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 2024-01-27.

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	19% 12% 4 8% 6% 3 5% 4 5% 6% 27%	74895
	Night	19% 13% 5% 9% 6% 3 5% 4 4 6% 27%	75147
Transparency	Day	58% 4 11% 18% 8% 1	74600
	Night	60% 3 11% 18% 8% 1	74976
Seeing	Day	28% 15% 28% 19% 8% 2	13618
	Night	27% 12% 26% 25% 9% 1	22632

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	17% 20% 9% 13% 9% 5% 6% 4 3 4 11%	1517
	11th to 20th	Day	19% 17% 6% 13% 10% 3 4 4 4 4 16%	1520
	21st to end	Day	11% 15% 7% 13% 8% 6% 7% 5% 4 5% 20%	1632
February	1st to 10th	Day	15% 18% 6% 14% 10% 6% 6% 3 3 3 15%	1503
	11th to 20th	Day	14% 17% 9% 14% 9% 3 5% 4 4 4 17%	1748
	21st to end	Day	13% 19% 8% 9% 8% 5% 6% 4 4 5% 21%	1480
March	1st to 10th	Day	15% 17% 5% 12% 7% 4 4 3 4 5% 24%	1797
	11th to 20th	Day	18% 14% 6% 8% 7% 4 4 4 3 5% 27%	1868
	21st to end	Day	18% 15% 5% 9% 7% 3 3 3 3 5% 28%	2335
April	1st to 10th	Day	23% 14% 4 9% 5% 3 4 4 4 5% 26%	2160
	11th to 20th	Day	21% 12% 3 6% 6% 3 4 4 4 4 35%	2174
	21st to end	Day	24% 13% 4 7% 4 3 4 4 4 6% 27%	2340
May	1st to 10th	Day	24% 9% 4 7% 5% 3 3 4 4 6% 32%	2366
	11th to 20th	Day	20% 8% 3 5% 6% 4 4 5% 5% 6% 34%	2478
	21st to end	Day	18% 7% 3 6% 7% 4 6% 6% 6% 8% 30%	2725
June	1st to 10th	Day	27% 8% 3 6% 4 3 5% 5% 6% 8% 25%	2520
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 5% 6% 8% 34%	2520
	21st to end	Day	21% 7% 4 6% 6% 4 5% 5% 6% 7% 28%	2520
July	1st to 10th	Day	17% 6% 3 7% 5% 3 5% 6% 5% 9% 34%	2520
	11th to 20th	Day	15% 6% 3 6% 6% 3 6% 7% 7% 9% 31%	2520
	21st to end	Day	17% 7% 3 7% 6% 3 6% 6% 6% 8% 31%	2744
August	1st to 10th	Day	18% 7% 3 6% 7% 4 5% 5% 7% 10% 30%	2481
	11th to 20th	Day	16% 7% 2 6% 6% 3 6% 6% 7% 9% 32%	2304
	21st to end	Day	17% 6% 2 5% 5% 3 5% 6% 6% 8% 36%	2399
September	1st to 10th	Day	16% 7% 3 5% 5% 3 5% 4 6% 6% 39%	2160
	11th to 20th	Day	20% 6% 2 5% 3 2 4 4 5% 6% 42%	2013
	21st to end	Day	20% 7% 2 5% 5% 3 4 5% 5% 9% 34%	1980
October	1st to 10th	Day	24% 10% 3 5% 6% 3 5% 4 6% 6% 28%	1966
	11th to 20th	Day	19% 12% 3 7% 7% 4 5% 5% 5% 6% 27%	1686
	21st to end	Day	27% 17% 4 7% 5% 3 4 3 4 4 22%	1782
November	1st to 10th	Day	20% 14% 5% 7% 5% 3 4 4 5% 4 28%	1671
	11th to 20th	Day	19% 15% 7% 10% 7% 2 4 4 5% 5% 22%	1710
	21st to end	Day	19% 18% 6% 10% 6% 3 4 5% 4 6% 19%	1558
December	1st to 10th	Day	22% 21% 9% 11% 6% 3 5% 3 3 4 14%	1520
	11th to 20th	Day	20% 18% 7% 10% 6% 3 4 4 4 4 20%	1906
	21st to end	Day	24% 19% 6% 13% 6% 3 4 4 3 3 16%	2772
January	1st to 10th	Night	17% 20% 8% 13% 9% 5% 5% 4 3 4 13%	2649
	11th to 20th	Night	16% 17% 8% 15% 8% 4 6% 4 4 3 16%	2660
	21st to end	Night	12% 15% 8% 13% 9% 5% 5% 5% 4 5% 19%	2868
February	1st to 10th	Night	14% 17% 7% 13% 9% 4 6% 4 3 5% 18%	2456
	11th to 20th	Night	13% 16% 8% 14% 9% 5% 5% 4 3 4 20%	2098
