	6215
February	1st to 10th	Day	17% 9% 2 5% 4 5% 4 4 5% 6% 40%	2100
	11th to 20th	Day	22% 9% 3 6% 4 4 3 2 4 4 39%	2048
	21st to end	Day	20% 8% 2 8% 4 3 4 3 4 5% 39%	1730
March	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 6% 33%	4580
	11th to 20th	Day	11% 6% 2 6% 3 4 3 3 5% 6% 49%	5090
	21st to end	Day	15% 6% 2 7% 5% 4 4 4 4 6% 43%	5586
April	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 6% 7% 45%	2475
	11th to 20th	Day	11% 7% 2 5% 4 4 4 3 5% 6% 49%	2494
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 4 6% 7% 46%	2623
May	1st to 10th	Day	15% 8% 3 7% 4 5% 5% 4 6% 8% 35%	7616
	11th to 20th	Day	13% 7% 3 8% 5% 5% 6% 5% 7% 8% 34%	7626
	21st to end	Day	16% 9% 4 9% 5% 5% 5% 4 6% 7% 30%	8391
June	1st to 10th	Day	10% 7% 3 7% 6% 6% 5% 5% 8% 9% 35%	2730
	11th to 20th	Day	5% 4 2 7% 5% 6% 6% 5% 8% 10% 42%	2730
	21st to end	Day	7% 6% 2 6% 6% 6% 7% 5% 7% 11% 39%	2730
July	1st to 10th	Day	5% 4 3 7% 6% 7% 7% 6% 9% 11% 35%	6030
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 10% 13% 35%	6030
	21st to end	Day	8% 8% 3 9% 7% 7% 7% 6% 9% 12% 24%	6554
August	1st to 10th	Day	5% 3 1 5% 4 5% 6% 6% 9% 12% 42%	4143
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 5% 7% 11% 41%	3849
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 7% 49%	4312
September	1st to 10th	Day	9% 5% 2 7% 6% 5% 7% 5% 7% 10% 39%	2265
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 3 6% 8% 41%	2200
	21st to end	Day	13% 6% 2 5% 3 4 4 4 6% 6% 47%	2200
October	1st to 10th	Day	10% 5% 2 6% 5% 4 3 3 5% 5% 53%	2200
	11th to 20th	Day	9% 6% 2 4 3 3 3 3 5% 6% 55%	2147
	21st to end	Day	13% 5% 2 4 3 3 4 3 4 5% 54%	2130
November	1st to 10th	Day	12% 6% 2 5% 3 4 3 2 4 5% 54%	1852
	11th to 20th	Day	12% 6% 2 5% 3 3 3 4 4 6% 52%	1890
	21st to end	Day	13% 6% 2 5% 4 4 4 3 6% 7% 45%	1890
December	1st to 10th	Day	18% 7% 2 6% 3 3 4 3 4 7% 43%	3020
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 3 3 4 40%	3463
	21st to end	Day	18% 8% 3 8% 4 4 4 3 4 7% 38%	4565
January	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 4 6% 41%	7635
	11th to 20th	Night	14% 9% 3 5% 3 3 3 3 4 5% 49%	7680
	21st to end	Night	20% 11% 3 7% 3 3 3 2 3 4 41%	7948
February	1st to 10th	Night	15% 9% 3 6% 4 3 3 3 5% 6% 43%	2520
	11th to 20th	Night	22% 8% 3 6% 3 3 4 3 4 4 41%	2458
	21st to end	Night	19% 8% 2 6% 4 4 3 3 6% 6% 39%	2076
March	1st to 10th	Night	21% 10% 3 7% 4 4 5% 3 4 6% 33%	5493
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 7% 50%	5090
	21st to end	Night	13% 7% 3 7% 5% 4 4 3 4 5% 44%	5588
April	1st to 10th	Night	11% 4 2 5% 4 4 4 5% 6% 9% 46%	2161
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 7% 51%	2075
	21st to end	Night	9% 6% 3 5% 4 4 3 3 5% 7% 50%	1995
May	1st to 10th	Night	13% 9% 3 8% 6% 5% 5% 4 6% 8% 34%	5237
