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 2024-12-02.

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	16% 12% 4 9% 6% 5% 5% 4 5% 7% 29%	101995
	Night	15% 13% 4 10% 6% 5% 5% 3 5% 6% 27%	97711
Transparency	Day	54% 6% 12% 19% 8% 1	105501
	Night	57% 6% 12% 17% 7% 1	102294
Seeing	Day	21% 13% 30% 23% 10% 3	15300
	Night	23% 9% 30% 29% 9% 1	24342

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% 20% 7% 15% 6% 8% 5% 3 5% 5% 12%	2208
	11th to 20th	Day	10% 14% 5% 18% 11% 6% 6% 3 4 6% 17%	2210
	21st to end	Day	13% 20% 9% 15% 8% 5% 4 3 4 5% 14%	2972
February	1st to 10th	Day	7% 17% 6% 13% 9% 7% 7% 4 5% 6% 19%	1710
	11th to 20th	Day	11% 18% 5% 10% 7% 6% 5% 4 5% 7% 22%	1663
	21st to end	Day	11% 18% 8% 11% 6% 5% 5% 3 4 5% 24%	1420
March	1st to 10th	Day	18% 14% 4 10% 6% 4 4 3 4 5% 27%	3360
	11th to 20th	Day	10% 18% 5% 16% 6% 4 4 3 3 4 25%	3480
	21st to end	Day	13% 14% 6% 11% 6% 5% 5% 3 3 5% 29%	3815
April	1st to 10th	Day	19% 16% 4 10% 6% 4 4 4 4 6% 25%	2252
	11th to 20th	Day	21% 12% 3 8% 5% 3 3 4 5% 6% 30%	2442
	21st to end	Day	21% 14% 3 7% 4 3 3 3 4 5% 33%	2470
May	1st to 10th	Day	18% 10% 3 7% 5% 5% 4 4 6% 6% 31%	4290
	11th to 20th	Day	15% 9% 3 8% 4 5% 4 4 5% 6% 36%	4398
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 5% 7% 37%	4993
June	1st to 10th	Day	18% 9% 3 8% 6% 5% 5% 4 6% 7% 30%	2850
	11th to 20th	Day	13% 7% 3 7% 6% 6% 5% 4 6% 7% 36%	2850
	21st to end	Day	18% 8% 3 8% 6% 5% 5% 4 6% 8% 29%	2850
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 31%	4760
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	4760
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 6% 9% 38%	4962
August	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 6% 9% 35%	4104
	11th to 20th	Day	17% 7% 3 6% 4 5% 5% 4 6% 7% 35%	4044
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 6% 7% 39%	4500
September	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	2280
	11th to 20th	Day	19% 7% 2 7% 4 4 4 4 7% 6% 36%	2161
	21st to end	Day	22% 9% 4 7% 5% 4 5% 3 4 7% 31%	2061
October	1st to 10th	Day	20% 11% 3 8% 4 4 5% 3 4 6% 30%	1900
	11th to 20th	Day	25% 16% 4 8% 5% 4 5% 3 4 5% 21%	1900
	21st to end	Day	25% 17% 5% 10% 5% 4 5% 3 5% 4 17%	1986
November	1st to 10th	Day	21% 15% 6% 9% 4 4 4 3 5% 7% 21%	1566
	11th to 20th	Day	18% 17% 5% 13% 8% 7% 6% 3 3 4 16%	1600
	21st to end	Day	21% 20% 8% 14% 9% 5% 4 3 4 3 10%	1600
December	1st to 10th	Day	15% 20% 8% 13% 7% 4 4 4 5% 6% 15%	1476
	11th to 20th	Day	16% 18% 6% 12% 7% 5% 4 4 4 5% 19%	1671
	21st to end	Day	16% 23% 8% 18% 8% 4 3 3 3 4 11%	2431
January	1st to 10th	Night	13% 17% 7% 15% 7% 7% 6% 4 5% 5% 14%	4748
	11th to 20th	Night	10% 16% 5% 14% 9% 7% 7% 4 5% 6% 17%	4760
	21st to end	Night	11% 20% 9% 14% 8% 7% 5% 3 4 4 14%	4684
February	1st to 10th	Night	10% 15% 5% 11% 9% 7% 7% 5% 5% 6% 19%	2470
