Bishop's University Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bishop's University Astronomical Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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	18% 12% 4 10% 5% 5% 5% 3 5% 6% 26%	108393
	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 25%	109864
Transparency	Day	58% 4 12% 18% 7% 1	111909
	Night	61% 3 11% 16% 7% 1	114512
Seeing	Day	28% 15% 28% 19% 8% 2	14905
	Night	27% 12% 26% 25% 9% 1	23915

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	15% 23% 9% 18% 7% 8% 3 2 2 3 11%	3597
	11th to 20th	Day	11% 17% 8% 24% 12% 4 4 2 3 3 11%	3570
	21st to end	Day	10% 17% 9% 15% 6% 10% 6% 3 3 4 16%	3960
February	1st to 10th	Day	14% 18% 5% 16% 8% 7% 5% 2 3 4 16%	1626
	11th to 20th	Day	13% 18% 9% 15% 9% 4 5% 3 3 4 17%	1848
	21st to end	Day	12% 20% 6% 10% 7% 6% 5% 3 4 5% 21%	1570
March	1st to 10th	Day	22% 17% 4 11% 6% 4 3 3 4 4 22%	3116
	11th to 20th	Day	16% 21% 6% 12% 6% 4 3 3 3 4 22%	3284
	21st to end	Day	17% 15% 6% 9% 4 4 3 2 3 4 31%	4097
April	1st to 10th	Day	23% 16% 3 9% 4 4 4 3 4 5% 26%	2280
	11th to 20th	Day	20% 13% 3 7% 5% 4 4 3 4 4 34%	2297
	21st to end	Day	23% 13% 4 8% 4 4 4 3 4 6% 27%	2470
May	1st to 10th	Day	24% 9% 3 8% 5% 4 5% 4 5% 6% 27%	4160
	11th to 20th	Day	18% 9% 3 8% 5% 6% 5% 4 6% 7% 29%	4398
	21st to end	Day	14% 7% 3 7% 5% 6% 6% 5% 7% 9% 31%	4837
June	1st to 10th	Day	26% 8% 2 7% 4 5% 5% 4 6% 8% 26%	2660
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 4 6% 8% 34%	2660
	21st to end	Day	21% 9% 3 7% 5% 6% 5% 3 6% 7% 28%	2660
July	1st to 10th	Day	14% 7% 3 8% 5% 5% 6% 5% 8% 9% 31%	4440
	11th to 20th	Day	14% 7% 2 7% 6% 6% 6% 6% 7% 9% 32%	4440
	21st to end	Day	15% 7% 2 7% 4 4 4 4 6% 7% 39%	4814
August	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 4 7% 8% 26%	4371
	11th to 20th	Day	18% 7% 3 8% 5% 5% 6% 5% 7% 10% 26%	4045
	21st to end	Day	18% 7% 3 6% 4 4 5% 4 6% 8% 36%	4217
September	1st to 10th	Day	15% 7% 2 6% 5% 5% 4 4 6% 6% 40%	2280
	11th to 20th	Day	19% 7% 2 6% 3 4 4 3 5% 7% 42%	2124
	21st to end	Day	20% 8% 2 6% 4 4 5% 3 5% 9% 34%	2090
October	1st to 10th	Day	23% 10% 2 6% 5% 5% 5% 4 6% 7% 28%	2075
	11th to 20th	Day	19% 12% 3 8% 5% 5% 4 4 6% 6% 28%	1779
	21st to end	Day	25% 18% 4 8% 4 3 4 3 4 4 24%	1881
November	1st to 10th	Day	19% 16% 4 8% 4 4 4 3 5% 4 28%	1761
	11th to 20th	Day	18% 17% 6% 10% 6% 3 4 4 5% 5% 23%	1800
	21st to end	Day	19% 19% 5% 11% 5% 4 4 3 4 6% 18%	1640
December	1st to 10th	Day	16% 22% 7% 19% 8% 5% 4 2 3 3 11%	2600
	11th to 20th	Day	18% 19% 7% 13% 5% 3 4 3 4 4 20%	2986
	21st to end	Day	19% 19% 5% 14% 4 6% 4 3 4 5% 18%	3960
January	1st to 10th	Night	13% 24% 9% 19% 8% 5% 3 2 3 3 12%	6349
	11th to 20th	Night	9% 21% 9% 22% 9% 6% 4 3 4 3 10%	6315
	21st to end	Night	10% 18% 7% 15% 8% 7% 5% 3 4 5% 18%	6996
February	1st to 10th	Night	14% 18% 6% 13% 8% 6% 5% 3 4 5% 18%	2664
