Crow's Nest Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Crow's Nest Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	17% 8% 2 6% 4 4 4 3 5% 7% 39%	148092
	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	142972
Transparency	Day	30% 34% 9% 16% 10% 1	151615
	Night	32% 34% 7% 15% 10% 2	147539
Seeing	Day	20% 9% 30% 27% 11% 2	17388
	Night	22% 10% 29% 27% 11% 1	26279

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	12% 8% 3 8% 5% 5% 5% 4 6% 6% 38%	4661
	11th to 20th	Day	16% 11% 3 5% 4 3 3 3 4 5% 43%	4650
	21st to end	Day	11% 10% 3 5% 3 3 4 3 4 5% 49%	5124
February	1st to 10th	Day	17% 9% 2 6% 4 5% 4 4 5% 6% 38%	1880
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 6% 7% 38%	2039
	21st to end	Day	15% 11% 3 6% 5% 5% 4 4 5% 5% 38%	1730
March	1st to 10th	Day	23% 10% 2 6% 4 5% 4 3 4 5% 33%	4576
	11th to 20th	Day	19% 11% 3 6% 4 4 4 3 5% 6% 37%	5031
	21st to end	Day	28% 7% 2 5% 3 3 4 3 5% 6% 34%	5679
April	1st to 10th	Day	20% 9% 3 6% 4 3 4 4 5% 6% 36%	2520
	11th to 20th	Day	19% 5% 2 7% 4 4 4 3 5% 7% 40%	2612
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 5% 37%	2730
May	1st to 10th	Day	29% 10% 3 7% 4 4 4 3 5% 6% 27%	7639
	11th to 20th	Day	18% 8% 3 7% 5% 5% 4 4 6% 8% 33%	7643
	21st to end	Day	21% 8% 3 7% 5% 4 4 4 5% 7% 33%	9060
June	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 8% 35%	2940
	11th to 20th	Day	17% 7% 2 5% 4 3 4 4 5% 7% 41%	2940
	21st to end	Day	21% 6% 2 6% 4 4 4 4 6% 6% 35%	2940
July	1st to 10th	Day	16% 8% 2 7% 5% 4 5% 4 6% 9% 33%	8280
	11th to 20th	Day	11% 7% 2 5% 3 4 4 4 6% 9% 44%	8280
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 8% 44%	8960
August	1st to 10th	Day	13% 7% 3 7% 4 4 5% 4 6% 9% 38%	6015
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 9% 39%	5953
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 49%	6345
September	1st to 10th	Day	12% 5% 2 4 4 3 4 4 5% 8% 49%	2457
	11th to 20th	Day	11% 6% 2 4 3 2 4 3 4 6% 55%	2233
	21st to end	Day	17% 6% 2 5% 3 3 3 3 5% 7% 45%	2200
October	1st to 10th	Day	17% 7% 3 6% 3 4 3 3 4 6% 44%	2200
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 3 4 5% 44%	1935
	21st to end	Day	21% 8% 2 5% 3 4 3 3 5% 6% 41%	1980
November	1st to 10th	Day	16% 7% 1 5% 4 3 3 3 4 6% 48%	1851
	11th to 20th	Day	19% 5% 2 4 3 3 3 3 5% 5% 47%	1890
	21st to end	Day	21% 8% 3 5% 3 3 3 3 3 5% 43%	1863
December	1st to 10th	Day	15% 9% 2 5% 3 3 3 3 5% 7% 45%	2452
	11th to 20th	Day	19% 7% 2 4 3 3 3 2 4 6% 48%	2756
	21st to end	Day	21% 10% 3 7% 4 4 4 4 4 5% 35%	4048
January	1st to 10th	Night	12% 9% 3 7% 4 4 4 3 5% 7% 42%	8234
	11th to 20th	Night	15% 12% 4 9% 5% 5% 4 4 5% 5% 32%	8235
	21st to end	Night	13% 9% 3 6% 4 4 4 3 5% 6% 42%	9057
February	1st to 10th	Night	16% 9% 2 7% 5% 5% 5% 4 5% 6% 36%	2738
	11th to 20th	Night	14% 9% 3 6% 5% 5% 4 4 5% 6% 40%	2463
