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 2025-05-11.

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	18% 8% 2 6% 4 4 4 3 5% 7% 39%	112853
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 38%	116077
Transparency	Day	26% 43% 7% 13% 9% 2	116422
	Night	29% 41% 7% 13% 9% 2	120673
Seeing	Day	20% 9% 30% 28% 11% 2	15606
	Night	22% 10% 29% 27% 11% 1	24685

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% 7% 3 7% 5% 5% 4 4 5% 6% 39%	3597
	11th to 20th	Day	15% 8% 3 5% 4 3 3 3 4 6% 46%	3570
	21st to end	Day	14% 10% 3 5% 3 3 4 3 4 4 47%	3960
February	1st to 10th	Day	18% 9% 2 6% 4 5% 4 4 5% 6% 37%	1790
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 6% 7% 38%	1939
	21st to end	Day	15% 11% 3 6% 5% 5% 4 4 5% 5% 37%	1650
March	1st to 10th	Day	23% 10% 2 6% 4 5% 4 3 4 5% 33%	4476
	11th to 20th	Day	19% 11% 3 6% 4 4 4 3 5% 6% 37%	4923
	21st to end	Day	29% 7% 2 5% 3 3 4 3 5% 6% 34%	5556
April	1st to 10th	Day	20% 9% 3 6% 4 3 4 4 5% 6% 36%	2400
	11th to 20th	Day	19% 5% 2 6% 4 4 4 3 5% 7% 41%	2486
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 5% 37%	2600
May	1st to 10th	Day	24% 9% 3 7% 5% 4 4 4 6% 6% 28%	4236
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 4 6% 8% 34%	4099
	21st to end	Day	17% 6% 2 5% 4 5% 4 4 6% 7% 39%	4837
June	1st to 10th	Day	22% 7% 3 6% 4 3 4 3 5% 7% 35%	2660
	11th to 20th	Day	17% 7% 2 5% 5% 3 4 4 5% 7% 41%	2660
	21st to end	Day	22% 6% 3 6% 5% 4 4 4 6% 6% 34%	2660
July	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 9% 34%	4440
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 10% 41%	4440
	21st to end	Day	19% 7% 2 5% 4 4 4 3 6% 8% 38%	4795
August	1st to 10th	Day	16% 6% 3 7% 4 4 5% 4 5% 8% 38%	4105
	11th to 20th	Day	16% 8% 2 7% 4 4 4 4 6% 8% 37%	4045
	21st to end	Day	8% 4 1 4 4 4 5% 4 6% 9% 50%	4358
September	1st to 10th	Day	12% 5% 2 4 4 3 4 3 5% 8% 50%	2223
	11th to 20th	Day	12% 6% 2 4 3 2 4 3 4 6% 54%	2090
	21st to end	Day	17% 7% 2 5% 3 3 3 3 5% 7% 45%	2090
October	1st to 10th	Day	18% 7% 3 6% 3 4 3 3 4 6% 42%	2090
	11th to 20th	Day	18% 6% 2 6% 5% 3 4 3 4 5% 44%	1838
	21st to end	Day	22% 8% 2 5% 3 4 3 3 5% 5% 41%	1881
November	1st to 10th	Day	16% 7% 1 5% 4 3 3 3 4 6% 48%	1761
	11th to 20th	Day	19% 4 2 4 3 3 3 3 5% 5% 47%	1800
	21st to end	Day	22% 8% 2 5% 3 3 3 3 3 5% 44%	1775
December	1st to 10th	Day	15% 9% 2 4 3 3 3 3 5% 7% 45%	2379
	11th to 20th	Day	19% 7% 2 4 3 3 3 2 4 6% 48%	2684
	21st to end	Day	21% 10% 3 7% 4 4 4 4 4 5% 35%	3960
January	1st to 10th	Night	12% 8% 3 6% 4 4 4 3 5% 7% 43%	6349
	11th to 20th	Night	13% 10% 4 10% 6% 5% 4 4 5% 5% 35%	6315
	21st to end	Night	15% 10% 3 6% 4 4 4 3 5% 6% 41%	6996
February	1st to 10th	Night	17% 10% 2 6% 5% 5% 5% 4 5% 6% 36%	2608
	11th to 20th	Night	14% 10% 3 6% 5% 5% 4 4 5% 5% 39%	2343
	21st to end	Night	15% 10% 3 7% 4 4 4 3 6% 5% 40%	1980
March	1st to 10th	Night	27% 9% 3 7% 4 4 3 3 4 5% 31%	5364
	11th to 20th	Night	16% 9% 4 8% 5% 5% 4 4 6% 7% 33%	5035
	21st to end	Night	25% 9% 3 5% 4 3 4 3 6% 6% 32%	5361
April	1st to 10th	Night	22% 10% 2 5% 4 4 4 3 4 6% 36%	2000
	11th to 20th	Night	20% 6% 2 5% 5% 4 5% 4 6% 7% 37%	1862
	21st to end	Night	23% 8% 2 6% 5% 4 4 3 5% 5% 36%	1800
May	1st to 10th	Night	24% 8% 3 7% 4 4 5% 4 7% 7% 27%	2924
	11th to 20th	Night	17% 9% 3 7% 6% 6% 6% 5% 6% 9% 26%	2820
	21st to end	Night	18% 9% 4 7% 4 4 4 4 6% 7% 34%	2747
June	1st to 10th	Night	22% 9% 3 6% 5% 5% 5% 3 5% 6% 31%	1520
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 6% 8% 38%	1520
	21st to end	Night	24% 8% 2 5% 4 4 5% 4 6% 7% 31%	1520
July	1st to 10th	Night	15% 8% 3 8% 5% 4 5% 5% 7% 9% 31%	2520
	11th to 20th	Night	13% 7% 2 8% 5% 5% 5% 4 6% 9% 35%	2520
	21st to end	Night	15% 6% 3 6% 5% 5% 4 4 7% 8% 37%	2739
August	1st to 10th	Night	21% 8% 3 8% 5% 5% 4 3 6% 7% 30%	2811
	11th to 20th	Night	16% 9% 3 7% 5% 4 5% 4 6% 8% 32%	2782
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 47%	3282
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 48%	1957
	11th to 20th	Night	15% 8% 2 4 3 2 4 2 4 6% 51%	2090
	21st to end	Night	15% 6% 2 5% 4 3 5% 3 4 6% 47%	2090
October	1st to 10th	Night	19% 6% 2 5% 4 4 4 3 6% 7% 39%	2090
	11th to 20th	Night	16% 6% 2 6% 3 4 4 3 6% 6% 43%	2337
	21st to end	Night	24% 7% 2 5% 4 4 3 4 5% 5% 38%	2717
November	1st to 10th	Night	16% 6% 2 5% 4 3 4 4 4 5% 46%	2537
	11th to 20th	Night	18% 5% 2 5% 4 3 5% 4 5% 5% 45%	2600
	21st to end	Night	16% 8% 2 6% 5% 3 4 3 5% 7% 42%	2620
December	1st to 10th	Night	17% 11% 3 5% 4 4 4 3 5% 6% 39%	4808
	11th to 20th	Night	23% 6% 2 5% 3 3 3 3 4 5% 43%	5517
	21st to end	Night	21% 12% 3 6% 4 3 3 2 4 5% 37%	6996

