Whitby Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whitby Ridge 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	21% 12% 4 9% 5% 4 4 3 5% 6% 28%	132875
	Night	21% 12% 4 9% 5% 4 4 3 5% 6% 28%	130396
Transparency	Day	36% 38% 7% 12% 6% 1	136457
	Night	37% 37% 6% 13% 7% 1	134901
Seeing	Day	27% 19% 31% 16% 5% 1	19485
	Night	27% 17% 30% 21% 6%	27074

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% 17% 6% 16% 9% 5% 4 3 3 4 19%	4690
	11th to 20th	Day	18% 18% 9% 16% 7% 5% 4 3 4 4 13%	4852
	21st to end	Day	13% 10% 4 10% 8% 7% 6% 4 5% 6% 26%	5781
February	1st to 10th	Day	19% 14% 5% 11% 7% 6% 5% 4 4 5% 20%	1890
	11th to 20th	Day	16% 15% 5% 12% 5% 5% 5% 4 4 5% 24%	1843
	21st to end	Day	13% 15% 4 9% 7% 5% 3 3 3 8% 31%	1595
March	1st to 10th	Day	23% 14% 4 12% 5% 5% 3 3 3 4 23%	5011
	11th to 20th	Day	18% 17% 4 9% 4 3 3 3 4 5% 29%	5090
	21st to end	Day	28% 16% 4 9% 5% 4 4 3 3 5% 20%	5587
April	1st to 10th	Day	26% 15% 3 8% 5% 3 3 2 4 5% 25%	2516
	11th to 20th	Day	23% 13% 3 6% 4 4 4 4 5% 6% 31%	2553
	21st to end	Day	22% 13% 3 7% 5% 4 4 3 4 7% 29%	2730
May	1st to 10th	Day	27% 14% 4 10% 5% 5% 5% 3 5% 5% 18%	6172
	11th to 20th	Day	23% 13% 4 8% 4 4 3 3 4 5% 28%	6458
	21st to end	Day	21% 12% 4 6% 4 3 4 4 6% 7% 29%	7102
June	1st to 10th	Day	25% 11% 3 8% 5% 5% 4 4 5% 6% 25%	2800
	11th to 20th	Day	22% 9% 3 7% 5% 4 4 3 5% 7% 32%	2814
	21st to end	Day	29% 10% 2 6% 5% 4 4 3 5% 6% 25%	3000
July	1st to 10th	Day	22% 12% 3 7% 4 4 5% 4 6% 8% 25%	6452
	11th to 20th	Day	18% 14% 3 8% 4 4 3 3 5% 8% 30%	6360
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 9% 36%	6714
August	1st to 10th	Day	26% 11% 3 7% 5% 3 4 4 5% 7% 27%	4235
	11th to 20th	Day	18% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	4175
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 45%	4477
September	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 4 8% 39%	2400
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 4 5% 43%	2334
	21st to end	Day	19% 7% 3 6% 3 4 3 2 5% 6% 41%	2130
October	1st to 10th	Day	18% 6% 3 6% 4 3 4 3 6% 7% 38%	2000
	11th to 20th	Day	20% 9% 2 7% 5% 4 4 3 6% 7% 33%	2000
	21st to end	Day	21% 10% 3 7% 5% 4 4 4 5% 6% 32%	2200
November	1st to 10th	Day	23% 10% 3 9% 5% 4 3 3 4 7% 29%	1718
	11th to 20th	Day	22% 13% 5% 9% 4 5% 4 3 4 6% 25%	1680
	21st to end	Day	19% 15% 4 9% 7% 4 5% 3 4 5% 24%	1680
December	1st to 10th	Day	22% 13% 5% 13% 5% 5% 3 4 5% 5% 21%	2690
	11th to 20th	Day	30% 11% 3 8% 4 5% 5% 3 4 4 25%	3076
	21st to end	Day	20% 14% 6% 12% 5% 3 3 3 4 7% 24%	4070
January	1st to 10th	Night	15% 16% 5% 14% 8% 6% 5% 3 5% 5% 19%	8234
	11th to 20th	Night	22% 14% 6% 17% 6% 5% 4 3 4 4 15%	8039
	21st to end	Night	12% 11% 4 12% 9% 7% 6% 4 6% 6% 22%	8400
February	1st to 10th	Night	19% 13% 4 11% 6% 6% 5% 4 5% 5% 22%	2730