	11th to 20th	Night	12% 17% 7% 16% 8% 6% 4 3 4 4 19%	2218
	21st to end	Night	14% 19% 6% 9% 6% 6% 5% 3 4 4 25%	1884
March	1st to 10th	Night	22% 17% 4 7% 5% 4 4 2 4 4 27%	3724
	11th to 20th	Night	17% 22% 6% 10% 6% 4 4 2 3 4 21%	3674
	21st to end	Night	16% 18% 5% 11% 6% 4 3 2 3 3 28%	3530
April	1st to 10th	Night	22% 14% 4 8% 5% 4 4 3 4 5% 27%	1900
	11th to 20th	Night	22% 11% 3 6% 5% 3 4 3 5% 5% 33%	1831
	21st to end	Night	22% 13% 3 7% 5% 4 4 2 4 6% 30%	1710
May	1st to 10th	Night	26% 10% 2 8% 5% 4 4 3 5% 6% 26%	2809
	11th to 20th	Night	20% 9% 3 8% 5% 4 5% 3 6% 6% 30%	2495
	21st to end	Night	16% 7% 2 7% 5% 5% 5% 4 5% 8% 37%	2738
June	1st to 10th	Night	27% 8% 2 6% 4 3 4 3 7% 8% 28%	1520
	11th to 20th	Night	18% 7% 3 5% 4 4 4 6% 6% 8% 35%	1520
	21st to end	Night	21% 7% 3 7% 5% 4 5% 5% 6% 6% 31%	1520
July	1st to 10th	Night	13% 8% 3 7% 6% 6% 7% 5% 6% 8% 32%	2520
	11th to 20th	Night	16% 8% 3 7% 5% 5% 5% 5% 6% 6% 35%	2520
	21st to end	Night	15% 7% 3 7% 4 4 5% 4 6% 6% 38%	2739
August	1st to 10th	Night	22% 9% 3 7% 5% 4 5% 4 5% 7% 30%	2527
	11th to 20th	Night	15% 7% 3 8% 6% 5% 6% 4 7% 9% 30%	2782
	21st to end	Night	18% 6% 3 6% 4 4 4 3 5% 7% 39%	3436
September	1st to 10th	Night	16% 6% 3 7% 4 5% 4 3 5% 6% 39%	1900
	11th to 20th	Night	16% 6% 2 5% 3 4 4 3 5% 7% 46%	2055
	21st to end	Night	20% 6% 2 5% 4 4 3 3 5% 8% 40%	2090
October	1st to 10th	Night	20% 9% 3 7% 4 4 5% 4 5% 6% 33%	2103
	11th to 20th	Night	22% 11% 3 9% 5% 5% 4 3 4 5% 29%	2399
	21st to end	Night	26% 12% 3 8% 4 4 4 3 4 6% 26%	2717
November	1st to 10th	Night	20% 15% 3 8% 5% 5% 4 3 5% 7% 26%	2537
	11th to 20th	Night	18% 15% 5% 10% 5% 4 4 3 4 5% 27%	2600
	21st to end	Night	20% 18% 5% 12% 5% 4 3 2 4 4 22%	2758
December	1st to 10th	Night	16% 25% 11% 18% 6% 4 3 2 2 3 9%	4580
	11th to 20th	Night	21% 20% 5% 15% 7% 4 3 3 4 4 14%	5208
	21st to end	Night	20% 19% 5% 11% 6% 5% 4 2 5% 6% 17%	6996

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	82% 2 9% 7% 1	3598
	11th to 20th	Day	80% 3 7% 8% 3 1	3600
	21st to end	Day	72% 3 8% 11% 6% 1	3960
February	1st to 10th	Day	74% 2 6% 12% 5% 1	1626
	11th to 20th	Day	73% 2 9% 10% 5% 1	1857
	21st to end	Day	67% 2 9% 14% 6% 2	1570
March	1st to 10th	Day	66% 3 8% 13% 8% 2	3119
	11th to 20th	Day	67% 3 8% 11% 9% 1	3284
	21st to end	Day	58% 2 7% 18% 14% 1	4097
April	1st to 10th	Day	63% 3 7% 18% 8% 2	2280
	11th to 20th	Day	55% 3 7% 21% 13% 1	2297
	21st to end	Day	59% 3 9% 17% 9% 3	2470
May	1st to 10th	Day	58% 2 12% 18% 8% 1	4174
	11th to 20th	Day	54% 3 19% 17% 7% 1	4402
	21st to end	Day	47% 5% 14% 23% 11% 1	4837
June	1st to 10th	Day	56% 4 12% 19% 8% 1	2660
	11th to 20th	Day	47% 4 13% 23% 11% 1	2660
	21st to end	Day	55% 4 13% 20% 8%	2660
July	1st to 10th	Day	45% 7% 16% 25% 8%	4370
	11th to 20th	Day	44% 8% 16% 28% 4	4365
	21st to end	Day	43% 6% 14% 30% 6%	4661
