Sunday Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunday Creek Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	15% 12% 4 10% 6% 5% 5% 4 5% 7% 28%	131061
	Night	14% 14% 5% 11% 6% 5% 5% 4 5% 6% 26%	130245
Transparency	Day	55% 6% 11% 19% 8% 1	134546
	Night	59% 6% 10% 17% 8% 1	134848
Seeing	Day	21% 13% 31% 23% 9% 3	16833
	Night	22% 10% 30% 28% 9% 1	26029

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	11% 20% 8% 17% 8% 8% 5% 3 5% 4 12%	4094
	11th to 20th	Day	8% 19% 7% 18% 9% 6% 5% 3 5% 5% 14%	4083
	21st to end	Day	7% 13% 7% 17% 8% 6% 6% 4 6% 6% 19%	5510
February	1st to 10th	Day	7% 16% 6% 13% 9% 7% 7% 4 5% 6% 19%	1890
	11th to 20th	Day	10% 18% 6% 10% 7% 6% 5% 4 5% 7% 22%	1843
	21st to end	Day	11% 18% 8% 11% 6% 5% 5% 2 4 5% 24%	1565
March	1st to 10th	Day	18% 15% 4 11% 6% 4 4 3 4 5% 25%	4852
	11th to 20th	Day	16% 16% 4 12% 7% 5% 4 3 4 5% 23%	4980
	21st to end	Day	14% 15% 6% 12% 5% 4 4 3 4 5% 28%	5465
April	1st to 10th	Day	18% 15% 4 10% 6% 4 4 4 5% 6% 25%	2371
	11th to 20th	Day	20% 12% 3 8% 4 3 3 4 5% 6% 31%	2572
	21st to end	Day	21% 14% 3 7% 4 3 3 3 4 5% 32%	2600
May	1st to 10th	Day	18% 11% 3 7% 5% 4 4 3 5% 5% 35%	6154
	11th to 20th	Day	13% 8% 3 8% 5% 5% 4 4 5% 7% 39%	6318
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 35%	7175
June	1st to 10th	Day	18% 9% 3 8% 6% 5% 5% 4 6% 7% 30%	3000
	11th to 20th	Day	13% 7% 3 8% 6% 6% 6% 4 6% 7% 35%	3000
	21st to end	Day	18% 8% 3 8% 6% 5% 5% 4 6% 8% 29%	3000
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 8% 11% 32%	6819
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 10% 36%	6820
	21st to end	Day	12% 7% 3 7% 4 5% 5% 4 7% 10% 37%	7130
August	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 6% 9% 35%	4234
	11th to 20th	Day	17% 7% 3 6% 5% 5% 6% 4 6% 7% 35%	4174
	21st to end	Day	13% 7% 3 7% 5% 4 5% 4 6% 7% 38%	4642
September	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 7% 34%	2400
	11th to 20th	Day	18% 7% 2 7% 4 4 4 3 6% 7% 37%	2275
	21st to end	Day	21% 9% 4 7% 5% 4 5% 3 4 7% 32%	2169
October	1st to 10th	Day	19% 11% 3 8% 4 4 5% 4 4 6% 31%	2000
	11th to 20th	Day	24% 16% 4 8% 5% 4 4 3 4 5% 22%	2000
	21st to end	Day	24% 17% 5% 10% 5% 4 5% 3 4 4 17%	2090
November	1st to 10th	Day	21% 15% 6% 9% 4 4 4 3 5% 7% 22%	1646
	11th to 20th	Day	18% 18% 6% 14% 8% 6% 5% 3 3 4 16%	1680
	21st to end	Day	21% 21% 8% 14% 8% 5% 3 3 4 3 10%	1680
December	1st to 10th	Day	13% 24% 10% 17% 8% 5% 3 3 4 4 10%	2586
	11th to 20th	Day	13% 19% 5% 15% 8% 5% 4 3 3 4 19%	2691
	21st to end	Day	18% 22% 7% 15% 7% 5% 4 3 4 5% 10%	3553
January	1st to 10th	Night	9% 18% 9% 19% 8% 7% 6% 3 4 4 13%	8831
	11th to 20th	Night	10% 19% 7% 15% 8% 6% 7% 5% 5% 4 14%	8832
	21st to end	Night	6% 13% 7% 15% 8% 9% 8% 5% 6% 5% 18%	8651
February	1st to 10th	Night	10% 15% 5% 12% 9% 7% 7% 5% 5% 6% 19%	2730
	11th to 20th	Night	10% 17% 6% 12% 8% 7% 5% 3 4 5% 22%	2663