	11th to 20th	Night	17% 12% 4 10% 6% 5% 5% 4 7% 6% 24%	2663
	21st to end	Night	14% 13% 4 10% 7% 5% 5% 4 5% 6% 27%	2210
March	1st to 10th	Night	24% 12% 4 11% 7% 5% 4 3 3 5% 21%	5033
	11th to 20th	Night	21% 15% 6% 8% 4 5% 4 3 4 4 27%	5089
	21st to end	Night	29% 14% 5% 10% 4 3 3 2 3 4 24%	5587
April	1st to 10th	Night	25% 13% 4 7% 4 4 4 3 5% 5% 27%	2103
	11th to 20th	Night	24% 12% 3 6% 5% 4 4 3 4 6% 30%	2014
	21st to end	Night	24% 11% 3 6% 5% 4 3 3 5% 6% 32%	1890
May	1st to 10th	Night	27% 16% 4 8% 4 3 3 3 4 5% 22%	3992
	11th to 20th	Night	24% 14% 3 7% 3 3 4 3 4 5% 30%	3676
	21st to end	Night	18% 16% 4 8% 5% 4 3 3 4 6% 29%	4036
June	1st to 10th	Night	28% 10% 3 7% 4 4 3 2 4 7% 28%	1601
	11th to 20th	Night	21% 10% 4 6% 4 3 4 3 5% 5% 35%	1592
	21st to end	Night	28% 12% 3 7% 5% 4 4 3 4 5% 24%	1400
July	1st to 10th	Night	20% 13% 4 11% 6% 5% 5% 4 5% 7% 22%	3471
	11th to 20th	Night	21% 12% 4 8% 5% 5% 4 4 4 6% 29%	3620
	21st to end	Night	18% 7% 2 7% 4 5% 4 4 5% 8% 36%	4135
August	1st to 10th	Night	24% 12% 3 6% 3 3 3 3 4 6% 34%	2915
	11th to 20th	Night	18% 10% 3 7% 4 4 5% 3 6% 8% 33%	2872
	21st to end	Night	12% 7% 2 5% 4 3 4 3 5% 7% 50%	3421
September	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 5% 6% 43%	2000
	11th to 20th	Night	18% 7% 2 6% 3 3 3 3 4 6% 45%	2063
	21st to end	Night	20% 9% 3 6% 3 3 3 3 5% 6% 38%	2270
October	1st to 10th	Night	19% 7% 2 5% 4 3 4 3 5% 6% 42%	2400
	11th to 20th	Night	20% 8% 2 6% 4 5% 3 3 5% 7% 38%	2400
	21st to end	Night	26% 9% 2 5% 4 3 4 3 4 6% 33%	2640
November	1st to 10th	Night	21% 10% 3 7% 4 4 4 3 5% 7% 33%	2797
	11th to 20th	Night	22% 12% 3 8% 5% 5% 3 3 5% 6% 27%	2940
	21st to end	Night	21% 11% 4 8% 6% 5% 5% 3 5% 5% 27%	2940
December	1st to 10th	Night	17% 11% 4 12% 6% 5% 5% 3 6% 6% 25%	4720
	11th to 20th	Night	27% 12% 3 7% 4 4 4 3 4 5% 27%	5353
	21st to end	Night	25% 11% 4 9% 5% 3 3 3 4 5% 28%	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	52% 24% 10% 9% 6%	4691
	11th to 20th	Day	56% 25% 6% 10% 3	4881
	21st to end	Day	42% 26% 5% 11% 13% 2	5781
February	1st to 10th	Day	29% 59% 2 5% 4 1	1890
	11th to 20th	Day	25% 58% 3 8% 5% 1	1851
	21st to end	Day	22% 59% 3 8% 7% 2	1595
March	1st to 10th	Day	47% 29% 6% 9% 6% 3	5015
	11th to 20th	Day	39% 33% 9% 14% 4	5090
	21st to end	Day	52% 28% 3 9% 7% 1	5587
April	1st to 10th	Day	25% 59% 3 5% 6% 2	2516
	11th to 20th	Day	25% 60% 3 8% 4	2553
	21st to end	Day	24% 60% 4 7% 5% 1	2730
May	1st to 10th	Day	48% 32% 7% 10% 3	6183
	11th to 20th	Day	44% 31% 6% 15% 3	6462
	21st to end	Day	40% 31% 10% 12% 7%	7102
June	1st to 10th	Day	21% 63% 4 8% 5%	2800
	11th to 20th	Day	18% 62% 5% 9% 5% 1	2814
	21st to end	Day	23% 60% 3 8% 6%	3000
July	1st to 10th	Day	44% 28% 10% 11% 7%	6382
	11th to 20th	Day	40% 27% 8% 18% 7%	6284
	21st to end	Day	31% 27% 13% 19% 9%	6522