	21st to end	Night	15% 10% 3 7% 4 4 4 3 6% 5% 41%	2076
March	1st to 10th	Night	27% 9% 3 7% 4 4 3 3 4 5% 32%	5484
	11th to 20th	Night	16% 9% 4 8% 4 5% 4 4 6% 7% 33%	5147
	21st to end	Night	25% 9% 3 6% 4 3 4 3 6% 6% 32%	5480
April	1st to 10th	Night	22% 9% 2 6% 4 4 4 3 4 6% 36%	2100
	11th to 20th	Night	20% 6% 2 5% 5% 4 5% 4 6% 6% 37%	1956
	21st to end	Night	23% 8% 2 6% 5% 4 4 3 5% 5% 37%	1890
May	1st to 10th	Night	30% 8% 3 6% 3 3 4 3 5% 6% 28%	5259
	11th to 20th	Night	19% 10% 3 7% 5% 5% 5% 4 6% 8% 31%	5235
	21st to end	Night	19% 10% 4 7% 4 4 4 4 6% 8% 31%	5137
June	1st to 10th	Night	20% 9% 3 6% 5% 5% 5% 3 5% 6% 33%	1680
	11th to 20th	Night	16% 8% 2 6% 4 4 4 4 5% 8% 38%	1680
	21st to end	Night	22% 8% 3 5% 4 4 5% 4 6% 7% 32%	1680
July	1st to 10th	Night	15% 9% 3 7% 4 5% 5% 4 7% 9% 32%	4680
	11th to 20th	Night	13% 9% 2 6% 4 4 5% 4 6% 8% 40%	4680
	21st to end	Night	14% 5% 2 5% 4 4 4 4 6% 8% 44%	5084
August	1st to 10th	Night	16% 7% 3 9% 5% 5% 5% 4 6% 8% 32%	4106
	11th to 20th	Night	15% 9% 3 7% 5% 4 5% 4 6% 7% 37%	4092
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 7% 9% 41%	4764
September	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 5% 6% 48%	2163
	11th to 20th	Night	14% 7% 2 4 3 2 4 2 4 6% 51%	2234
	21st to end	Night	15% 6% 2 5% 4 3 5% 4 4 6% 46%	2200
October	1st to 10th	Night	18% 6% 2 5% 4 4 4 3 6% 7% 41%	2200
	11th to 20th	Night	17% 6% 2 6% 3 4 4 3 6% 6% 43%	2460
	21st to end	Night	23% 7% 2 5% 4 4 3 4 5% 5% 38%	2860
November	1st to 10th	Night	17% 6% 2 5% 4 3 4 4 4 5% 46%	2667
	11th to 20th	Night	17% 6% 2 5% 4 3 5% 4 5% 5% 45%	2730
	21st to end	Night	16% 8% 2 6% 5% 3 4 3 5% 7% 42%	2751
December	1st to 10th	Night	17% 10% 3 5% 4 4 4 3 5% 6% 39%	4955
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 43%	5665
	21st to end	Night	21% 12% 3 6% 4 3 3 2 4 5% 37%	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	36% 22% 11% 18% 12%	4662
	11th to 20th	Day	35% 22% 8% 20% 13% 2	4680
	21st to end	Day	29% 24% 7% 19% 17% 4	5124
February	1st to 10th	Day	19% 59% 5% 9% 6% 1	1880
	11th to 20th	Day	19% 58% 5% 8% 8% 1	2048
	21st to end	Day	17% 59% 5% 9% 8% 2	1730
March	1st to 10th	Day	41% 29% 5% 7% 10% 7%	4579
	11th to 20th	Day	35% 30% 11% 14% 9% 2	5031
	21st to end	Day	41% 29% 3 12% 12% 4	5679
April	1st to 10th	Day	21% 59% 4 8% 7% 2	2520
	11th to 20th	Day	19% 60% 5% 8% 6% 2	2612
	21st to end	Day	21% 59% 3 8% 8%	2730
May	1st to 10th	Day	46% 27% 9% 12% 5%	7651
	11th to 20th	Day	39% 26% 12% 16% 6% 1	7647
	21st to end	Day	39% 25% 10% 16% 9%	9061
June	1st to 10th	Day	17% 59% 5% 12% 6%	2940
	11th to 20th	Day	17% 60% 5% 9% 8% 1	2940
	21st to end	Day	18% 59% 5% 12% 6% 1	2940
July	1st to 10th	Day	39% 23% 11% 19% 8%	8210
	11th to 20th	Day	29% 22% 13% 23% 12% 1	8205
	21st to end	Day	31% 22% 10% 22% 14%	8762
August	1st to 10th	Day	34% 29% 14% 16% 6% 2	6002
