Whitby
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whitby clear sky chart from 2005-11-06 to 2026-07-01.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	138225
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	134108
Transparency	Day	47% 5% 13% 22% 11% 1	141816
	Night	48% 5% 12% 21% 12% 1	138605
Seeing	Day	21% 11% 29% 24% 11% 4	16492
	Night	20% 9% 28% 29% 12% 1	25865

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% 11% 4 11% 6% 5% 5% 3 4 5% 29%	4661
	11th to 20th	Day	10% 19% 7% 17% 9% 6% 5% 4 4 4 15%	4981
	21st to end	Day	14% 15% 5% 15% 6% 6% 5% 3 5% 5% 22%	5780
February	1st to 10th	Day	15% 14% 4 12% 6% 7% 4 4 5% 5% 23%	1890
	11th to 20th	Day	13% 13% 6% 13% 6% 6% 5% 2 5% 6% 26%	1843
	21st to end	Day	17% 14% 5% 10% 6% 4 4 4 4 4 27%	1606
March	1st to 10th	Day	19% 9% 4 9% 5% 4 4 3 5% 6% 32%	5042
	11th to 20th	Day	12% 11% 5% 8% 5% 6% 6% 4 6% 8% 30%	5090
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 4 5% 35%	5586
April	1st to 10th	Day	18% 10% 4 9% 6% 4 4 4 6% 6% 30%	2505
	11th to 20th	Day	20% 9% 3 6% 4 4 4 4 4 6% 35%	2541
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 3 5% 5% 34%	2730
May	1st to 10th	Day	18% 9% 3 9% 4 5% 5% 4 6% 7% 31%	7718
	11th to 20th	Day	14% 8% 3 8% 5% 4 4 4 6% 9% 35%	8221
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 8% 43%	9060
June	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 37%	2940
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	2940
	21st to end	Day	13% 6% 2 5% 4 5% 5% 4 6% 9% 40%	2940
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 10% 42%	6360
	11th to 20th	Day	8% 4 2 6% 4 4 5% 4 7% 8% 48%	6360
	21st to end	Day	6% 4 2 5% 4 5% 6% 5% 7% 9% 47%	6593
August	1st to 10th	Day	15% 7% 2 6% 4 4 5% 3 6% 8% 39%	4234
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 7% 10% 38%	4174
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 44%	4454
September	1st to 10th	Day	13% 4 2 5% 5% 4 5% 4 6% 7% 45%	2400
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 7% 8% 48%	2320
	21st to end	Day	14% 5% 3 7% 5% 5% 5% 4 6% 8% 39%	2135
October	1st to 10th	Day	15% 8% 2 6% 5% 5% 4 4 5% 6% 38%	2000
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 6% 6% 32%	2000
	21st to end	Day	22% 10% 3 9% 5% 5% 5% 4 5% 6% 26%	2200
November	1st to 10th	Day	15% 8% 3 9% 5% 6% 4 3 5% 7% 33%	1767
	11th to 20th	Day	15% 11% 3 9% 7% 5% 5% 4 7% 6% 26%	1680
	21st to end	Day	19% 11% 4 10% 6% 5% 5% 4 5% 7% 24%	1680
December	1st to 10th	Day	18% 15% 4 12% 7% 7% 6% 4 6% 6% 14%	2680
	11th to 20th	Day	20% 14% 4 10% 5% 5% 4 3 4 6% 25%	3066
	21st to end	Day	27% 15% 6% 12% 6% 4 4 3 3 5% 16%	4048
January	1st to 10th	Night	15% 13% 4 10% 6% 5% 5% 4 6% 6% 26%	8234
	11th to 20th	Night	13% 16% 5% 14% 8% 6% 5% 4 5% 6% 18%	7901
	21st to end	Night	13% 13% 5% 10% 6% 5% 6% 4 6% 7% 25%	8401
February	1st to 10th	Night	13% 14% 5% 10% 6% 5% 5% 4 5% 5% 27%	2730
