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 2026-06-01.

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% 6% 5% 5% 4 5% 6% 26%	132011
	Night	17% 14% 5% 11% 6% 5% 4 3 4 5% 24%	131162
Transparency	Day	58% 4 12% 18% 7% 1	135479
	Night	62% 4 11% 16% 7% 1	135781
Seeing	Day	28% 15% 28% 19% 8% 2	16623
	Night	27% 12% 26% 25% 9% 1	25551

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	14% 20% 9% 16% 8% 8% 4 2 3 4 12%	4661
	11th to 20th	Day	10% 20% 8% 24% 12% 4 4 3 3 3 10%	4650
	21st to end	Day	8% 15% 8% 15% 10% 12% 6% 4 4 4 14%	5124
February	1st to 10th	Day	14% 17% 5% 17% 8% 7% 5% 2 4 4 17%	1754
	11th to 20th	Day	12% 18% 9% 16% 9% 4 5% 3 3 4 17%	2048
	21st to end	Day	12% 19% 6% 11% 7% 6% 5% 3 4 5% 21%	1730
March	1st to 10th	Day	20% 16% 6% 13% 6% 5% 4 3 3 4 19%	4576
	11th to 20th	Day	15% 17% 5% 10% 6% 5% 4 3 4 5% 27%	4804
	21st to end	Day	20% 15% 5% 10% 5% 4 3 2 3 5% 26%	6003
April	1st to 10th	Day	22% 16% 4 9% 4 4 4 3 4 5% 26%	2520
	11th to 20th	Day	20% 13% 4 7% 5% 5% 4 3 3 4 33%	2542
	21st to end	Day	23% 13% 4 8% 4 4 4 3 4 6% 27%	2730
May	1st to 10th	Day	28% 11% 4 9% 5% 4 5% 3 5% 5% 21%	6099
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	6458
	21st to end	Day	19% 10% 3 7% 5% 5% 5% 4 6% 8% 29%	7103
June	1st to 10th	Day	26% 8% 2 7% 4 5% 5% 4 6% 7% 26%	2800
	11th to 20th	Day	17% 7% 3 6% 5% 4 5% 4 6% 8% 34%	2800
	21st to end	Day	21% 8% 3 7% 5% 6% 5% 3 6% 7% 29%	2800
July	1st to 10th	Day	16% 7% 3 9% 6% 6% 6% 5% 8% 10% 26%	6360
	11th to 20th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 34%	6360
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 9% 41%	6926
August	1st to 10th	Day	17% 8% 3 8% 5% 5% 6% 4 7% 8% 28%	4509
	11th to 20th	Day	18% 7% 3 7% 5% 5% 6% 5% 7% 10% 27%	4175
	21st to end	Day	18% 7% 3 6% 4 4 5% 4 6% 8% 36%	4350
September	1st to 10th	Day	15% 7% 2 6% 5% 4 4 4 6% 6% 41%	2400
	11th to 20th	Day	18% 6% 2 6% 3 4 4 3 5% 7% 43%	2236
	21st to end	Day	20% 8% 2 5% 4 4 5% 3 5% 9% 35%	2200
October	1st to 10th	Day	22% 9% 2 6% 5% 5% 4 4 6% 7% 30%	2184
	11th to 20th	Day	18% 12% 3 9% 5% 5% 4 4 5% 7% 28%	1872
	21st to end	Day	25% 18% 4 8% 4 3 4 3 4 4 23%	1980
November	1st to 10th	Day	19% 17% 4 8% 4 4 4 4 5% 4 27%	1851
	11th to 20th	Day	18% 18% 6% 10% 5% 3 3 3 4 4 23%	1890
	21st to end	Day	20% 20% 6% 11% 5% 4 4 3 4 6% 17%	1722
December	1st to 10th	Day	16% 22% 7% 19% 8% 5% 4 2 3 3 11%	2680
	11th to 20th	Day	18% 18% 7% 13% 5% 3 4 3 4 4 20%	3066
	21st to end	Day	19% 18% 5% 14% 5% 6% 4 3 4 5% 18%	4048
January	1st to 10th	Night	12% 22% 9% 17% 8% 5% 4 3 3 3 14%	8234
	11th to 20th	Night	10% 21% 9% 22% 8% 7% 4 3 4 3 10%	8235
	21st to end	Night	8% 15% 6% 16% 11% 9% 5% 4 4 5% 17%	9057
February	1st to 10th	Night	13% 17% 6% 14% 8% 6% 4 3 4 5% 19%	2864
	11th to 20th	Night	12% 17% 7% 16% 8% 6% 4 3 3 4 19%	2458
