Garden Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Garden Bay 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	20% 9% 3 7% 4 4 4 3 5% 6% 34%	134693
	Night	25% 10% 3 8% 5% 4 4 3 4 5% 29%	135174
Transparency	Day	51% 8% 15% 18% 7% 1	134393
	Night	57% 7% 13% 16% 6% 1	135052
Seeing	Day	24% 5% 18% 26% 21% 6%	17152
	Night	31% 5% 13% 24% 22% 6%	25384

By Month

Percentage Cloud Cover NW (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	37% 16% 4 8% 4 4 4 3 4 4 12%	4125
	11th to 20th	Day	20% 11% 4 8% 3 4 4 3 6% 6% 31%	4119
	21st to end	Day	36% 11% 4 9% 4 4 3 2 3 3 19%	5023
February	1st to 10th	Day	29% 15% 4 8% 6% 4 4 4 4 7% 14%	1890
	11th to 20th	Day	26% 12% 4 9% 5% 4 4 3 5% 6% 22%	1845
	21st to end	Day	23% 13% 4 8% 6% 7% 5% 3 5% 5% 20%	1557
March	1st to 10th	Day	22% 14% 5% 11% 5% 5% 5% 4 5% 6% 18%	4739
	11th to 20th	Day	20% 14% 4 9% 5% 5% 5% 3 5% 6% 24%	5000
	21st to end	Day	24% 13% 4 11% 6% 5% 5% 4 5% 6% 17%	5546
April	1st to 10th	Day	24% 12% 3 9% 6% 4 5% 5% 6% 6% 21%	2400
	11th to 20th	Day	20% 10% 3 7% 6% 5% 4 3 5% 6% 29%	2534
	21st to end	Day	22% 10% 4 7% 5% 4 4 3 5% 6% 30%	2625
May	1st to 10th	Day	13% 7% 3 7% 5% 4 4 3 4 6% 43%	6320
	11th to 20th	Day	18% 8% 3 7% 5% 5% 4 4 6% 10% 30%	6277
	21st to end	Day	21% 9% 3 6% 5% 5% 5% 4 5% 7% 30%	7266
June	1st to 10th	Day	20% 9% 2 7% 5% 4 4 4 6% 7% 31%	3000
	11th to 20th	Day	16% 9% 3 8% 6% 5% 6% 4 6% 8% 29%	3000
	21st to end	Day	15% 8% 2 7% 6% 4 4 3 5% 7% 38%	3000
July	1st to 10th	Day	8% 3 1 4 3 3 3 3 4 6% 61%	6780
	11th to 20th	Day	5% 2 1 3 2 2 2 2 4 6% 71%	6780
	21st to end	Day	7% 4 2 4 3 3 3 2 4 6% 61%	7201
August	1st to 10th	Day	7% 3 1 3 2 3 2 2 4 6% 67%	4430
	11th to 20th	Day	11% 8% 3 7% 4 4 4 4 6% 8% 41%	4156
	21st to end	Day	15% 6% 2 7% 4 4 5% 4 5% 7% 41%	4636
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 48%	2400
	11th to 20th	Day	17% 7% 3 7% 5% 4 5% 3 5% 7% 38%	2343
	21st to end	Day	22% 7% 3 8% 4 5% 5% 3 5% 7% 31%	2140
October	1st to 10th	Day	23% 8% 3 9% 5% 5% 5% 4 5% 7% 26%	3280
	11th to 20th	Day	31% 9% 3 8% 5% 5% 5% 3 4 5% 21%	3261
	21st to end	Day	27% 12% 4 10% 6% 5% 4 4 5% 7% 17%	3245
November	1st to 10th	Day	35% 10% 3 9% 5% 5% 4 3 5% 5% 17%	1648
	11th to 20th	Day	32% 13% 4 9% 5% 4 4 3 4 5% 16%	1680
	21st to end	Day	35% 13% 4 9% 5% 4 3 3 4 4 15%	1680
December	1st to 10th	Day	24% 13% 5% 9% 5% 5% 4 4 5% 8% 20%	2477
	11th to 20th	Day	43% 15% 3 9% 4 3 3 3 4 3 9%	2715
	21st to end	Day	37% 15% 5% 10% 5% 4 3 2 3 4 11%	3575
January	1st to 10th	Night	36% 14% 4 10% 5% 4 4 3 3 4 13%	8770
	11th to 20th	Night	24% 8% 3 6% 4 3 4 3 5% 6% 34%	8779
	21st to end	Night	34% 10% 3 7% 4 4 4 3 3 4 24%	9150
February	1st to 10th	Night	30% 15% 5% 7% 5% 4 3 3 4 5% 20%	2730
	11th to 20th	Night	24% 14% 4 8% 5% 4 4 3 4 5% 26%	2662
	21st to end	Night	28% 14% 3 7% 5% 4 4 3 4 5% 22%	2249