	11th to 20th	Night	15% 12% 5% 10% 7% 5% 4 3 5% 6% 27%	2663
	21st to end	Night	14% 11% 4 10% 6% 5% 5% 5% 6% 6% 29%	2195
March	1st to 10th	Night	17% 10% 4 8% 5% 5% 5% 3 5% 6% 32%	5018
	11th to 20th	Night	11% 8% 4 7% 5% 5% 5% 4 7% 8% 36%	5089
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 5% 39%	5588
April	1st to 10th	Night	19% 9% 3 6% 5% 4 4 4 5% 6% 33%	2114
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 5% 6% 38%	2045
	21st to end	Night	16% 8% 2 7% 5% 4 5% 4 6% 7% 36%	1890
May	1st to 10th	Night	22% 8% 3 6% 4 4 4 4 5% 6% 35%	5180
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 5% 7% 7% 36%	4639
	21st to end	Night	18% 5% 2 5% 4 4 4 4 5% 7% 44%	5114
June	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 7% 8% 43%	1680
	11th to 20th	Night	8% 4 3 7% 4 5% 4 5% 6% 8% 45%	1680
	21st to end	Night	11% 6% 2 6% 4 4 4 5% 6% 9% 42%	1680
July	1st to 10th	Night	7% 6% 2 6% 4 5% 5% 4 6% 9% 46%	3600
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 6% 8% 49%	3600
	21st to end	Night	9% 4 1 5% 3 3 4 4 6% 8% 53%	4243
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 48%	2915
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 8% 50%	2875
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 9% 47%	3410
September	1st to 10th	Night	10% 5% 3 5% 4 4 5% 4 6% 7% 48%	2000
	11th to 20th	Night	10% 4 2 4 4 3 3 3 4 7% 56%	2076
	21st to end	Night	17% 6% 2 7% 4 3 4 4 4 6% 43%	2265
October	1st to 10th	Night	15% 6% 3 7% 5% 4 4 4 5% 6% 42%	2400
	11th to 20th	Night	19% 9% 3 7% 5% 4 5% 4 5% 6% 34%	2400
	21st to end	Night	20% 9% 3 6% 5% 4 5% 4 5% 6% 32%	2640
November	1st to 10th	Night	17% 8% 2 7% 5% 4 4 3 6% 7% 37%	2745
	11th to 20th	Night	16% 11% 3 7% 6% 6% 5% 4 6% 7% 29%	2940
	21st to end	Night	20% 11% 3 8% 5% 5% 6% 4 6% 7% 25%	2940
December	1st to 10th	Night	17% 13% 5% 10% 6% 6% 6% 4 6% 7% 19%	4720
	11th to 20th	Night	17% 15% 4 9% 6% 5% 5% 4 5% 6% 24%	5348
	21st to end	Night	25% 14% 6% 11% 5% 5% 5% 3 5% 5% 16%	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	57% 4 13% 15% 10%	4662
	11th to 20th	Day	72% 4 8% 10% 5% 1	5014
	21st to end	Day	64% 4 10% 13% 9%	5780
February	1st to 10th	Day	62% 4 10% 17% 6% 1	1890
	11th to 20th	Day	59% 5% 9% 17% 8% 1	1852
	21st to end	Day	58% 5% 12% 18% 7% 1	1606
March	1st to 10th	Day	53% 4 13% 15% 15% 1	5045
	11th to 20th	Day	51% 7% 16% 16% 8% 2	5090
	21st to end	Day	50% 4 9% 20% 15% 1	5586
April	1st to 10th	Day	54% 5% 11% 18% 11% 2	2505
	11th to 20th	Day	49% 5% 9% 22% 13% 1	2541
	21st to end	Day	50% 4 11% 20% 12% 3	2730
May	1st to 10th	Day	50% 6% 13% 18% 10% 3	7727
	11th to 20th	Day	45% 6% 14% 19% 15% 1	8225
	21st to end	Day	40% 4 12% 29% 13% 2	9061
June	1st to 10th	Day	42% 6% 13% 22% 16% 1	2940
	11th to 20th	Day	38% 6% 14% 26% 14% 2	2940
	21st to end	Day	39% 6% 16% 25% 13% 1	2940
July	1st to 10th	Day	37% 6% 15% 26% 15% 2	6290
	11th to 20th	Day	31% 6% 13% 34% 15% 1	6285