	21st to end	Night	14% 19% 6% 10% 6% 5% 5% 3 4 4 25%	2076
March	1st to 10th	Night	21% 17% 5% 10% 5% 6% 4 2 3 4 22%	5484
	11th to 20th	Night	16% 17% 5% 10% 6% 5% 5% 3 5% 5% 23%	5374
	21st to end	Night	17% 18% 5% 11% 6% 5% 4 3 3 3 24%	5166
April	1st to 10th	Night	22% 14% 4 8% 5% 4 4 3 4 5% 27%	2100
	11th to 20th	Night	23% 12% 3 6% 5% 3 4 3 4 5% 32%	2026
	21st to end	Night	21% 12% 3 8% 5% 4 4 2 4 6% 31%	1890
May	1st to 10th	Night	28% 11% 3 8% 5% 4 4 3 5% 6% 23%	4104
	11th to 20th	Night	24% 8% 3 7% 5% 5% 5% 4 5% 6% 28%	3655
	21st to end	Night	19% 9% 3 7% 5% 4 5% 3 5% 7% 32%	4014
June	1st to 10th	Night	26% 8% 2 6% 4 4 4 3 7% 8% 28%	1600
	11th to 20th	Night	19% 7% 3 5% 4 4 4 6% 6% 7% 34%	1600
	21st to end	Night	21% 7% 3 7% 5% 4 5% 5% 6% 7% 31%	1600
July	1st to 10th	Night	14% 8% 3 8% 6% 6% 7% 4 7% 9% 29%	3600
	11th to 20th	Night	17% 7% 3 7% 4 5% 5% 4 5% 6% 37%	3600
	21st to end	Night	13% 6% 3 7% 4 4 5% 4 6% 8% 39%	3926
August	1st to 10th	Night	21% 9% 3 7% 4 4 5% 3 5% 7% 32%	2608
	11th to 20th	Night	15% 7% 3 8% 6% 5% 6% 4 7% 9% 31%	2872
	21st to end	Night	18% 6% 3 6% 4 4 4 3 5% 7% 39%	3545
September	1st to 10th	Night	16% 6% 3 7% 4 5% 4 3 5% 6% 40%	2000
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 47%	2163
	21st to end	Night	21% 6% 2 5% 4 4 3 3 5% 8% 40%	2200
October	1st to 10th	Night	20% 8% 3 6% 4 4 4 4 5% 6% 35%	2214
	11th to 20th	Night	21% 11% 3 9% 5% 5% 4 3 4 5% 30%	2526
	21st to end	Night	25% 12% 3 7% 4 4 4 3 4 6% 26%	2860
November	1st to 10th	Night	20% 15% 3 8% 4 5% 4 3 5% 6% 26%	2667
	11th to 20th	Night	18% 16% 5% 10% 5% 4 4 3 4 5% 26%	2730
	21st to end	Night	20% 19% 6% 12% 5% 4 3 2 4 4 21%	2896
December	1st to 10th	Night	16% 24% 11% 18% 6% 4 3 2 2 3 9%	4720
	11th to 20th	Night	20% 20% 5% 16% 7% 4 3 3 3 4 14%	5348
	21st to end	Night	19% 19% 5% 11% 6% 5% 4 2 5% 6% 17%	7150

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	79% 2 10% 7% 1	4662
	11th to 20th	Day	80% 3 6% 8% 3	4680
	21st to end	Day	73% 3 7% 10% 7%	5124
February	1st to 10th	Day	73% 2 6% 13% 5% 1	1754
	11th to 20th	Day	72% 2 9% 11% 5% 1	2057
	21st to end	Day	67% 2 9% 14% 6% 2	1730
March	1st to 10th	Day	70% 3 6% 10% 8% 3	4579
	11th to 20th	Day	60% 4 12% 13% 10% 1	4804
	21st to end	Day	62% 3 7% 15% 13% 1	6003
April	1st to 10th	Day	63% 3 7% 18% 8% 2	2520
	11th to 20th	Day	56% 3 7% 20% 13% 1	2542
	21st to end	Day	58% 3 9% 18% 9% 2	2730
May	1st to 10th	Day	65% 3 10% 15% 6%	6110
	11th to 20th	Day	54% 5% 16% 17% 7% 1	6462
	21st to end	Day	52% 5% 13% 20% 10% 1	7103
June	1st to 10th	Day	56% 4 12% 19% 8% 1	2800
	11th to 20th	Day	47% 5% 13% 23% 11% 1	2800
	21st to end	Day	54% 4 13% 20% 8%	2800
July	1st to 10th	Day	49% 7% 16% 21% 7%	6290
	11th to 20th	Day	42% 8% 15% 28% 7%	6285
	21st to end	Day	39% 6% 15% 32% 7%	6728
August	1st to 10th	Day	51% 7% 16% 20% 5%	4495