	21st to end	Night	14% 17% 7% 9% 7% 5% 5% 4 3 4 24%	1776
March	1st to 10th	Night	15% 16% 7% 9% 7% 3 5% 3 3 4 25%	2150
	11th to 20th	Night	18% 14% 6% 8% 6% 3 5% 3 4 5% 28%	2090
	21st to end	Night	19% 13% 6% 9% 5% 3 4 3 4 4 30%	2016
April	1st to 10th	Night	22% 13% 5% 8% 6% 3 4 3 5% 5% 27%	1800
	11th to 20th	Night	22% 9% 3 5% 6% 3 4 4 4 5% 34%	1734
	21st to end	Night	23% 12% 4 6% 5% 3 4 3 4 6% 29%	1620
May	1st to 10th	Night	23% 10% 3 7% 5% 3 4 4 4 6% 30%	1609
	11th to 20th	Night	19% 8% 4 6% 5% 4 4 4 5% 6% 35%	1415
	21st to end	Night	18% 6% 3 6% 6% 3 5% 5% 5% 8% 34%	1550
June	1st to 10th	Night	26% 7% 3 5% 5% 2 5% 4 6% 8% 28%	1440
	11th to 20th	Night	19% 6% 3 5% 4 3 4 7% 6% 8% 35%	1440
	21st to end	Night	20% 6% 3 7% 5% 3 5% 7% 6% 7% 32%	1440
July	1st to 10th	Night	15% 7% 3 5% 5% 3 6% 5% 6% 8% 37%	1440
	11th to 20th	Night	12% 6% 3 5% 6% 4 6% 7% 6% 8% 37%	1440
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 7% 7% 38%	1574
August	1st to 10th	Night	16% 5% 2 5% 5% 3 5% 6% 7% 10% 37%	1439
	11th to 20th	Night	14% 7% 3 7% 6% 4 5% 4 5% 9% 36%	1596
	21st to end	Night	16% 6% 3 6% 5% 3 4 4 5% 8% 40%	1954
September	1st to 10th	Night	16% 7% 3 6% 6% 4 4 5% 5% 6% 39%	1800
	11th to 20th	Night	16% 6% 2 5% 3 3 4 5% 4 7% 46%	1946
	21st to end	Night	21% 6% 2 4 4 3 3 4 5% 8% 40%	1980
October	1st to 10th	Night	21% 8% 3 6% 6% 2 5% 5% 5% 6% 33%	1992
	11th to 20th	Night	22% 10% 3 8% 6% 4 5% 4 4 5% 28%	2272
	21st to end	Night	27% 12% 3 6% 5% 3 4 4 4 6% 25%	2574
November	1st to 10th	Night	20% 14% 3 8% 5% 4 4 4 4 7% 26%	2407
	11th to 20th	Night	19% 13% 6% 9% 7% 3 4 3 4 5% 26%	2470
	21st to end	Night	20% 16% 6% 11% 6% 3 3 3 4 4 23%	2620
December	1st to 10th	Night	22% 20% 8% 10% 8% 3 4 4 3 4 13%	2660
	11th to 20th	Night	21% 19% 7% 14% 9% 3 4 4 3 3 14%	3288
	21st to end	Night	24% 20% 5% 11% 6% 3 4 3 3 4 17%	4884

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 8% 9% 2	1518
	11th to 20th	Day	73% 4 7% 11% 5% 1	1520
	21st to end	Day	69% 2 9% 14% 5% 1	1632
February	1st to 10th	Day	76% 2 6% 11% 4 1	1503
	11th to 20th	Day	73% 2 10% 10% 5% 1	1757
	21st to end	Day	68% 2 9% 14% 6% 2	1480
March	1st to 10th	Day	66% 3 8% 16% 6% 3	1800
	11th to 20th	Day	61% 3 10% 13% 11% 1	1868
	21st to end	Day	61% 2 7% 19% 9% 2	2335
April	1st to 10th	Day	63% 3 7% 17% 8% 2	2160
	11th to 20th	Day	54% 3 7% 21% 13% 1	2174
	21st to end	Day	60% 3 9% 17% 9% 3	2340
May	1st to 10th	Day	55% 2 8% 20% 14% 1	2366
	11th to 20th	Day	52% 3 11% 21% 12% 1	2482
	21st to end	Day	52% 5% 13% 19% 11% 1	2725
June	1st to 10th	Day	57% 4 12% 18% 7% 1	2520
	11th to 20th	Day	48% 4 12% 23% 11% 1	2520
	21st to end	Day	55% 4 13% 20% 8%	2520
July	1st to 10th	Day	46% 5% 15% 25% 9%	2450
	11th to 20th	Day	46% 5% 16% 26% 6%	2445
	21st to end	Day	50% 4 17% 23% 6% 1	2591
August	1st to 10th	Day	49% 6% 17% 23% 5%	2467