	21st to end	Day	31% 7% 16% 31% 15%	6405
August	1st to 10th	Day	42% 6% 15% 24% 12% 1	4221
	11th to 20th	Day	39% 6% 16% 29% 10%	4213
	21st to end	Day	33% 6% 14% 37% 9% 1	4454
September	1st to 10th	Day	37% 7% 14% 31% 11% 1	2400
	11th to 20th	Day	32% 5% 16% 32% 14% 1	2320
	21st to end	Day	42% 7% 14% 26% 10%	2135
October	1st to 10th	Day	36% 5% 15% 28% 15% 1	3260
	11th to 20th	Day	38% 5% 17% 24% 15%	3240
	21st to end	Day	50% 6% 15% 22% 7%	3538
November	1st to 10th	Day	49% 6% 13% 21% 9% 1	1767
	11th to 20th	Day	55% 6% 14% 16% 9% 1	1680
	21st to end	Day	60% 4 15% 17% 4	1680
December	1st to 10th	Day	68% 6% 14% 9% 3	2680
	11th to 20th	Day	61% 6% 12% 12% 9%	3066
	21st to end	Day	71% 5% 7% 10% 6% 1	4048
January	1st to 10th	Night	56% 6% 12% 18% 7%	8234
	11th to 20th	Night	65% 5% 11% 12% 7% 1	7943
	21st to end	Night	57% 4 12% 14% 12% 1	8401
February	1st to 10th	Night	57% 5% 11% 20% 6% 1	2730
	11th to 20th	Night	56% 5% 10% 18% 9%	2665
	21st to end	Night	53% 5% 13% 20% 8% 1	2195
March	1st to 10th	Night	53% 4 10% 18% 13% 3	5028
	11th to 20th	Night	44% 5% 17% 23% 10% 2	5090
	21st to end	Night	46% 4 9% 21% 18% 2	5588
April	1st to 10th	Night	52% 4 9% 21% 13% 1	2114
	11th to 20th	Night	47% 5% 10% 20% 17% 2	2045
	21st to end	Night	46% 5% 11% 22% 15% 2	1890
May	1st to 10th	Night	50% 4 10% 17% 15% 5%	5187
	11th to 20th	Night	45% 5% 14% 19% 15% 2	4640
	21st to end	Night	41% 4 11% 26% 15% 2	5115
June	1st to 10th	Night	38% 7% 10% 26% 17% 2	1680
	11th to 20th	Night	36% 5% 13% 23% 21% 2	1680
	21st to end	Night	38% 6% 14% 26% 15% 1	1680
July	1st to 10th	Night	34% 5% 14% 25% 19% 2	3560
	11th to 20th	Night	31% 6% 13% 32% 18% 1	3552
	21st to end	Night	29% 6% 12% 30% 22% 1	4117
August	1st to 10th	Night	34% 4 15% 29% 16% 2	2904
	11th to 20th	Night	33% 4 17% 32% 13% 1	2902
	21st to end	Night	33% 6% 12% 34% 15% 1	3410
September	1st to 10th	Night	34% 5% 15% 30% 14% 1	2000
	11th to 20th	Night	29% 4 13% 35% 18%	2076
	21st to end	Night	42% 5% 11% 28% 13% 1	2265
October	1st to 10th	Night	32% 4 9% 30% 24% 1	3920
	11th to 20th	Night	41% 5% 14% 30% 10%	3899
	21st to end	Night	40% 5% 15% 28% 11% 1	4252
November	1st to 10th	Night	46% 4 13% 24% 13% 1	2745
	11th to 20th	Night	52% 5% 14% 18% 10% 1	2940
	21st to end	Night	56% 6% 18% 16% 4	2940
December	1st to 10th	Night	62% 5% 15% 14% 4	4720
	11th to 20th	Night	61% 7% 10% 13% 10%	5348
	21st to end	Night	70% 5% 12% 8% 5%	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	24% 20% 31% 19% 6%	220
	11th to 20th	Day	29% 22% 29% 15% 4 1	281
	21st to end	Day	31% 17% 34% 15% 3 1	363
February	1st to 10th	Day	28% 22% 32% 13% 4 1	330
	11th to 20th	Day	27% 23% 37% 12% 2	330
	21st to end	Day	31% 20% 35% 13% 1	317
March	1st to 10th	Day	27% 16% 33% 19% 5%	540
	11th to 20th	Day	29% 16% 32% 18% 5%	540
	21st to end	Day	31% 13% 30% 19% 6% 1	594
April	1st to 10th	Day	25% 16% 36% 19% 5% 1	540
	11th to 20th	Day	30% 13% 27% 19% 8% 2	532