August	1st to 10th	Day	39% 38% 6% 11% 6%	4222
	11th to 20th	Day	32% 38% 8% 18% 4	4214
	21st to end	Day	22% 37% 7% 21% 13%	4477
September	1st to 10th	Day	19% 57% 6% 11% 5% 1	2400
	11th to 20th	Day	20% 58% 5% 11% 6%	2334
	21st to end	Day	16% 57% 4 13% 8%	2130
October	1st to 10th	Day	31% 38% 6% 16% 10%	3260
	11th to 20th	Day	24% 37% 5% 18% 15% 1	3240
	21st to end	Day	27% 36% 10% 16% 10% 1	3538
November	1st to 10th	Day	28% 55% 4 8% 4 1	1718
	11th to 20th	Day	30% 56% 5% 7% 3 1	1680
	21st to end	Day	30% 55% 5% 6% 3	1680
December	1st to 10th	Day	39% 37% 6% 15% 3	2690
	11th to 20th	Day	41% 32% 6% 10% 11%	3075
	21st to end	Day	45% 27% 10% 12% 7%	4070
January	1st to 10th	Night	48% 26% 8% 15% 4	8234
	11th to 20th	Night	55% 25% 6% 10% 4	8084
	21st to end	Night	44% 27% 5% 12% 11% 2	8400
February	1st to 10th	Night	29% 59% 3 4 4 1	2730
	11th to 20th	Night	23% 58% 3 9% 5%	2666
	21st to end	Night	23% 59% 3 9% 6% 1	2210
March	1st to 10th	Night	49% 29% 3 8% 8% 1	5042
	11th to 20th	Night	43% 31% 6% 13% 6%	5090
	21st to end	Night	51% 28% 4 11% 5%	5587
April	1st to 10th	Night	26% 59% 3 5% 6% 2	2103
	11th to 20th	Night	23% 60% 3 9% 4 1	2014
	21st to end	Night	22% 59% 3 10% 5% 2	1890
May	1st to 10th	Night	49% 31% 7% 8% 5%	3998
	11th to 20th	Night	43% 30% 6% 15% 5% 1	3677
	21st to end	Night	43% 29% 7% 15% 5%	4037
June	1st to 10th	Night	22% 61% 3 10% 4	1601
	11th to 20th	Night	18% 62% 3 11% 5%	1592
	21st to end	Night	23% 60% 4 7% 5%	1400
July	1st to 10th	Night	49% 27% 6% 11% 6% 1	3432
	11th to 20th	Night	42% 27% 8% 14% 9% 1	3572
	21st to end	Night	34% 27% 9% 19% 11% 1	4014
August	1st to 10th	Night	36% 37% 7% 16% 3 1	2903
	11th to 20th	Night	30% 39% 11% 16% 4	2899
	21st to end	Night	19% 37% 9% 21% 14%	3420
September	1st to 10th	Night	16% 58% 5% 13% 8% 1	2000
	11th to 20th	Night	19% 56% 4 13% 7%	2063
	21st to end	Night	17% 57% 4 16% 6%	2270
October	1st to 10th	Night	32% 36% 8% 16% 8% 1	3920
	11th to 20th	Night	20% 38% 6% 19% 17%	3899
	21st to end	Night	24% 36% 8% 18% 13%	4252
November	1st to 10th	Night	25% 54% 7% 8% 5%	2797
	11th to 20th	Night	28% 56% 4 8% 5%	2940
	21st to end	Night	27% 55% 5% 7% 5% 1	2940
December	1st to 10th	Night	33% 37% 6% 19% 5%	4720
	11th to 20th	Night	39% 32% 6% 12% 10% 1	5355
	21st to end	Night	41% 28% 7% 14% 9%	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% 25% 30% 13% 1	430
	11th to 20th	Day	34% 20% 30% 12% 4	430
	21st to end	Day	25% 23% 36% 13% 3	473
February	1st to 10th	Day	30% 24% 28% 15% 3	430
	11th to 20th	Day	31% 27% 28% 11% 2	426
	21st to end	Day	25% 24% 34% 12% 5%	355
March	1st to 10th	Day	28% 28% 32% 10% 2	532
	11th to 20th	Day	29% 27% 30% 10% 4	540
	21st to end	Day	34% 26% 27% 10% 2 1	594
April	1st to 10th	Day	37% 28% 27% 6% 2	540
	11th to 20th	Day	31% 27% 29% 10% 2	532
	21st to end	Day	28% 25% 30% 12% 5% 1	540