	11th to 20th	Day	47% 5% 19% 21% 9%	2304
	21st to end	Day	45% 5% 15% 24% 12%	2399
September	1st to 10th	Day	45% 5% 14% 26% 9% 1	2160
	11th to 20th	Day	43% 3 11% 28% 14%	2013
	21st to end	Day	47% 4 14% 24% 10% 1	1980
October	1st to 10th	Day	56% 4 12% 19% 8%	1966
	11th to 20th	Day	57% 4 15% 19% 5%	1686
	21st to end	Day	67% 3 9% 15% 6% 1	1782
November	1st to 10th	Day	59% 4 12% 18% 7% 1	1671
	11th to 20th	Day	65% 3 11% 13% 7% 1	1710
	21st to end	Day	67% 4 14% 13% 2	1558
December	1st to 10th	Day	76% 3 9% 9% 3	1520
	11th to 20th	Day	69% 3 8% 15% 5%	1906
	21st to end	Day	75% 3 9% 9% 5%	2772
January	1st to 10th	Night	77% 2 8% 9% 3	2649
	11th to 20th	Night	75% 2 8% 11% 3	2660
	21st to end	Night	68% 3 8% 15% 5% 1	2868
February	1st to 10th	Night	72% 3 7% 13% 5%	2456
	11th to 20th	Night	70% 2 8% 14% 6%	2100
	21st to end	Night	65% 2 8% 14% 9% 1	1776
March	1st to 10th	Night	64% 3 8% 17% 7% 2	2160
	11th to 20th	Night	61% 2 9% 15% 10% 2	2091
	21st to end	Night	59% 2 7% 20% 9% 3	2016
April	1st to 10th	Night	62% 2 8% 17% 11% 1	1800
	11th to 20th	Night	52% 3 9% 22% 13% 1	1734
	21st to end	Night	58% 3 9% 18% 10% 2	1620
May	1st to 10th	Night	56% 3 6% 19% 14% 1	1609
	11th to 20th	Night	51% 4 11% 18% 14% 2	1416
	21st to end	Night	48% 6% 13% 20% 12% 1	1551
June	1st to 10th	Night	53% 4 12% 23% 8%	1440
	11th to 20th	Night	45% 5% 12% 25% 10% 3	1440
	21st to end	Night	50% 5% 15% 20% 9%	1440
July	1st to 10th	Night	44% 6% 16% 22% 12% 1	1400
	11th to 20th	Night	42% 6% 17% 27% 8%	1392
	21st to end	Night	43% 5% 19% 25% 9%	1486
August	1st to 10th	Night	41% 6% 19% 28% 6%	1429
	11th to 20th	Night	45% 4 18% 23% 10%	1596
	21st to end	Night	43% 4 15% 27% 10%	1954
September	1st to 10th	Night	46% 4 13% 25% 12% 1	1800
	11th to 20th	Night	40% 4 16% 29% 11% 1	1946
	21st to end	Night	43% 6% 14% 26% 11% 1	1980
October	1st to 10th	Night	52% 3 11% 23% 11% 1	1992
	11th to 20th	Night	58% 4 13% 20% 5%	2272
	21st to end	Night	62% 3 11% 16% 7% 1	2574
November	1st to 10th	Night	59% 3 12% 18% 7% 1	2407
	11th to 20th	Night	61% 3 11% 15% 8% 2	2470
	21st to end	Night	67% 3 13% 14% 4	2620
December	1st to 10th	Night	77% 2 9% 10% 2	2660
	11th to 20th	Night	76% 2 6% 11% 4	3288
	21st to end	Night	74% 2 9% 13% 2	4884

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	29% 16% 33% 20% 2 1	180
	11th to 20th	Day	34% 22% 28% 10% 4 1	180
	21st to end	Day	28% 19% 31% 14% 7% 1	188
February	1st to 10th	Day	40% 24% 22% 11% 2	193
	11th to 20th	Day	29% 24% 31% 13% 2 1	416
	21st to end	Day	31% 21% 27% 20% 2	346
March	1st to 10th	Day	34% 17% 28% 18% 3	420
	11th to 20th	Day	32% 21% 25% 16% 4 1	420
	21st to end	Day	32% 19% 24% 15% 8% 2	462
April	1st to 10th	Day	39% 14% 25% 15% 5% 2	420
	11th to 20th	Day	36% 17% 26% 12% 8% 2	412
	21st to end	Day	36% 14% 30% 12% 6% 1	420
May	1st to 10th	Day	37% 12% 30% 14% 7% 1	423