	21st to end	Night	9% 16% 7% 13% 7% 6% 6% 4 4 6% 21%	2239
March	1st to 10th	Night	18% 16% 4 10% 5% 4 5% 3 4 4 27%	5032
	11th to 20th	Night	17% 17% 5% 13% 7% 5% 5% 3 4 5% 20%	4979
	21st to end	Night	16% 14% 6% 11% 5% 4 4 3 4 6% 27%	5467
April	1st to 10th	Night	18% 16% 5% 9% 5% 4 3 3 5% 6% 27%	2030
	11th to 20th	Night	19% 10% 3 9% 6% 5% 4 3 5% 5% 32%	1792
	21st to end	Night	19% 8% 4 8% 5% 4 4 4 4 6% 35%	1800
May	1st to 10th	Night	16% 10% 3 8% 4 5% 4 3 5% 5% 36%	3787
	11th to 20th	Night	13% 7% 2 7% 5% 4 4 3 5% 9% 42%	3575
	21st to end	Night	16% 9% 3 6% 4 4 4 4 6% 7% 36%	3707
June	1st to 10th	Night	18% 9% 1 7% 5% 5% 6% 4 7% 7% 32%	1400
	11th to 20th	Night	16% 7% 2 5% 4 4 4 2 6% 7% 44%	1400
	21st to end	Night	16% 7% 2 6% 6% 4 5% 4 6% 6% 38%	1400
July	1st to 10th	Night	16% 8% 3 6% 5% 4 5% 4 6% 6% 37%	3160
	11th to 20th	Night	12% 6% 3 6% 5% 4 5% 4 7% 7% 40%	3160
	21st to end	Night	11% 6% 2 5% 4 4 4 4 6% 8% 45%	3729
August	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 4 7% 10% 40%	2915
	11th to 20th	Night	14% 7% 2 6% 5% 4 5% 4 5% 8% 39%	2875
	21st to end	Night	14% 6% 2 7% 4 4 4 3 5% 6% 45%	3223
September	1st to 10th	Night	15% 7% 3 6% 5% 3 4 4 5% 7% 41%	2000
	11th to 20th	Night	16% 7% 2 6% 3 4 3 3 4 6% 44%	2121
	21st to end	Night	17% 9% 2 6% 3 4 4 4 5% 7% 38%	2230
October	1st to 10th	Night	19% 10% 3 8% 4 5% 4 3 4 7% 33%	2400
	11th to 20th	Night	23% 14% 4 8% 6% 4 5% 4 5% 5% 22%	2400
	21st to end	Night	21% 15% 4 9% 6% 5% 4 3 5% 6% 22%	2748
November	1st to 10th	Night	17% 15% 5% 9% 5% 5% 5% 4 4 6% 24%	2872
	11th to 20th	Night	19% 17% 6% 13% 7% 5% 5% 3 4 5% 17%	2940
	21st to end	Night	22% 20% 6% 12% 5% 5% 4 3 5% 5% 13%	2940
December	1st to 10th	Night	13% 22% 8% 17% 9% 6% 5% 3 3 4 10%	4814
	11th to 20th	Night	14% 18% 5% 13% 9% 6% 5% 4 4 4 16%	5736
	21st to end	Night	21% 20% 7% 15% 6% 4 3 2 4 4 16%	7667

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	76% 5% 8% 8% 3	4095
	11th to 20th	Day	72% 4 6% 11% 7%	4110
	21st to end	Day	64% 4 7% 17% 8%	5510
February	1st to 10th	Day	65% 5% 10% 13% 7%	1890
	11th to 20th	Day	62% 5% 10% 14% 8% 1	1852
	21st to end	Day	63% 3 7% 18% 9% 1	1565
March	1st to 10th	Day	61% 5% 8% 13% 12% 2	4855
	11th to 20th	Day	63% 4 11% 14% 7%	4980
	21st to end	Day	58% 4 9% 18% 9% 2	5465
April	1st to 10th	Day	59% 5% 9% 16% 10% 1	2371
	11th to 20th	Day	53% 5% 9% 21% 10% 1	2572
	21st to end	Day	55% 4 9% 17% 13% 2	2600
May	1st to 10th	Day	48% 6% 9% 18% 17% 3	6163
	11th to 20th	Day	44% 7% 14% 19% 12% 3	6322
	21st to end	Day	48% 6% 10% 20% 14% 2	7175
June	1st to 10th	Day	51% 7% 12% 21% 9% 1	3000
	11th to 20th	Day	46% 7% 11% 21% 14% 1	3000
	21st to end	Day	51% 8% 12% 19% 9% 1	3000
July	1st to 10th	Day	44% 7% 16% 28% 6%	6746
	11th to 20th	Day	40% 8% 19% 26% 7%	6739
	21st to end	Day	42% 6% 14% 31% 7%	6926