March	1st to 10th	Night	24% 14% 4 9% 5% 5% 4 3 4 5% 23%	5194
	11th to 20th	Night	21% 13% 4 9% 5% 5% 4 3 4 5% 27%	4999
	21st to end	Night	29% 14% 4 11% 6% 4 4 3 4 4 17%	5416
April	1st to 10th	Night	23% 12% 3 8% 7% 6% 6% 3 5% 6% 22%	2000
	11th to 20th	Night	19% 10% 3 9% 7% 4 5% 4 5% 7% 28%	1814
	21st to end	Night	24% 10% 3 6% 5% 3 4 4 5% 6% 30%	1775
May	1st to 10th	Night	17% 9% 3 6% 3 3 3 2 4 6% 44%	3640
	11th to 20th	Night	20% 7% 3 7% 5% 4 4 4 6% 9% 31%	3616
	21st to end	Night	21% 12% 4 11% 7% 5% 4 3 4 5% 25%	3606
June	1st to 10th	Night	19% 10% 3 6% 6% 5% 5% 4 5% 5% 33%	1400
	11th to 20th	Night	18% 9% 3 8% 6% 4 4 4 6% 7% 30%	1400
	21st to end	Night	16% 8% 3 6% 4 4 4 3 5% 7% 40%	1400
July	1st to 10th	Night	8% 4 2 5% 4 3 3 3 5% 6% 57%	3180
	11th to 20th	Night	3 2 1 2 2 2 3 2 4 5% 73%	3180
	21st to end	Night	7% 5% 2 5% 3 3 3 3 4 8% 58%	3640
August	1st to 10th	Night	4 2 1 2 2 2 3 2 4 5% 72%	2707
	11th to 20th	Night	14% 5% 2 6% 4 4 5% 4 5% 8% 44%	2902
	21st to end	Night	15% 6% 3 7% 4 4 3 3 4 6% 46%	3273
September	1st to 10th	Night	8% 4 2 6% 4 4 3 3 4 7% 56%	2000
	11th to 20th	Night	18% 7% 2 5% 4 4 3 3 5% 6% 43%	2055
	21st to end	Night	23% 7% 3 6% 3 3 3 3 4 6% 39%	2259
October	1st to 10th	Night	25% 9% 3 7% 5% 5% 5% 3 4 6% 28%	3920
	11th to 20th	Night	31% 10% 3 7% 4 3 3 3 4 6% 27%	3897
	21st to end	Night	28% 10% 3 8% 5% 5% 4 3 4 5% 25%	4556
November	1st to 10th	Night	36% 10% 3 8% 5% 4 4 3 4 5% 17%	2870
	11th to 20th	Night	34% 12% 3 9% 5% 4 4 3 3 4 19%	2940
	21st to end	Night	33% 11% 4 8% 5% 4 4 3 4 4 19%	2940
December	1st to 10th	Night	27% 11% 3 8% 6% 5% 5% 5% 5% 5% 20%	4922
	11th to 20th	Night	41% 15% 4 9% 4 4 3 2 3 3 12%	5710
	21st to end	Night	35% 15% 4 10% 5% 5% 4 3 3 4 13%	7623

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	75% 6% 9% 8% 1	4125
	11th to 20th	Day	52% 10% 11% 18% 8%	4140
	21st to end	Day	70% 7% 6% 15% 2	5024
February	1st to 10th	Day	71% 7% 8% 8% 4 2	1890
	11th to 20th	Day	64% 7% 10% 12% 5% 1	1854
	21st to end	Day	64% 8% 12% 10% 5% 2	1557
March	1st to 10th	Day	65% 9% 10% 12% 5%	4739
	11th to 20th	Day	61% 7% 10% 18% 3	5000
	21st to end	Day	67% 10% 14% 8% 2	5546
April	1st to 10th	Day	62% 8% 14% 11% 4	2400
	11th to 20th	Day	56% 7% 11% 16% 10% 1	2534
	21st to end	Day	56% 7% 13% 17% 8% 1	2625
May	1st to 10th	Day	42% 6% 16% 24% 11% 1	6320
	11th to 20th	Day	47% 10% 19% 20% 3	6280
	21st to end	Day	53% 9% 19% 15% 5%	7267
June	1st to 10th	Day	51% 8% 17% 16% 5% 2	3000
	11th to 20th	Day	52% 9% 18% 16% 4 1	3000
	21st to end	Day	46% 7% 19% 18% 8% 1	3000
July	1st to 10th	Day	26% 7% 19% 24% 18% 6%	6706
	11th to 20th	Day	17% 6% 20% 36% 18% 2	6699
	21st to end	Day	24% 7% 29% 27% 10% 2	6997
August	1st to 10th	Day	19% 6% 15% 47% 12% 1	4413
	11th to 20th	Day	42% 11% 21% 17% 8% 1	4195
	21st to end	Day	39% 12% 19% 22% 8% 1	4635