May	1st to 10th	Day	32% 23% 31% 11% 2 1	608
	11th to 20th	Day	25% 20% 31% 19% 5% 1	774
	21st to end	Day	29% 17% 34% 17% 3 1	845
June	1st to 10th	Day	31% 16% 31% 16% 5%	730
	11th to 20th	Day	24% 17% 33% 18% 6% 1	730
	21st to end	Day	34% 12% 28% 19% 6% 1	730
July	1st to 10th	Day	23% 11% 34% 23% 7% 1	720
	11th to 20th	Day	22% 10% 34% 23% 10% 1	718
	21st to end	Day	27% 10% 33% 23% 7% 1	709
August	1st to 10th	Day	20% 9% 35% 27% 7% 2	526
	11th to 20th	Day	21% 12% 32% 23% 10% 2	519
	21st to end	Day	18% 13% 35% 23% 8% 3	583
September	1st to 10th	Day	18% 12% 38% 23% 7% 2	530
	11th to 20th	Day	24% 16% 30% 18% 9% 3	530
	21st to end	Day	20% 15% 32% 24% 7% 2	491
October	1st to 10th	Day	22% 13% 34% 19% 10% 2	420
	11th to 20th	Day	21% 20% 34% 15% 7% 2	420
	21st to end	Day	26% 23% 32% 13% 5% 1	462
November	1st to 10th	Day	28% 21% 29% 15% 6% 1	425
	11th to 20th	Day	36% 21% 25% 13% 5%	430
	21st to end	Day	30% 27% 27% 13% 2 1	430
December	1st to 10th	Day	34% 21% 31% 13% 2	430
	11th to 20th	Day	37% 26% 26% 10% 1	430
	21st to end	Day	29% 28% 29% 11% 3 1	473
January	1st to 10th	Night	28% 22% 34% 14% 2	950
	11th to 20th	Night	31% 22% 29% 15% 3	914
	21st to end	Night	23% 25% 34% 16% 2	924
February	1st to 10th	Night	28% 21% 31% 18% 2	840
	11th to 20th	Night	28% 25% 31% 14% 2	823
	21st to end	Night	21% 22% 34% 19% 3	692
March	1st to 10th	Night	25% 24% 33% 16% 2	840
	11th to 20th	Night	25% 20% 30% 20% 5%	840
	21st to end	Night	27% 22% 30% 17% 4	924
April	1st to 10th	Night	34% 19% 29% 16% 3	633
	11th to 20th	Night	29% 20% 32% 16% 3	624
	21st to end	Night	29% 14% 33% 20% 4 1	630
May	1st to 10th	Night	32% 14% 31% 18% 5%	630
	11th to 20th	Night	28% 14% 29% 22% 7% 1	622
	21st to end	Night	24% 14% 34% 23% 4	682
June	1st to 10th	Night	33% 10% 29% 23% 4	601
	11th to 20th	Night	24% 10% 36% 24% 5% 1	592
	21st to end	Night	33% 8% 23% 28% 9%	400
July	1st to 10th	Night	22% 8% 30% 32% 8%	515
	11th to 20th	Night	25% 6% 28% 31% 10%	582
	21st to end	Night	22% 7% 28% 32% 10%	656
August	1st to 10th	Night	20% 4 28% 33% 14% 1	598
	11th to 20th	Night	23% 5% 23% 34% 14% 1	592
	21st to end	Night	19% 7% 27% 31% 14% 1	660
September	1st to 10th	Night	22% 7% 31% 29% 11% 1	600
	11th to 20th	Night	21% 11% 25% 29% 13% 1	663
	21st to end	Night	25% 12% 27% 26% 9% 1	800
October	1st to 10th	Night	21% 13% 29% 27% 10% 1	800
	11th to 20th	Night	23% 16% 30% 22% 9% 1	800
	21st to end	Night	31% 15% 31% 19% 4	880
November	1st to 10th	Night	30% 14% 31% 20% 4 1	922
	11th to 20th	Night	29% 20% 27% 18% 5%	950
	21st to end	Night	30% 22% 28% 16% 4	950
December	1st to 10th	Night	32% 18% 32% 14% 4	950
	11th to 20th	Night	33% 23% 30% 12% 2	950
	21st to end	Night	29% 22% 31% 15% 2	1045

Page generated at 2026-05-29 02:53:30. Version ac07

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