	11th to 20th	Day	49% 8% 18% 20% 5%	4214
	21st to end	Day	44% 6% 15% 21% 12% 2	4350
September	1st to 10th	Day	42% 5% 13% 30% 9% 1	2400
	11th to 20th	Day	41% 4 12% 29% 14%	2236
	21st to end	Day	47% 5% 14% 23% 10% 1	2200
October	1st to 10th	Day	48% 6% 14% 24% 8%	3561
	11th to 20th	Day	52% 4 15% 24% 5%	3023
	21st to end	Day	54% 3 14% 21% 7%	3180
November	1st to 10th	Day	60% 5% 11% 18% 6% 1	1851
	11th to 20th	Day	65% 3 10% 13% 8% 1	1890
	21st to end	Day	70% 4 13% 11% 2	1722
December	1st to 10th	Day	79% 4 8% 7% 2	2680
	11th to 20th	Day	68% 3 8% 16% 5%	3066
	21st to end	Day	70% 3 8% 10% 8%	4048
January	1st to 10th	Night	77% 2 10% 8% 2	8234
	11th to 20th	Night	80% 3 7% 7% 3	8280
	21st to end	Night	70% 3 8% 11% 8%	9057
February	1st to 10th	Night	70% 3 8% 14% 5%	2864
	11th to 20th	Night	70% 2 8% 14% 5%	2460
	21st to end	Night	64% 2 9% 15% 9% 1	2076
March	1st to 10th	Night	66% 3 8% 13% 8% 2	5494
	11th to 20th	Night	62% 4 13% 13% 7% 1	5375
	21st to end	Night	65% 2 6% 16% 9% 1	5166
April	1st to 10th	Night	61% 2 8% 17% 11% 1	2100
	11th to 20th	Night	55% 3 8% 21% 12% 1	2026
	21st to end	Night	56% 3 9% 18% 12% 2	1890
May	1st to 10th	Night	64% 3 10% 15% 7% 1	4110
	11th to 20th	Night	55% 6% 13% 19% 7% 1	3656
	21st to end	Night	51% 5% 13% 18% 13%	4015
June	1st to 10th	Night	54% 4 11% 22% 9% 1	1600
	11th to 20th	Night	46% 5% 13% 23% 11% 2	1600
	21st to end	Night	50% 5% 15% 20% 9% 1	1600
July	1st to 10th	Night	49% 8% 17% 19% 7%	3560
	11th to 20th	Night	45% 7% 14% 26% 8%	3552
	21st to end	Night	41% 7% 17% 27% 8%	3809
August	1st to 10th	Night	51% 5% 14% 25% 5%	2598
	11th to 20th	Night	48% 8% 17% 19% 7%	2899
	21st to end	Night	44% 5% 13% 24% 13% 1	3545
September	1st to 10th	Night	45% 4 13% 26% 12% 1	2000
	11th to 20th	Night	38% 4 16% 30% 12% 1	2163
	21st to end	Night	43% 6% 13% 25% 12% 1	2200
October	1st to 10th	Night	50% 5% 11% 23% 9%	3617
	11th to 20th	Night	53% 4 12% 24% 7%	4114
	21st to end	Night	51% 3 14% 22% 8% 1	4610
November	1st to 10th	Night	60% 3 12% 18% 6% 1	2667
	11th to 20th	Night	62% 3 10% 15% 8% 1	2730
	21st to end	Night	68% 3 12% 13% 4	2896
December	1st to 10th	Night	82% 3 7% 7% 1	4720
	11th to 20th	Night	75% 2 6% 12% 5%	5348
	21st to end	Night	69% 2 9% 13% 6%	7150

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	30% 17% 31% 19% 2	220
	11th to 20th	Day	31% 25% 29% 10% 4 1	220
	21st to end	Day	29% 20% 30% 14% 7%	242
February	1st to 10th	Day	33% 23% 27% 13% 4	264
	11th to 20th	Day	28% 23% 32% 15% 2 1	536
	21st to end	Day	30% 21% 30% 17% 2	446
March	1st to 10th	Day	34% 19% 27% 16% 4	540
	11th to 20th	Day	35% 20% 26% 15% 4 1	540
	21st to end	Day	34% 21% 24% 13% 6% 2	594
April	1st to 10th	Day	39% 14% 26% 14% 4 2	540
	11th to 20th	Day	38% 17% 26% 11% 7% 2	532
	21st to end	Day	35% 15% 29% 13% 6% 2	540
May	1st to 10th	Day	37% 13% 29% 14% 6% 1	543