	11th to 20th	Day	33% 29% 15% 16% 7%	5992
	21st to end	Day	23% 29% 13% 20% 15% 1	6345
September	1st to 10th	Day	14% 56% 6% 15% 8%	2457
	11th to 20th	Day	17% 56% 5% 12% 10% 1	2233
	21st to end	Day	16% 57% 4 14% 9% 1	2200
October	1st to 10th	Day	29% 37% 7% 15% 10% 1	3590
	11th to 20th	Day	18% 36% 6% 21% 18% 1	3125
	21st to end	Day	20% 36% 8% 19% 14% 2	3180
November	1st to 10th	Day	19% 54% 7% 11% 8% 1	1851
	11th to 20th	Day	18% 54% 4 10% 11% 3	1890
	21st to end	Day	21% 54% 7% 11% 7% 1	1863
December	1st to 10th	Day	23% 35% 10% 22% 9%	2452
	11th to 20th	Day	26% 31% 8% 13% 13% 8%	2756
	21st to end	Day	38% 28% 5% 14% 12% 3	4048
January	1st to 10th	Night	34% 22% 9% 23% 11% 1	8234
	11th to 20th	Night	42% 23% 8% 14% 11% 2	8280
	21st to end	Night	35% 23% 4 17% 15% 5%	9057
February	1st to 10th	Night	20% 59% 5% 9% 6% 2	2738
	11th to 20th	Night	19% 58% 4 9% 9% 1	2465
	21st to end	Night	19% 59% 4 9% 7% 2	2076
March	1st to 10th	Night	43% 29% 6% 7% 10% 6%	5494
	11th to 20th	Night	36% 30% 12% 13% 7% 1	5148
	21st to end	Night	41% 29% 3 9% 14% 4	5480
April	1st to 10th	Night	22% 59% 2 7% 8% 1	2100
	11th to 20th	Night	22% 60% 4 8% 7% 1	1956
	21st to end	Night	22% 59% 3 7% 7% 2	1890
May	1st to 10th	Night	46% 26% 10% 9% 8%	5265
	11th to 20th	Night	40% 25% 10% 20% 5%	5236
	21st to end	Night	39% 26% 9% 18% 7%	5138
June	1st to 10th	Night	20% 60% 4 11% 6%	1680
	11th to 20th	Night	16% 60% 7% 10% 6%	1680
	21st to end	Night	20% 58% 5% 11% 5% 1	1680
July	1st to 10th	Night	39% 23% 11% 18% 9%	4640
	11th to 20th	Night	34% 22% 9% 22% 13%	4632
	21st to end	Night	29% 22% 12% 26% 11%	4967
August	1st to 10th	Night	39% 29% 11% 14% 6% 2	4095
	11th to 20th	Night	35% 29% 11% 19% 7%	4119
	21st to end	Night	27% 30% 13% 18% 11%	4764
September	1st to 10th	Night	16% 56% 6% 14% 8% 1	2163
	11th to 20th	Night	19% 56% 4 12% 9% 1	2234
	21st to end	Night	17% 58% 4 15% 6% 1	2200
October	1st to 10th	Night	27% 37% 9% 15% 10% 1	3590
	11th to 20th	Night	16% 36% 6% 21% 20% 1	4009
	21st to end	Night	22% 36% 7% 20% 13% 2	4610
November	1st to 10th	Night	20% 54% 4 13% 8% 1	2667
	11th to 20th	Night	19% 54% 4 10% 11% 2	2730
	21st to end	Night	19% 55% 7% 10% 8% 1	2751
December	1st to 10th	Night	30% 35% 5% 22% 9%	4955
	11th to 20th	Night	32% 31% 7% 12% 15% 3	5666
	21st to end	Night	39% 27% 7% 12% 14% 1	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	21% 22% 31% 20% 5%	220
	11th to 20th	Day	25% 19% 30% 22% 5% 1	220
	21st to end	Day	15% 18% 40% 20% 7%	242
February	1st to 10th	Day	21% 15% 38% 23% 4	420
	11th to 20th	Day	19% 17% 43% 20% 2	532
	21st to end	Day	19% 16% 39% 21% 5%	446
March	1st to 10th	Day	22% 15% 37% 23% 3	540
	11th to 20th	Day	19% 15% 41% 20% 4	540
	21st to end	Day	24% 15% 35% 17% 6% 2	594
April	1st to 10th	Day	25% 15% 34% 19% 6% 1	540
	11th to 20th	Day	23% 11% 36% 21% 8% 1	532
	21st to end	Day	26% 9% 32% 25% 7% 1	540