August	1st to 10th	Day	52% 7% 16% 20% 5%	4357
	11th to 20th	Day	50% 8% 18% 19% 5%	4084
	21st to end	Day	44% 6% 15% 20% 12% 2	4217
September	1st to 10th	Day	43% 5% 14% 28% 9% 1	2280
	11th to 20th	Day	42% 4 12% 29% 13%	2124
	21st to end	Day	47% 5% 14% 24% 9% 1	2090
October	1st to 10th	Day	49% 6% 14% 24% 7%	3452
	11th to 20th	Day	53% 4 15% 24% 4	2930
	21st to end	Day	53% 3 15% 21% 7%	3081
November	1st to 10th	Day	59% 4 11% 18% 7% 1	1761
	11th to 20th	Day	64% 3 10% 13% 8% 1	1800
	21st to end	Day	68% 4 13% 12% 2	1640
December	1st to 10th	Day	79% 4 8% 7% 2	2600
	11th to 20th	Day	68% 3 8% 16% 5%	2986
	21st to end	Day	70% 3 8% 10% 8%	3960
January	1st to 10th	Night	81% 1 9% 6% 2	6349
	11th to 20th	Night	80% 2 9% 7% 2	6360
	21st to end	Night	69% 3 9% 10% 9%	6996
February	1st to 10th	Night	70% 3 8% 13% 5%	2664
	11th to 20th	Night	70% 2 8% 14% 5%	2220
	21st to end	Night	64% 2 9% 15% 9% 1	1884
March	1st to 10th	Night	62% 3 10% 16% 7% 3	3734
	11th to 20th	Night	68% 2 8% 13% 8% 1	3675
	21st to end	Night	64% 1 6% 18% 10% 2	3530
April	1st to 10th	Night	61% 2 8% 17% 11% 1	1900
	11th to 20th	Night	53% 3 9% 22% 13% 1	1831
	21st to end	Night	57% 3 9% 18% 11% 2	1710
May	1st to 10th	Night	59% 2 12% 18% 8% 1	2815
	11th to 20th	Night	54% 4 15% 18% 8% 1	2496
	21st to end	Night	45% 4 15% 18% 17% 1	2739
June	1st to 10th	Night	53% 4 12% 23% 9%	1520
	11th to 20th	Night	45% 5% 13% 24% 10% 2	1520
	21st to end	Night	51% 5% 15% 20% 9% 1	1520
July	1st to 10th	Night	47% 7% 15% 21% 10%	2480
	11th to 20th	Night	46% 7% 15% 26% 6%	2472
	21st to end	Night	44% 7% 16% 25% 9%	2651
August	1st to 10th	Night	52% 5% 14% 24% 4	2517
	11th to 20th	Night	49% 8% 17% 18% 7%	2809
	21st to end	Night	43% 5% 13% 24% 14% 1	3436
September	1st to 10th	Night	46% 4 13% 25% 12% 1	1900
	11th to 20th	Night	38% 4 16% 29% 11% 1	2055
	21st to end	Night	43% 6% 13% 26% 11% 1	2090
October	1st to 10th	Night	52% 5% 11% 23% 9%	3506
	11th to 20th	Night	54% 4 12% 23% 6%	3987
	21st to end	Night	51% 3 14% 22% 8% 1	4467
November	1st to 10th	Night	59% 3 13% 18% 6% 1	2537
	11th to 20th	Night	61% 3 10% 16% 9% 2	2600
	21st to end	Night	67% 3 13% 13% 4	2758
December	1st to 10th	Night	82% 3 7% 6% 1	4580
	11th to 20th	Night	74% 2 6% 12% 6%	5208
	21st to end	Night	69% 2 9% 13% 6%	6996

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	31% 17% 32% 18% 2 1	200
	11th to 20th	Day	32% 24% 29% 11% 4 1	200
	21st to end	Day	28% 20% 30% 14% 7%	220
February	1st to 10th	Day	37% 25% 23% 12% 2	226
	11th to 20th	Day	29% 24% 32% 12% 2 1	456
	21st to end	Day	31% 20% 27% 19% 2	382
March	1st to 10th	Day	34% 17% 27% 18% 4	460
	11th to 20th	Day	33% 20% 26% 15% 4 1	460
	21st to end	Day	32% 21% 25% 14% 7% 2	506
April	1st to 10th	Day	39% 14% 25% 15% 5% 2	460
	11th to 20th	Day	35% 18% 26% 11% 8% 2	452
	21st to end	Day	36% 14% 31% 11% 6% 1	460