August	1st to 10th	Day	43% 8% 16% 27% 6%	4221
	11th to 20th	Day	45% 8% 14% 25% 6% 1	4213
	21st to end	Day	43% 7% 13% 29% 7% 2	4642
September	1st to 10th	Day	47% 8% 15% 22% 8%	2400
	11th to 20th	Day	45% 7% 16% 25% 6%	2275
	21st to end	Day	53% 7% 13% 20% 7%	2169
October	1st to 10th	Day	54% 5% 11% 23% 6%	3260
	11th to 20th	Day	58% 5% 11% 18% 8%	3240
	21st to end	Day	67% 5% 13% 9% 6%	3354
November	1st to 10th	Day	63% 6% 14% 12% 5%	1646
	11th to 20th	Day	73% 5% 8% 11% 3 1	1680
	21st to end	Day	79% 5% 8% 6% 2	1680
December	1st to 10th	Day	79% 4 8% 6% 3	2586
	11th to 20th	Day	69% 4 9% 16% 2	2691
	21st to end	Day	78% 4 5% 9% 4	3553
January	1st to 10th	Night	74% 5% 8% 9% 2	8831
	11th to 20th	Night	72% 5% 7% 10% 6%	8880
	21st to end	Night	66% 5% 6% 16% 7%	8651
February	1st to 10th	Night	63% 6% 10% 13% 8% 1	2730
	11th to 20th	Night	64% 5% 11% 14% 6%	2665
	21st to end	Night	63% 5% 9% 15% 8% 1	2239
March	1st to 10th	Night	61% 3 9% 13% 9% 4	5042
	11th to 20th	Night	67% 5% 9% 12% 6% 1	4980
	21st to end	Night	58% 4 8% 15% 10% 3	5467
April	1st to 10th	Night	57% 5% 8% 17% 12% 1	2030
	11th to 20th	Night	53% 4 9% 18% 14% 2	1792
	21st to end	Night	50% 6% 10% 14% 18% 2	1800
May	1st to 10th	Night	48% 6% 8% 19% 17% 1	3794
	11th to 20th	Night	39% 8% 15% 22% 14% 3	3576
	21st to end	Night	45% 6% 12% 20% 15% 3	3708
June	1st to 10th	Night	48% 9% 10% 21% 11% 2	1400
	11th to 20th	Night	40% 5% 12% 25% 15% 2	1400
	21st to end	Night	45% 6% 14% 20% 13% 1	1400
July	1st to 10th	Night	45% 8% 11% 27% 10%	3125
	11th to 20th	Night	40% 7% 14% 30% 8%	3118
	21st to end	Night	36% 6% 16% 32% 9%	3619
August	1st to 10th	Night	37% 7% 16% 32% 8%	2904
	11th to 20th	Night	41% 8% 16% 30% 5%	2902
	21st to end	Night	40% 6% 14% 28% 10% 1	3223
September	1st to 10th	Night	42% 8% 17% 23% 8% 2	2000
	11th to 20th	Night	41% 6% 17% 28% 7%	2121
	21st to end	Night	44% 7% 16% 24% 8%	2230
October	1st to 10th	Night	49% 6% 12% 24% 9%	3920
	11th to 20th	Night	57% 6% 12% 19% 6%	3899
	21st to end	Night	59% 6% 16% 13% 5%	4434
November	1st to 10th	Night	60% 7% 13% 15% 4	2872
	11th to 20th	Night	69% 7% 10% 10% 3	2940
	21st to end	Night	74% 7% 10% 7% 2	2940
December	1st to 10th	Night	79% 5% 6% 8% 2	4814
	11th to 20th	Night	70% 5% 8% 11% 5%	5735
	21st to end	Night	74% 3 6% 12% 5%	7667

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	22% 15% 35% 19% 7% 1	220
	11th to 20th	Day	18% 13% 34% 26% 7% 2	220
	21st to end	Day	17% 12% 35% 26% 9% 1	355
February	1st to 10th	Day	21% 19% 37% 17% 5% 1	330
	11th to 20th	Day	19% 20% 38% 19% 3 1	330
	21st to end	Day	23% 20% 41% 15% 1	273
March	1st to 10th	Day	19% 16% 40% 20% 5%	454
	11th to 20th	Day	22% 16% 34% 22% 6% 1	500
	21st to end	Day	25% 16% 31% 22% 5% 1	550
April	1st to 10th	Day	23% 17% 28% 21% 9% 2	500
	11th to 20th	Day	30% 13% 24% 22% 7% 4	492
	21st to end	Day	30% 17% 26% 17% 8% 3	500