	11th to 20th	Day	27% 15% 31% 20% 6% 2	447
	21st to end	Day	22% 19% 31% 21% 6% 1	485
June	1st to 10th	Day	30% 16% 31% 16% 6% 1	450
	11th to 20th	Day	22% 17% 28% 22% 9% 1	450
	21st to end	Day	24% 12% 35% 17% 9% 2	450
July	1st to 10th	Day	21% 10% 30% 26% 12% 2	445
	11th to 20th	Day	18% 8% 39% 25% 8% 2	444
	21st to end	Day	20% 8% 34% 28% 9% 1	488
August	1st to 10th	Day	20% 9% 26% 27% 16% 2	446
	11th to 20th	Day	21% 8% 26% 27% 15% 5%	439
	21st to end	Day	23% 11% 23% 24% 12% 7%	495
September	1st to 10th	Day	19% 9% 30% 23% 12% 6%	450
	11th to 20th	Day	21% 12% 22% 20% 17% 8%	450
	21st to end	Day	19% 13% 25% 24% 14% 5%	450
October	1st to 10th	Day	27% 14% 25% 15% 14% 5%	450
	11th to 20th	Day	30% 16% 24% 17% 11% 2	392
	21st to end	Day	41% 13% 22% 17% 7%	396
November	1st to 10th	Day	36% 13% 26% 20% 5% 1	365
	11th to 20th	Day	31% 18% 26% 18% 6% 1	370
	21st to end	Day	30% 20% 28% 18% 3 1	218
December	1st to 10th	Day	36% 18% 23% 18% 4 2	180
	11th to 20th	Day	35% 22% 28% 10% 4 1	180
	21st to end	Day	42% 17% 23% 14% 3 1	198
January	1st to 10th	Night	31% 18% 28% 19% 3 1	760
	11th to 20th	Night	28% 18% 30% 20% 5%	760
	21st to end	Night	23% 16% 30% 24% 6%	820
February	1st to 10th	Night	26% 19% 31% 19% 4	720
	11th to 20th	Night	25% 19% 32% 21% 3 1	703
	21st to end	Night	28% 16% 30% 23% 3	592
March	1st to 10th	Night	29% 14% 30% 23% 4	720
	11th to 20th	Night	29% 15% 29% 19% 6%	720
	21st to end	Night	29% 14% 27% 21% 8% 1	792
April	1st to 10th	Night	34% 12% 26% 21% 6%	720
	11th to 20th	Night	30% 12% 25% 24% 8%	670
	21st to end	Night	31% 14% 28% 20% 6%	540
May	1st to 10th	Night	30% 8% 28% 24% 9%	529
	11th to 20th	Night	23% 9% 30% 30% 7% 1	354
	21st to end	Night	21% 8% 31% 31% 8% 1	389
June	1st to 10th	Night	31% 7% 22% 32% 8% 1	360
	11th to 20th	Night	22% 8% 23% 33% 15%	360
	21st to end	Night	19% 5% 29% 36% 11% 1	360
July	1st to 10th	Night	17% 4 25% 39% 14% 1	350
	11th to 20th	Night	14% 2 27% 39% 16% 2	348
	21st to end	Night	15% 4 22% 41% 18%	394
August	1st to 10th	Night	18% 4 13% 43% 21% 1	365
	11th to 20th	Night	19% 4 19% 39% 18% 1	532
	21st to end	Night	19% 3 23% 33% 19% 2	766
September	1st to 10th	Night	21% 6% 23% 34% 15% 2	720
	11th to 20th	Night	20% 6% 22% 29% 20% 3	720
	21st to end	Night	24% 8% 24% 26% 16% 2	720
October	1st to 10th	Night	27% 9% 22% 26% 15% 1	720
	11th to 20th	Night	30% 10% 24% 25% 10% 2	720
	21st to end	Night	34% 12% 24% 24% 5%	792
November	1st to 10th	Night	31% 12% 28% 21% 7%	740
	11th to 20th	Night	28% 16% 27% 18% 9% 1	760
	21st to end	Night	33% 17% 24% 19% 6% 1	760
December	1st to 10th	Night	35% 17% 25% 18% 5%	760
	11th to 20th	Night	32% 15% 27% 21% 4 1	760
	21st to end	Night	37% 16% 24% 19% 4	836

Page generated at 2024-01-24 09:37:26. Version ac07

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