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 7% 9% 34%	5234
	21st to end	Night	13% 8% 3 9% 6% 5% 5% 4 6% 9% 31%	5761
June	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 6% 7% 12% 39%	1890
	11th to 20th	Night	4 3 2 5% 6% 5% 5% 4 7% 11% 48%	1890
	21st to end	Night	5% 5% 2 5% 4 4 5% 5% 7% 12% 46%	1890
July	1st to 10th	Night	5% 5% 2 7% 6% 5% 6% 6% 9% 12% 35%	4148
	11th to 20th	Night	5% 5% 2 6% 6% 5% 6% 6% 10% 13% 37%	4147
	21st to end	Night	8% 7% 3 9% 6% 7% 6% 5% 8% 11% 31%	4517
August	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 5% 7% 13% 43%	3001
	11th to 20th	Night	7% 5% 3 6% 5% 4 5% 5% 5% 9% 47%	3200
	21st to end	Night	8% 5% 2 5% 3 3 4 3 6% 8% 53%	3586
September	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	2115
	11th to 20th	Night	13% 5% 2 6% 4 5% 4 3 6% 7% 44%	2200
	21st to end	Night	10% 5% 2 5% 3 4 3 3 6% 8% 51%	2200
October	1st to 10th	Night	9% 5% 1 5% 3 3 3 3 4 6% 58%	2200
	11th to 20th	Night	10% 5% 2 4 4 2 3 3 4 5% 57%	2250
	21st to end	Night	12% 5% 1 5% 4 3 3 3 4 4 56%	2706
November	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 5% 6% 53%	2666
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 7% 54%	2730
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 6% 47%	2730
December	1st to 10th	Night	17% 7% 3 6% 3 3 3 2 4 5% 46%	4380
	11th to 20th	Night	18% 9% 3 6% 3 3 3 2 3 5% 43%	4952
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 7% 39%	6633

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	47% 3 7% 23% 18% 2	5259
	11th to 20th	Day	33% 3 5% 20% 31% 8%	5280
	21st to end	Day	52% 2 6% 14% 18% 7%	6215
February	1st to 10th	Day	46% 4 7% 19% 21% 3	2100
	11th to 20th	Day	49% 3 5% 19% 19% 5%	2058
	21st to end	Day	46% 4 8% 13% 22% 7%	1730
March	1st to 10th	Day	50% 4 9% 11% 18% 7%	4580
	11th to 20th	Day	36% 4 6% 15% 24% 15%	5090
	21st to end	Day	42% 4 9% 22% 20% 3	5586
April	1st to 10th	Day	38% 5% 11% 22% 19% 5%	2475
	11th to 20th	Day	36% 4 6% 19% 26% 7%	2494
	21st to end	Day	35% 7% 11% 25% 20% 3	2623
May	1st to 10th	Day	45% 6% 12% 22% 13% 2	7651
	11th to 20th	Day	45% 8% 15% 21% 10% 1	7630
	21st to end	Day	52% 8% 9% 17% 13% 2	8405
June	1st to 10th	Day	41% 11% 14% 21% 12% 2	2730
	11th to 20th	Day	33% 10% 19% 23% 14% 1	2730
	21st to end	Day	36% 11% 17% 22% 12% 2	2730
July	1st to 10th	Day	36% 13% 20% 22% 9% 1	5965
	11th to 20th	Day	34% 14% 29% 17% 5% 1	5961
	21st to end	Day	49% 12% 19% 16% 4	6370
August	1st to 10th	Day	29% 10% 14% 34% 13%	4130
	11th to 20th	Day	32% 9% 17% 26% 13% 3	3885
	21st to end	Day	33% 8% 12% 21% 22% 4	4311
September	1st to 10th	Day	39% 10% 14% 16% 16% 5%	2265
	11th to 20th	Day	40% 9% 10% 21% 17% 2	2200
	21st to end	Day	37% 6% 11% 17% 22% 7%	2200
October	1st to 10th	Day	27% 4 7% 11% 37% 13%	3590