	21st to end	Day	28% 13% 34% 16% 9% 2	540
May	1st to 10th	Day	23% 10% 32% 22% 8% 5%	543
	11th to 20th	Day	17% 9% 29% 28% 13% 3	567
	21st to end	Day	16% 9% 27% 28% 16% 5%	617
June	1st to 10th	Day	17% 8% 32% 28% 13% 2	570
	11th to 20th	Day	14% 8% 29% 29% 17% 2	570
	21st to end	Day	17% 6% 29% 30% 13% 4	570
July	1st to 10th	Day	10% 4 24% 35% 21% 6%	525
	11th to 20th	Day	14% 4 23% 33% 19% 8%	524
	21st to end	Day	8% 3 31% 36% 16% 6%	576
August	1st to 10th	Day	12% 4 22% 33% 18% 10%	526
	11th to 20th	Day	13% 3 21% 29% 26% 7%	519
	21st to end	Day	13% 6% 23% 32% 17% 10%	583
September	1st to 10th	Day	14% 7% 23% 28% 17% 10%	530
	11th to 20th	Day	11% 4 19% 28% 23% 15%	530
	21st to end	Day	16% 7% 29% 27% 15% 6%	530
October	1st to 10th	Day	20% 9% 25% 26% 15% 5%	530
	11th to 20th	Day	21% 14% 34% 22% 8% 1	530
	21st to end	Day	28% 12% 30% 22% 7% 1	583
November	1st to 10th	Day	26% 15% 27% 23% 8% 1	320
	11th to 20th	Day	23% 19% 35% 17% 6%	220
	21st to end	Day	28% 18% 36% 14% 4	220
December	1st to 10th	Day	26% 16% 33% 18% 5% 2	220
	11th to 20th	Day	31% 19% 30% 16% 5%	220
	21st to end	Day	31% 18% 34% 14% 2	242
January	1st to 10th	Night	23% 17% 37% 17% 5% 1	840
	11th to 20th	Night	28% 15% 31% 21% 3 1	840
	21st to end	Night	20% 13% 35% 26% 6%	924
February	1st to 10th	Night	21% 15% 38% 23% 3	840
	11th to 20th	Night	22% 15% 35% 22% 5%	823
	21st to end	Night	22% 14% 38% 21% 4	692
March	1st to 10th	Night	21% 13% 35% 26% 4 1	840
	11th to 20th	Night	23% 12% 35% 24% 5% 1	840
	21st to end	Night	22% 11% 31% 27% 9%	924
April	1st to 10th	Night	25% 11% 32% 27% 5%	840
	11th to 20th	Night	19% 11% 33% 27% 8% 1	801
	21st to end	Night	21% 7% 30% 32% 10%	630
May	1st to 10th	Night	23% 4 26% 29% 16% 2	604
	11th to 20th	Night	16% 5% 29% 37% 13%	414
	21st to end	Night	13% 4 23% 39% 20% 1	455
June	1st to 10th	Night	15% 4 25% 39% 17% 1	420
	11th to 20th	Night	11% 6% 25% 39% 18%	420
	21st to end	Night	12% 5% 23% 42% 17%	420
July	1st to 10th	Night	6% 3 18% 47% 24% 1	390
	11th to 20th	Night	12% 2 18% 41% 26% 1	388
	21st to end	Night	12% 1 16% 46% 22% 2	576
August	1st to 10th	Night	10% 3 13% 42% 30% 3	597
	11th to 20th	Night	9% 1 13% 41% 30% 5%	594
	21st to end	Night	11% 4 20% 37% 25% 3	769
September	1st to 10th	Night	11% 6% 19% 37% 23% 5%	800
	11th to 20th	Night	13% 4 17% 33% 28% 6%	800
	21st to end	Night	19% 7% 18% 32% 20% 4	800
October	1st to 10th	Night	18% 5% 26% 29% 21% 2	800
	11th to 20th	Night	24% 9% 28% 28% 9% 1	800
	21st to end	Night	27% 7% 29% 32% 5%	880
November	1st to 10th	Night	23% 10% 25% 30% 10% 1	820
	11th to 20th	Night	22% 13% 30% 26% 6% 2	840
	21st to end	Night	26% 12% 30% 25% 7%	840
December	1st to 10th	Night	28% 13% 32% 22% 5%	840
	11th to 20th	Night	24% 14% 36% 22% 4	840
	21st to end	Night	31% 11% 35% 20% 3	924

Page generated at 2026-06-28 02:10:44. Version ac07

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