	11th to 20th	Night	11% 17% 6% 11% 8% 7% 4 4 4 5% 23%	2403
	21st to end	Night	10% 16% 7% 13% 6% 6% 6% 4 4 6% 22%	2032
March	1st to 10th	Night	17% 15% 3 8% 5% 4 5% 3 4 5% 31%	3479
	11th to 20th	Night	14% 17% 5% 14% 7% 5% 5% 3 4 5% 23%	3479
	21st to end	Night	14% 11% 6% 13% 5% 5% 4 3 4 5% 30%	3817
April	1st to 10th	Night	18% 16% 4 8% 5% 4 3 3 5% 6% 27%	1929
	11th to 20th	Night	19% 10% 3 9% 6% 5% 4 3 5% 5% 32%	1701
	21st to end	Night	19% 8% 4 7% 5% 4 4 4 4 6% 36%	1710
May	1st to 10th	Night	18% 10% 4 8% 5% 5% 5% 3 5% 5% 32%	2651
	11th to 20th	Night	14% 8% 2 7% 5% 4 5% 3 6% 9% 36%	2495
	21st to end	Night	14% 8% 3 6% 4 4 5% 4 6% 8% 38%	2584
June	1st to 10th	Night	18% 9% 1 7% 5% 5% 5% 4 7% 7% 32%	1330
	11th to 20th	Night	17% 6% 2 5% 4 4 4 2 6% 7% 43%	1330
	21st to end	Night	16% 7% 2 6% 6% 5% 5% 4 6% 6% 38%	1330
July	1st to 10th	Night	15% 7% 3 5% 5% 4 5% 4 6% 6% 40%	2210
	11th to 20th	Night	12% 7% 3 6% 6% 5% 6% 4 7% 8% 37%	2210
	21st to end	Night	14% 6% 2 5% 4 5% 4 4 6% 8% 43%	2593
August	1st to 10th	Night	9% 5% 2 7% 5% 6% 6% 4 7% 10% 40%	2825
	11th to 20th	Night	15% 7% 2 6% 5% 4 5% 4 5% 8% 39%	2785
	21st to end	Night	14% 5% 2 7% 4 4 4 3 5% 6% 45%	3124
September	1st to 10th	Night	15% 7% 3 6% 4 3 4 4 5% 7% 42%	1900
	11th to 20th	Night	17% 7% 2 7% 3 5% 3 3 4 6% 43%	2015
	21st to end	Night	18% 9% 2 6% 3 4 4 4 5% 8% 38%	2118
October	1st to 10th	Night	20% 10% 3 8% 4 4 3 4 4 6% 32%	2280
	11th to 20th	Night	24% 14% 4 8% 6% 4 5% 4 4 5% 22%	2280
	21st to end	Night	22% 15% 4 9% 6% 5% 4 3 5% 6% 21%	2610
November	1st to 10th	Night	17% 15% 5% 8% 5% 5% 5% 4 4 6% 25%	2732
	11th to 20th	Night	19% 16% 5% 12% 7% 5% 5% 3 4 5% 18%	2800
	21st to end	Night	22% 20% 6% 11% 6% 5% 4 3 5% 5% 14%	2800
December	1st to 10th	Night	17% 19% 6% 14% 8% 6% 5% 3 4 5% 13%	2735
	11th to 20th	Night	17% 18% 5% 12% 8% 6% 5% 4 4 5% 16%	3526
	21st to end	Night	19% 21% 7% 17% 7% 4 3 2 3 4 12%	5236

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	73% 7% 11% 7% 2	2209
	11th to 20th	Day	68% 6% 9% 14% 4	2210
	21st to end	Day	72% 6% 8% 10% 4	2972
February	1st to 10th	Day	64% 5% 11% 13% 7%	1710
	11th to 20th	Day	61% 5% 10% 14% 8% 1	1672
	21st to end	Day	63% 3 8% 17% 9% 1	1420
March	1st to 10th	Day	59% 7% 9% 18% 6% 1	3363
	11th to 20th	Day	62% 5% 9% 17% 7%	3480
	21st to end	Day	56% 5% 9% 17% 10% 2	3815
April	1st to 10th	Day	59% 6% 9% 16% 9% 1	2252
	11th to 20th	Day	53% 5% 10% 21% 9% 1	2442
	21st to end	Day	54% 4 9% 17% 13% 2	2470
May	1st to 10th	Day	49% 8% 11% 20% 10% 1	4302
	11th to 20th	Day	46% 8% 14% 22% 10% 1	4402
	21st to end	Day	45% 7% 12% 18% 16% 2	4993
June	1st to 10th	Day	52% 7% 12% 20% 8% 1	2850
	11th to 20th	Day	46% 7% 12% 21% 14% 1	2850
	21st to end	Day	51% 8% 11% 19% 9% 1	2850
July	1st to 10th	Day	47% 7% 16% 24% 6%	4686
	11th to 20th	Day	40% 7% 18% 27% 9%	4679
	21st to end	Day	42% 6% 12% 32% 7%	4804