September	1st to 10th	Day	35% 10% 17% 23% 12% 2	2400
	11th to 20th	Day	46% 10% 16% 16% 11% 1	2343
	21st to end	Day	52% 9% 16% 15% 8% 1	2140
October	1st to 10th	Day	56% 9% 16% 14% 5%	3280
	11th to 20th	Day	65% 6% 12% 12% 4	3261
	21st to end	Day	66% 10% 12% 9% 3	3245
November	1st to 10th	Day	71% 7% 9% 8% 4 1	1648
	11th to 20th	Day	72% 6% 9% 9% 3 1	1680
	21st to end	Day	72% 6% 8% 9% 5%	1680
December	1st to 10th	Day	62% 8% 11% 15% 3 1	2477
	11th to 20th	Day	80% 6% 7% 5% 2	2718
	21st to end	Day	79% 6% 9% 4 2 1	3575
January	1st to 10th	Night	76% 6% 7% 8% 2	8770
	11th to 20th	Night	48% 11% 13% 16% 11%	8820
	21st to end	Night	64% 6% 8% 19% 2	9150
February	1st to 10th	Night	68% 6% 9% 10% 6% 2	2730
	11th to 20th	Night	62% 7% 12% 12% 7% 1	2664
	21st to end	Night	66% 5% 9% 13% 5% 1	2249
March	1st to 10th	Night	61% 7% 13% 14% 5%	5194
	11th to 20th	Night	58% 6% 12% 20% 3	5000
	21st to end	Night	70% 8% 11% 10% 2	5416
April	1st to 10th	Night	63% 6% 13% 15% 3	2000
	11th to 20th	Night	56% 7% 10% 17% 8% 2	1814
	21st to end	Night	55% 7% 12% 17% 8%	1775
May	1st to 10th	Night	41% 6% 14% 27% 11%	3637
	11th to 20th	Night	46% 9% 21% 20% 4	3616
	21st to end	Night	62% 6% 14% 16% 3	3606
June	1st to 10th	Night	53% 6% 15% 17% 7% 2	1400
	11th to 20th	Night	53% 6% 18% 18% 5%	1400
	21st to end	Night	43% 7% 21% 19% 8% 2	1400
July	1st to 10th	Night	27% 7% 20% 25% 16% 5%	3145
	11th to 20th	Night	13% 7% 22% 37% 19% 2	3138
	21st to end	Night	24% 6% 29% 25% 14% 2	3539
August	1st to 10th	Night	16% 4 13% 45% 20% 2	2699
	11th to 20th	Night	40% 9% 26% 16% 8% 2	2929
	21st to end	Night	40% 7% 22% 23% 8%	3273
September	1st to 10th	Night	30% 6% 19% 30% 13% 3	2000
	11th to 20th	Night	44% 7% 13% 17% 15% 3	2055
	21st to end	Night	50% 6% 16% 19% 7% 2	2259
October	1st to 10th	Night	57% 7% 17% 13% 6%	3920
	11th to 20th	Night	61% 4 12% 19% 4	3897
	21st to end	Night	61% 8% 14% 13% 4	4556
November	1st to 10th	Night	70% 7% 10% 8% 4 1	2870
	11th to 20th	Night	70% 6% 10% 9% 3 1	2940
	21st to end	Night	69% 6% 8% 10% 5% 1	2940
December	1st to 10th	Night	63% 9% 13% 12% 3 1	4922
	11th to 20th	Night	79% 5% 7% 6% 2	5706
	21st to end	Night	76% 6% 9% 7% 2	7623

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	40% 10% 15% 21% 8% 5%	220
	11th to 20th	Day	35% 10% 12% 26% 15% 1	220
	21st to end	Day	41% 5% 16% 22% 12% 4	302
February	1st to 10th	Day	39% 7% 21% 20% 13% 1	430
	11th to 20th	Day	34% 5% 21% 25% 15% 1	425
	21st to end	Day	31% 11% 23% 20% 10% 5%	355
March	1st to 10th	Day	32% 5% 21% 23% 17% 3	493
	11th to 20th	Day	31% 7% 17% 23% 17% 4	500
	21st to end	Day	31% 6% 25% 24% 11% 3	550
April	1st to 10th	Day	31% 8% 22% 24% 12% 3	500
	11th to 20th	Day	25% 5% 28% 23% 16% 3	492
	21st to end	Day	23% 7% 23% 28% 17% 4	500
May	1st to 10th	Day	17% 4 22% 28% 25% 4	503
	11th to 20th	Day	19% 5% 16% 29% 25% 6%	528