May	1st to 10th	Day	24% 11% 33% 19% 11% 2	503
	11th to 20th	Day	21% 13% 31% 22% 10% 3	527
	21st to end	Day	22% 11% 31% 26% 9% 1	669
June	1st to 10th	Day	18% 13% 35% 24% 8% 2	730
	11th to 20th	Day	17% 14% 31% 27% 10% 2	730
	21st to end	Day	19% 10% 28% 29% 11% 3	730
July	1st to 10th	Day	13% 11% 32% 28% 12% 3	721
	11th to 20th	Day	14% 11% 31% 27% 13% 3	718
	21st to end	Day	14% 9% 28% 30% 14% 5%	684
August	1st to 10th	Day	13% 8% 29% 26% 17% 6%	526
	11th to 20th	Day	15% 7% 24% 27% 18% 9%	519
	21st to end	Day	18% 10% 29% 22% 12% 10%	583
September	1st to 10th	Day	18% 9% 30% 25% 15% 4	530
	11th to 20th	Day	18% 9% 24% 31% 13% 5%	530
	21st to end	Day	23% 12% 23% 24% 15% 2	530
October	1st to 10th	Day	26% 12% 28% 22% 10% 2	530
	11th to 20th	Day	30% 16% 31% 18% 5% 1	530
	21st to end	Day	31% 15% 30% 19% 4 1	477
November	1st to 10th	Day	35% 15% 35% 13% 2	220
	11th to 20th	Day	24% 21% 30% 20% 5%	220
	21st to end	Day	37% 20% 28% 10% 4 1	220
December	1st to 10th	Day	21% 18% 34% 19% 8% 1	220
	11th to 20th	Day	26% 15% 36% 15% 6% 2	220
	21st to end	Day	27% 14% 36% 16% 5% 2	242
January	1st to 10th	Night	22% 10% 35% 26% 6% 1	1050
	11th to 20th	Night	24% 11% 31% 26% 8% 1	1050
	21st to end	Night	18% 8% 28% 33% 11% 1	1007
February	1st to 10th	Night	18% 12% 33% 29% 8%	840
	11th to 20th	Night	19% 11% 35% 29% 5%	823
	21st to end	Night	18% 14% 36% 25% 7%	692
March	1st to 10th	Night	20% 12% 36% 26% 5% 1	809
	11th to 20th	Night	22% 15% 34% 22% 7%	800
	21st to end	Night	19% 12% 33% 29% 7% 1	880
April	1st to 10th	Night	26% 12% 29% 27% 6% 1	800
	11th to 20th	Night	25% 9% 30% 27% 9% 1	608
	21st to end	Night	24% 9% 32% 27% 7% 1	600
May	1st to 10th	Night	19% 8% 30% 36% 7%	490
	11th to 20th	Night	20% 7% 31% 32% 9% 1	394
	21st to end	Night	19% 9% 26% 38% 7%	433
June	1st to 10th	Night	22% 8% 28% 34% 8%	400
	11th to 20th	Night	18% 7% 31% 34% 11% 1	400
	21st to end	Night	19% 6% 27% 36% 12% 1	400
July	1st to 10th	Night	14% 5% 34% 38% 10%	390
	11th to 20th	Night	15% 5% 27% 39% 13% 1	388
	21st to end	Night	12% 4 26% 41% 15% 1	444
August	1st to 10th	Night	14% 4 25% 37% 18% 2	597
	11th to 20th	Night	15% 4 23% 36% 21% 2	594
	21st to end	Night	18% 6% 26% 31% 17% 2	660
September	1st to 10th	Night	20% 6% 29% 29% 15% 1	800
	11th to 20th	Night	18% 7% 25% 30% 19% 1	800
	21st to end	Night	22% 7% 23% 32% 15% 2	800
October	1st to 10th	Night	26% 9% 28% 28% 9% 1	800
	11th to 20th	Night	29% 12% 31% 20% 7% 1	800
	21st to end	Night	30% 9% 28% 27% 6% 1	880
November	1st to 10th	Night	25% 10% 35% 24% 5% 1	820
	11th to 20th	Night	27% 12% 31% 23% 6%	840
	21st to end	Night	31% 12% 29% 23% 4	840
December	1st to 10th	Night	26% 10% 32% 24% 7% 1	895
	11th to 20th	Night	27% 10% 33% 23% 6% 1	1050
	21st to end	Night	30% 12% 26% 23% 8% 1	1155

Page generated at 2026-02-28 01:40:21. Version ac07

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