May	1st to 10th	Day	28% 7% 33% 24% 7% 1	543
	11th to 20th	Day	20% 5% 37% 28% 10% 1	567
	21st to end	Day	20% 6% 32% 31% 9% 1	617
June	1st to 10th	Day	21% 5% 26% 35% 12% 2	570
	11th to 20th	Day	17% 7% 30% 32% 11% 3	570
	21st to end	Day	21% 4 28% 31% 14% 2	570
July	1st to 10th	Day	18% 3 21% 39% 17% 2	565
	11th to 20th	Day	16% 4 22% 37% 18% 3	564
	21st to end	Day	14% 3 19% 39% 20% 4	620
August	1st to 10th	Day	13% 3 20% 37% 23% 4	566
	11th to 20th	Day	16% 3 21% 34% 20% 7%	559
	21st to end	Day	11% 4 22% 38% 21% 4	627
September	1st to 10th	Day	14% 5% 25% 32% 20% 4	570
	11th to 20th	Day	13% 6% 25% 30% 19% 7%	542
	21st to end	Day	18% 5% 27% 28% 17% 5%	530
October	1st to 10th	Day	19% 7% 23% 29% 17% 5%	530
	11th to 20th	Day	21% 8% 29% 26% 13% 3	420
	21st to end	Day	26% 10% 29% 25% 9%	462
November	1st to 10th	Day	18% 16% 31% 25% 9% 2	425
	11th to 20th	Day	22% 13% 36% 20% 9% 1	430
	21st to end	Day	24% 17% 31% 21% 7% 1	430
December	1st to 10th	Day	24% 14% 37% 16% 9%	283
	11th to 20th	Day	25% 19% 35% 16% 5%	220
	21st to end	Day	24% 22% 29% 19% 6% 1	242
January	1st to 10th	Night	21% 19% 37% 19% 4	840
	11th to 20th	Night	23% 20% 34% 18% 4	840
	21st to end	Night	20% 19% 38% 19% 4	924
February	1st to 10th	Night	22% 19% 34% 21% 4	840
	11th to 20th	Night	20% 18% 40% 20% 3	823
	21st to end	Night	19% 15% 38% 22% 5%	692
March	1st to 10th	Night	23% 16% 37% 21% 3	840
	11th to 20th	Night	23% 12% 34% 23% 8%	840
	21st to end	Night	25% 12% 32% 25% 6% 1	924
April	1st to 10th	Night	27% 11% 32% 24% 5%	840
	11th to 20th	Night	22% 8% 34% 28% 8% 1	712
	21st to end	Night	26% 7% 28% 27% 10% 1	630
May	1st to 10th	Night	25% 3 27% 32% 11% 2	630
	11th to 20th	Night	23% 5% 27% 32% 12% 1	622
	21st to end	Night	27% 3 22% 32% 14% 2	465
June	1st to 10th	Night	24% 5% 18% 35% 16% 3	420
	11th to 20th	Night	19% 4 21% 38% 16% 2	420
	21st to end	Night	24% 4 15% 41% 15% 1	420
July	1st to 10th	Night	20% 2 11% 44% 20% 1	410
	11th to 20th	Night	19% 3 15% 38% 23% 2	408
	21st to end	Night	16% 2 15% 40% 24% 3	460
August	1st to 10th	Night	15% 3 13% 42% 24% 2	612
	11th to 20th	Night	15% 2 14% 42% 23% 4	622
	21st to end	Night	15% 3 16% 37% 26% 3	796
September	1st to 10th	Night	15% 3 19% 37% 24% 2	840
	11th to 20th	Night	18% 3 22% 28% 24% 5%	813
	21st to end	Night	18% 4 23% 33% 19% 3	800
October	1st to 10th	Night	21% 7% 21% 29% 20% 2	800
	11th to 20th	Night	19% 9% 30% 29% 12% 2	800
	21st to end	Night	27% 10% 29% 26% 8%	880
November	1st to 10th	Night	21% 9% 34% 27% 8% 1	820
	11th to 20th	Night	20% 15% 34% 22% 7% 1	840
	21st to end	Night	19% 18% 34% 22% 6%	851
December	1st to 10th	Night	27% 12% 37% 17% 7%	950
	11th to 20th	Night	24% 19% 35% 18% 3	931
	21st to end	Night	28% 15% 34% 19% 4	924

Page generated at 2026-09-11 04:11:53. Version ac07

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