May	1st to 10th	Day	36% 12% 29% 15% 7% 1	463
	11th to 20th	Day	26% 14% 29% 21% 8% 2	487
	21st to end	Day	22% 19% 31% 22% 5% 1	529
June	1st to 10th	Day	29% 15% 31% 18% 6% 1	490
	11th to 20th	Day	22% 17% 29% 21% 10% 1	490
	21st to end	Day	26% 14% 33% 17% 8% 2	490
July	1st to 10th	Day	20% 10% 31% 26% 11% 1	485
	11th to 20th	Day	18% 9% 39% 25% 7% 2	484
	21st to end	Day	20% 8% 33% 29% 9% 1	532
August	1st to 10th	Day	20% 9% 26% 28% 15% 2	486
	11th to 20th	Day	20% 8% 26% 27% 14% 5%	479
	21st to end	Day	22% 11% 24% 24% 13% 6%	539
September	1st to 10th	Day	19% 10% 30% 23% 12% 6%	490
	11th to 20th	Day	20% 11% 20% 21% 19% 9%	490
	21st to end	Day	19% 12% 24% 24% 14% 5%	490
October	1st to 10th	Day	26% 14% 25% 17% 14% 4	490
	11th to 20th	Day	31% 15% 24% 17% 11% 2	425
	21st to end	Day	40% 13% 23% 17% 7%	429
November	1st to 10th	Day	36% 14% 26% 19% 5% 1	395
	11th to 20th	Day	31% 18% 26% 19% 6% 1	400
	21st to end	Day	31% 20% 28% 16% 3 1	240
December	1st to 10th	Day	35% 17% 25% 18% 5% 2	200
	11th to 20th	Day	35% 22% 29% 10% 5% 1	200
	21st to end	Day	40% 16% 23% 16% 4 1	220
January	1st to 10th	Night	30% 19% 29% 19% 3 1	800
	11th to 20th	Night	28% 18% 30% 20% 5%	800
	21st to end	Night	23% 16% 31% 25% 6%	880
February	1st to 10th	Night	26% 18% 31% 20% 4	781
	11th to 20th	Night	25% 19% 32% 21% 3 1	743
	21st to end	Night	27% 16% 31% 22% 4	628
March	1st to 10th	Night	30% 14% 29% 23% 4	760
	11th to 20th	Night	29% 15% 29% 20% 6%	760
	21st to end	Night	29% 14% 27% 21% 8% 1	836
April	1st to 10th	Night	34% 12% 26% 22% 6%	760
	11th to 20th	Night	31% 12% 26% 23% 8%	707
	21st to end	Night	30% 15% 28% 20% 6%	570
May	1st to 10th	Night	30% 8% 28% 25% 8%	558
	11th to 20th	Night	23% 9% 29% 30% 8% 1	374
	21st to end	Night	21% 8% 31% 32% 8% 1	411
June	1st to 10th	Night	31% 7% 21% 32% 8% 1	380
	11th to 20th	Night	22% 8% 23% 32% 15%	380
	21st to end	Night	19% 6% 28% 35% 10% 1	380
July	1st to 10th	Night	16% 5% 25% 40% 13% 1	370
	11th to 20th	Night	15% 3 26% 39% 15% 2	368
	21st to end	Night	16% 4 21% 41% 18%	416
August	1st to 10th	Night	19% 4 13% 42% 21% 1	386
	11th to 20th	Night	19% 4 20% 38% 18% 1	562
	21st to end	Night	19% 3 22% 33% 20% 2	809
September	1st to 10th	Night	20% 7% 23% 34% 14% 2	760
	11th to 20th	Night	19% 6% 21% 29% 23% 3	760
	21st to end	Night	24% 8% 23% 26% 17% 2	760
October	1st to 10th	Night	26% 10% 21% 26% 15% 1	760
	11th to 20th	Night	30% 10% 23% 25% 10% 1	760
	21st to end	Night	33% 12% 25% 24% 5%	836
November	1st to 10th	Night	32% 13% 27% 21% 7%	780
	11th to 20th	Night	28% 16% 27% 19% 9% 1	800
	21st to end	Night	34% 17% 24% 19% 6% 1	800
December	1st to 10th	Night	34% 17% 25% 18% 5%	800
	11th to 20th	Night	32% 15% 27% 21% 4 1	800
	21st to end	Night	36% 16% 24% 19% 4 1	880

Page generated at 2025-02-03 03:05:53. Version ac07

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