	21st to end	Day	17% 6% 20% 30% 22% 5%	728
June	1st to 10th	Day	19% 6% 22% 29% 21% 4	730
	11th to 20th	Day	15% 5% 24% 30% 22% 4	730
	21st to end	Day	18% 3 17% 31% 27% 5%	730
July	1st to 10th	Day	14% 3 15% 31% 31% 7%	721
	11th to 20th	Day	8% 3 16% 34% 32% 8%	718
	21st to end	Day	6% 4 13% 27% 38% 13%	720
August	1st to 10th	Day	12% 2 9% 25% 33% 18%	527
	11th to 20th	Day	12% 3 12% 26% 33% 13%	517
	21st to end	Day	15% 3 15% 29% 30% 9%	583
September	1st to 10th	Day	13% 2 17% 32% 27% 9%	530
	11th to 20th	Day	24% 6% 15% 25% 22% 9%	530
	21st to end	Day	27% 5% 20% 25% 16% 7%	530
October	1st to 10th	Day	27% 6% 19% 22% 18% 9%	530
	11th to 20th	Day	36% 6% 15% 19% 16% 7%	530
	21st to end	Day	35% 8% 19% 23% 11% 4	438
November	1st to 10th	Day	37% 8% 25% 20% 9% 2	220
	11th to 20th	Day	38% 8% 13% 21% 11% 8%	220
	21st to end	Day	45% 6% 18% 15% 9% 7%	220
December	1st to 10th	Day	37% 5% 20% 14% 14% 10%	220
	11th to 20th	Day	43% 13% 15% 17% 10% 3	220
	21st to end	Day	46% 7% 17% 21% 7% 2	242
January	1st to 10th	Night	47% 7% 15% 18% 12% 2	950
	11th to 20th	Night	38% 5% 14% 22% 18% 2	950
	21st to end	Night	38% 3 16% 23% 16% 4	986
February	1st to 10th	Night	38% 6% 15% 28% 12% 2	840
	11th to 20th	Night	31% 5% 17% 27% 19% 1	824
	21st to end	Night	35% 8% 17% 23% 14% 3	692
March	1st to 10th	Night	33% 5% 17% 23% 17% 4	808
	11th to 20th	Night	39% 5% 14% 20% 17% 5%	800
	21st to end	Night	34% 4 16% 24% 18% 5%	880
April	1st to 10th	Night	31% 5% 14% 30% 16% 6%	800
	11th to 20th	Night	25% 5% 19% 30% 19% 3	626
	21st to end	Night	28% 5% 13% 31% 20% 4	575
May	1st to 10th	Night	22% 4 12% 30% 27% 6%	400
	11th to 20th	Night	21% 5% 9% 24% 34% 8%	394
	21st to end	Night	20% 2 13% 30% 28% 8%	433
June	1st to 10th	Night	24% 4 11% 30% 28% 4	400
	11th to 20th	Night	22% 4 12% 29% 25% 8%	400
	21st to end	Night	20% 2 9% 29% 33% 8%	400
July	1st to 10th	Night	12% 2 10% 24% 39% 13%	390
	11th to 20th	Night	9% 2 9% 26% 44% 11%	388
	21st to end	Night	8% 2 5% 20% 47% 18%	438
August	1st to 10th	Night	9% 1 5% 21% 42% 22%	457
	11th to 20th	Night	11% 1 5% 22% 47% 14%	594
	21st to end	Night	14% 3 10% 20% 40% 13%	675
September	1st to 10th	Night	11% 2 7% 30% 36% 14%	800
	11th to 20th	Night	21% 4 10% 18% 31% 16%	800
	21st to end	Night	26% 3 11% 27% 26% 7%	800
October	1st to 10th	Night	26% 4 9% 27% 26% 9%	800
	11th to 20th	Night	35% 4 11% 23% 21% 6%	800
	21st to end	Night	37% 5% 15% 25% 14% 4	880
November	1st to 10th	Night	43% 6% 14% 23% 13%	820
	11th to 20th	Night	42% 5% 15% 23% 13% 3	840
	21st to end	Night	41% 5% 15% 23% 13% 3	840
December	1st to 10th	Night	36% 5% 15% 22% 17% 4	909
	11th to 20th	Night	48% 6% 15% 20% 9% 1	950
	21st to end	Night	42% 8% 18% 21% 9% 1	1045

Page generated at 2026-02-27 22:40:22. Version ac07

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