	11th to 20th	Day	26% 16% 29% 20% 7% 2	567
	21st to end	Day	23% 20% 31% 20% 5% 1	617
June	1st to 10th	Day	28% 14% 32% 18% 6% 1	530
	11th to 20th	Day	22% 17% 29% 21% 10% 1	530
	21st to end	Day	25% 14% 35% 16% 8% 2	530
July	1st to 10th	Day	21% 11% 31% 25% 11% 1	525
	11th to 20th	Day	17% 9% 40% 25% 7% 2	524
	21st to end	Day	19% 10% 34% 28% 9% 1	576
August	1st to 10th	Day	19% 9% 25% 29% 17% 2	526
	11th to 20th	Day	20% 8% 26% 26% 16% 5%	519
	21st to end	Day	22% 12% 24% 24% 13% 6%	583
September	1st to 10th	Day	19% 10% 29% 24% 13% 6%	530
	11th to 20th	Day	18% 11% 21% 21% 19% 10%	530
	21st to end	Day	20% 12% 24% 24% 14% 5%	530
October	1st to 10th	Day	25% 14% 25% 17% 14% 5%	530
	11th to 20th	Day	29% 15% 24% 18% 11% 2	458
	21st to end	Day	40% 13% 23% 17% 7%	462
November	1st to 10th	Day	37% 15% 26% 18% 4	425
	11th to 20th	Day	31% 17% 27% 18% 6% 1	430
	21st to end	Day	33% 21% 27% 15% 3 1	262
December	1st to 10th	Day	33% 17% 27% 17% 5% 1	220
	11th to 20th	Day	34% 22% 29% 10% 4	220
	21st to end	Day	39% 17% 23% 17% 4 1	242
January	1st to 10th	Night	30% 18% 29% 20% 3	840
	11th to 20th	Night	28% 18% 29% 20% 5%	840
	21st to end	Night	23% 16% 30% 24% 6%	924
February	1st to 10th	Night	25% 18% 31% 21% 5%	840
	11th to 20th	Night	25% 19% 31% 22% 3	823
	21st to end	Night	27% 16% 31% 23% 3	692
March	1st to 10th	Night	30% 16% 29% 21% 4	840
	11th to 20th	Night	30% 16% 29% 20% 6%	840
	21st to end	Night	29% 15% 27% 21% 7% 1	924
April	1st to 10th	Night	35% 13% 26% 21% 6%	840
	11th to 20th	Night	32% 13% 25% 23% 7%	782
	21st to end	Night	29% 15% 29% 20% 6%	630
May	1st to 10th	Night	30% 9% 28% 25% 8%	616
	11th to 20th	Night	24% 9% 30% 30% 7% 1	414
	21st to end	Night	22% 8% 30% 32% 7% 1	455
June	1st to 10th	Night	31% 7% 23% 32% 8% 1	400
	11th to 20th	Night	22% 8% 23% 32% 15%	400
	21st to end	Night	19% 7% 29% 35% 10% 1	400
July	1st to 10th	Night	16% 4 25% 40% 13% 1	390
	11th to 20th	Night	15% 3 26% 40% 15% 2	388
	21st to end	Night	16% 4 21% 41% 17%	438
August	1st to 10th	Night	18% 4 13% 43% 22% 1	407
	11th to 20th	Night	18% 4 19% 38% 20% 1	592
	21st to end	Night	18% 4 23% 33% 20% 2	852
September	1st to 10th	Night	20% 7% 23% 34% 15% 2	800
	11th to 20th	Night	18% 6% 20% 29% 24% 3	800
	21st to end	Night	24% 8% 23% 27% 17% 2	800
October	1st to 10th	Night	25% 10% 21% 28% 15% 1	800
	11th to 20th	Night	29% 10% 24% 26% 10% 1	800
	21st to end	Night	33% 12% 25% 25% 5%	880
November	1st to 10th	Night	32% 13% 27% 20% 7%	820
	11th to 20th	Night	29% 16% 26% 19% 9% 1	840
	21st to end	Night	33% 18% 24% 19% 6% 1	840
December	1st to 10th	Night	33% 18% 26% 18% 4	840
	11th to 20th	Night	32% 16% 27% 20% 4 1	840
	21st to end	Night	36% 16% 24% 19% 4 1	924

Page generated at 2026-05-29 01:53:24. Version ac07

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