August	1st to 10th	Day	44% 8% 15% 27% 6%	4091
	11th to 20th	Day	46% 8% 14% 25% 6% 1	4083
	21st to end	Day	43% 7% 13% 29% 7% 2	4500
September	1st to 10th	Day	47% 8% 15% 23% 8%	2280
	11th to 20th	Day	47% 8% 16% 24% 6%	2161
	21st to end	Day	54% 7% 14% 19% 7%	2061
October	1st to 10th	Day	55% 6% 11% 22% 6%	3160
	11th to 20th	Day	58% 5% 11% 18% 7%	3140
	21st to end	Day	67% 5% 14% 8% 6%	3250
November	1st to 10th	Day	63% 6% 14% 11% 5%	1566
	11th to 20th	Day	72% 5% 8% 11% 3 1	1600
	21st to end	Day	79% 5% 8% 6% 2	1600
December	1st to 10th	Day	71% 5% 11% 10% 4	1476
	11th to 20th	Day	66% 6% 10% 17% 2	1671
	21st to end	Day	80% 5% 6% 8% 2	2431
January	1st to 10th	Night	69% 8% 13% 9% 2	4748
	11th to 20th	Night	67% 7% 8% 14% 4	4760
	21st to end	Night	73% 6% 6% 10% 4	4684
February	1st to 10th	Night	63% 6% 10% 13% 7%	2470
	11th to 20th	Night	63% 5% 12% 15% 6%	2405
	21st to end	Night	63% 5% 9% 14% 8% 1	2032
March	1st to 10th	Night	55% 5% 12% 16% 10% 2	3489
	11th to 20th	Night	63% 6% 9% 14% 7% 1	3480
	21st to end	Night	55% 6% 9% 14% 12% 3	3817
April	1st to 10th	Night	57% 5% 8% 17% 12% 1	1929
	11th to 20th	Night	54% 4 9% 18% 13% 2	1701
	21st to end	Night	48% 6% 11% 15% 18% 2	1710
May	1st to 10th	Night	49% 8% 10% 19% 12% 1	2658
	11th to 20th	Night	41% 9% 15% 23% 11% 1	2496
	21st to end	Night	41% 7% 14% 20% 14% 3	2585
June	1st to 10th	Night	48% 9% 10% 20% 11% 2	1330
	11th to 20th	Night	40% 5% 13% 25% 15% 2	1330
	21st to end	Night	44% 7% 14% 20% 13% 1	1330
July	1st to 10th	Night	42% 8% 13% 26% 10%	2175
	11th to 20th	Night	42% 8% 16% 26% 8%	2168
	21st to end	Night	40% 7% 14% 27% 12%	2511
August	1st to 10th	Night	38% 7% 16% 32% 8%	2814
	11th to 20th	Night	41% 8% 16% 30% 5%	2812
	21st to end	Night	40% 6% 14% 28% 10% 1	3124
September	1st to 10th	Night	41% 8% 18% 23% 9% 2	1900
	11th to 20th	Night	42% 6% 18% 26% 7%	2015
	21st to end	Night	45% 7% 16% 24% 7%	2118
October	1st to 10th	Night	49% 6% 11% 24% 9%	3800
	11th to 20th	Night	56% 6% 12% 20% 6%	3779
	21st to end	Night	60% 6% 16% 13% 5%	4296
November	1st to 10th	Night	59% 7% 14% 15% 4	2732
	11th to 20th	Night	69% 7% 11% 10% 3	2800
	21st to end	Night	74% 7% 10% 7% 2	2800
December	1st to 10th	Night	74% 6% 8% 9% 2	2735
	11th to 20th	Night	71% 5% 9% 12% 3	3525
	21st to end	Night	78% 4 6% 8% 3	5236

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	23% 16% 30% 20% 9% 2	180
	11th to 20th	Day	17% 14% 32% 27% 7% 2	180
	21st to end	Day	21% 8% 34% 28% 9% 1	290
February	1st to 10th	Day	19% 21% 38% 17% 4 1	270
	11th to 20th	Day	18% 18% 39% 20% 3 1	270
	21st to end	Day	24% 21% 42% 12%	225
March	1st to 10th	Day	18% 15% 41% 20% 6%	410
	11th to 20th	Day	19% 16% 35% 23% 6% 1	460
	21st to end	Day	25% 15% 33% 22% 5% 1	506
April	1st to 10th	Day	23% 16% 29% 21% 9% 1	460
	11th to 20th	Day	30% 13% 25% 22% 7% 3	452