	11th to 20th	Day	21% 3 5% 17% 30% 25%	3478
	21st to end	Day	25% 4 6% 23% 30% 11%	3419
November	1st to 10th	Day	34% 3 7% 18% 30% 8%	1852
	11th to 20th	Day	33% 4 8% 19% 27% 9%	1890
	21st to end	Day	37% 5% 7% 19% 26% 6%	1890
December	1st to 10th	Day	42% 3 6% 18% 26% 5%	3020
	11th to 20th	Day	48% 3 8% 18% 17% 5%	3460
	21st to end	Day	46% 4 8% 26% 15% 1	4565
January	1st to 10th	Night	44% 4 9% 24% 17% 3	7636
	11th to 20th	Night	38% 3 6% 19% 28% 7%	7680
	21st to end	Night	49% 2 4 16% 22% 7%	7948
February	1st to 10th	Night	42% 5% 6% 20% 21% 6%	2520
	11th to 20th	Night	48% 3 6% 18% 18% 7%	2459
	21st to end	Night	45% 4 8% 16% 18% 8%	2076
March	1st to 10th	Night	51% 4 8% 14% 18% 4	5493
	11th to 20th	Night	33% 4 8% 16% 24% 15%	5090
	21st to end	Night	42% 3 7% 22% 20% 6%	5588
April	1st to 10th	Night	33% 6% 13% 21% 20% 7%	2161
	11th to 20th	Night	33% 4 7% 22% 25% 9%	2075
	21st to end	Night	34% 5% 11% 26% 20% 4	1995
May	1st to 10th	Night	46% 8% 12% 18% 14% 2	5264
	11th to 20th	Night	44% 7% 15% 22% 12%	5235
	21st to end	Night	47% 9% 11% 19% 13% 1	5771
June	1st to 10th	Night	34% 10% 17% 21% 15% 2	1890
	11th to 20th	Night	28% 9% 21% 25% 16% 2	1890
	21st to end	Night	28% 11% 19% 22% 15% 4	1890
July	1st to 10th	Night	37% 12% 19% 23% 8% 1	4105
	11th to 20th	Night	32% 14% 28% 19% 5% 1	4096
	21st to end	Night	45% 13% 19% 18% 4 1	4387
August	1st to 10th	Night	31% 9% 16% 34% 10% 1	2990
	11th to 20th	Night	31% 9% 15% 27% 16% 2	3230
	21st to end	Night	29% 8% 14% 17% 29% 3	3586
September	1st to 10th	Night	36% 10% 16% 17% 14% 6%	2115
	11th to 20th	Night	37% 8% 12% 22% 19% 2	2200
	21st to end	Night	31% 7% 12% 20% 23% 8%	2200
October	1st to 10th	Night	24% 4 8% 14% 36% 14%	3590
	11th to 20th	Night	19% 3 4 18% 33% 24%	3658
	21st to end	Night	24% 3 4 24% 32% 14%	4367
November	1st to 10th	Night	33% 5% 8% 19% 25% 11%	2666
	11th to 20th	Night	31% 4 6% 21% 28% 10%	2730
	21st to end	Night	38% 4 7% 20% 25% 6%	2730
December	1st to 10th	Night	41% 4 6% 20% 27% 3	4380
	11th to 20th	Night	45% 4 7% 17% 24% 4	4954
	21st to end	Night	45% 4 7% 27% 15% 1	6633

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	26% 26% 35% 11% 2	330
	11th to 20th	Day	27% 24% 30% 14% 5%	330
	21st to end	Day	26% 27% 35% 9% 3 1	363
February	1st to 10th	Day	24% 22% 36% 15% 3	330
	11th to 20th	Day	36% 22% 31% 9% 1	330
	21st to end	Day	30% 23% 35% 9% 3	273
March	1st to 10th	Day	26% 20% 33% 16% 4 1	330
	11th to 20th	Day	25% 19% 31% 19% 6% 1	540
	21st to end	Day	19% 19% 37% 19% 6%	594
April	1st to 10th	Day	22% 14% 33% 18% 12% 1	540
	11th to 20th	Day	17% 16% 30% 23% 12% 2	532
	21st to end	Day	16% 11% 28% 29% 12% 4	540
May	1st to 10th	Day	17% 12% 28% 26% 13% 4	543
	11th to 20th	Day	12% 7% 25% 32% 16% 9%	567