	21st to end	Day	28% 17% 27% 17% 9% 3	460
May	1st to 10th	Day	24% 11% 32% 19% 11% 2	463
	11th to 20th	Day	22% 13% 30% 22% 10% 3	487
	21st to end	Day	20% 11% 33% 25% 9% 1	620
June	1st to 10th	Day	19% 13% 34% 25% 8% 1	680
	11th to 20th	Day	16% 14% 31% 26% 11% 2	680
	21st to end	Day	19% 9% 27% 30% 12% 3	680
July	1st to 10th	Day	13% 11% 32% 29% 13% 3	671
	11th to 20th	Day	13% 10% 31% 28% 14% 3	668
	21st to end	Day	15% 8% 27% 30% 16% 5%	635
August	1st to 10th	Day	14% 8% 27% 27% 18% 6%	486
	11th to 20th	Day	16% 7% 22% 28% 17% 9%	479
	21st to end	Day	18% 10% 28% 22% 12% 11%	539
September	1st to 10th	Day	18% 9% 29% 25% 15% 4	490
	11th to 20th	Day	19% 9% 24% 30% 13% 4	490
	21st to end	Day	24% 11% 23% 23% 15% 2	490
October	1st to 10th	Day	27% 12% 26% 22% 10% 2	490
	11th to 20th	Day	30% 16% 30% 18% 5% 1	490
	21st to end	Day	32% 16% 31% 17% 3 1	439
November	1st to 10th	Day	35% 15% 35% 13% 2	200
	11th to 20th	Day	24% 21% 30% 22% 5% 1	200
	21st to end	Day	35% 20% 29% 11% 5% 2	200
December	1st to 10th	Day	21% 18% 32% 20% 8% 1	182
	11th to 20th	Day	26% 15% 36% 15% 7% 2	180
	21st to end	Day	25% 15% 35% 18% 5% 3	198
January	1st to 10th	Night	22% 10% 34% 27% 6% 1	950
	11th to 20th	Night	24% 10% 31% 26% 7% 1	950
	21st to end	Night	19% 8% 26% 33% 12% 2	912
February	1st to 10th	Night	19% 11% 33% 30% 7%	760
	11th to 20th	Night	20% 10% 35% 29% 6% 1	743
	21st to end	Night	19% 14% 37% 24% 7%	628
March	1st to 10th	Night	20% 12% 36% 26% 6% 1	760
	11th to 20th	Night	22% 14% 34% 23% 7%	760
	21st to end	Night	18% 12% 33% 30% 6% 1	836
April	1st to 10th	Night	26% 12% 29% 27% 5% 1	760
	11th to 20th	Night	25% 9% 30% 28% 8% 1	577
	21st to end	Night	24% 9% 31% 28% 8% 1	570
May	1st to 10th	Night	20% 8% 29% 36% 7%	466
	11th to 20th	Night	21% 7% 30% 32% 9% 1	374
	21st to end	Night	19% 9% 26% 39% 6%	411
June	1st to 10th	Night	23% 8% 28% 34% 8%	380
	11th to 20th	Night	19% 6% 30% 33% 11% 1	380
	21st to end	Night	18% 6% 26% 37% 12% 1	380
July	1st to 10th	Night	14% 4 34% 39% 10%	370
	11th to 20th	Night	14% 5% 26% 40% 14% 1	368
	21st to end	Night	13% 4 24% 42% 16% 1	421
August	1st to 10th	Night	14% 4 25% 37% 18% 2	567
	11th to 20th	Night	16% 3 22% 35% 22% 2	564
	21st to end	Night	18% 6% 26% 31% 17% 2	627
September	1st to 10th	Night	20% 6% 28% 29% 15% 1	760
	11th to 20th	Night	19% 7% 26% 30% 18% 1	760
	21st to end	Night	23% 7% 23% 32% 14% 2	760
October	1st to 10th	Night	26% 9% 27% 28% 9% 1	760
	11th to 20th	Night	29% 11% 31% 21% 7% 1	760
	21st to end	Night	31% 9% 29% 26% 5% 1	836
November	1st to 10th	Night	25% 10% 35% 24% 5% 1	780
	11th to 20th	Night	27% 12% 32% 22% 7% 1	800
	21st to end	Night	32% 11% 30% 23% 4	800
December	1st to 10th	Night	28% 9% 31% 24% 6% 1	817
	11th to 20th	Night	27% 10% 32% 23% 6% 1	950
	21st to end	Night	30% 11% 26% 23% 9% 1	1045

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