	21st to end	Day	14% 6% 22% 29% 19% 10%	617
June	1st to 10th	Day	12% 5% 22% 30% 20% 11%	570
	11th to 20th	Day	7% 4 19% 28% 23% 19%	570
	21st to end	Day	9% 4 15% 27% 27% 17%	570
July	1st to 10th	Day	7% 2 11% 28% 30% 21%	565
	11th to 20th	Day	7% 2 8% 25% 32% 26%	564
	21st to end	Day	9% 1 12% 26% 35% 17%	616
August	1st to 10th	Day	11% 2 13% 26% 29% 17%	526
	11th to 20th	Day	10% 3 10% 30% 28% 19%	519
	21st to end	Day	14% 2 11% 25% 29% 19%	583
September	1st to 10th	Day	8% 4 15% 31% 27% 15%	530
	11th to 20th	Day	15% 5% 13% 27% 28% 12%	530
	21st to end	Day	14% 4 20% 27% 26% 9%	530
October	1st to 10th	Day	13% 5% 23% 32% 20% 8%	530
	11th to 20th	Day	11% 10% 25% 28% 22% 3	530
	21st to end	Day	17% 11% 25% 27% 18% 2	513
November	1st to 10th	Day	19% 11% 31% 25% 12% 2	330
	11th to 20th	Day	18% 15% 41% 17% 8% 1	330
	21st to end	Day	14% 21% 38% 19% 7% 2	330
December	1st to 10th	Day	29% 20% 30% 16% 3 1	330
	11th to 20th	Day	25% 15% 32% 22% 5% 1	330
	21st to end	Day	28% 17% 32% 17% 6%	363
January	1st to 10th	Night	22% 19% 30% 24% 5%	840
	11th to 20th	Night	24% 15% 32% 24% 6%	840
	21st to end	Night	22% 16% 36% 23% 2	924
February	1st to 10th	Night	20% 15% 31% 29% 4	840
	11th to 20th	Night	27% 18% 31% 22% 2	823
	21st to end	Night	24% 19% 34% 20% 3	692
March	1st to 10th	Night	21% 14% 32% 27% 5%	840
	11th to 20th	Night	18% 15% 34% 25% 8% 1	840
	21st to end	Night	18% 14% 31% 29% 8%	924
April	1st to 10th	Night	14% 14% 34% 28% 11%	691
	11th to 20th	Night	14% 12% 31% 30% 12% 1	623
	21st to end	Night	11% 10% 25% 37% 16% 1	630
May	1st to 10th	Night	12% 7% 26% 34% 19% 2	630
	11th to 20th	Night	12% 7% 21% 33% 23% 4	621
	21st to end	Night	11% 4 14% 35% 30% 6%	683
June	1st to 10th	Night	8% 3 12% 41% 31% 5%	630
	11th to 20th	Night	4 2 12% 32% 38% 12%	630
	21st to end	Night	6% 2 9% 31% 43% 8%	630
July	1st to 10th	Night	8% 3 8% 22% 46% 12%	615
	11th to 20th	Night	7% 1 6% 26% 47% 13%	612
	21st to end	Night	8% 2 8% 28% 43% 10%	687
August	1st to 10th	Night	8% 1 6% 32% 44% 10%	597
	11th to 20th	Night	10% 2 7% 30% 41% 10%	594
	21st to end	Night	13% 3 8% 28% 38% 10%	660
September	1st to 10th	Night	9% 3 10% 29% 40% 9%	715
	11th to 20th	Night	14% 3 13% 30% 36% 4	800
	21st to end	Night	13% 4 15% 34% 32% 3	800
October	1st to 10th	Night	11% 4 22% 33% 25% 5%	800
	11th to 20th	Night	13% 6% 25% 33% 22% 2	800
	21st to end	Night	15% 8% 22% 36% 16% 2	880
November	1st to 10th	Night	15% 8% 23% 34% 19% 1	820
	11th to 20th	Night	15% 11% 30% 32% 12% 1	840
	21st to end	Night	18% 12% 28% 31% 11% 1	840
December	1st to 10th	Night	22% 14% 30% 26% 7%	840
	11th to 20th	Night	23% 11% 30% 28% 8%	840
	21st to end	Night	25% 13% 29% 27% 7%	924

Page generated at 2026-07-28 03:25:21. Version ac07

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