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	33% 28% 8% 19% 12%	3598
	11th to 20th	Day	29% 28% 9% 22% 11% 2	3600
	21st to end	Day	30% 30% 6% 15% 14% 5%	3960
February	1st to 10th	Day	19% 62% 5% 7% 6% 1	1790
	11th to 20th	Day	19% 61% 5% 7% 7% 1	1948
	21st to end	Day	16% 62% 4 9% 7% 2	1650
March	1st to 10th	Day	41% 30% 5% 7% 10% 7%	4479
	11th to 20th	Day	34% 31% 11% 14% 9% 2	4923
	21st to end	Day	40% 29% 3 11% 12% 4	5556
April	1st to 10th	Day	20% 61% 3 7% 6% 2	2400
	11th to 20th	Day	17% 63% 4 8% 6% 2	2486
	21st to end	Day	20% 62% 3 8% 7%	2600
May	1st to 10th	Day	34% 46% 9% 9% 3	4250
	11th to 20th	Day	30% 44% 11% 11% 3	4103
	21st to end	Day	24% 43% 11% 12% 9%	4837
June	1st to 10th	Day	15% 65% 5% 9% 4	2660
	11th to 20th	Day	14% 65% 4 8% 7% 1	2660
	21st to end	Day	16% 64% 4 10% 6%	2660
July	1st to 10th	Day	30% 39% 8% 15% 8%	4370
	11th to 20th	Day	23% 38% 12% 18% 8%	4365
	21st to end	Day	30% 37% 9% 15% 9% 1	4642
August	1st to 10th	Day	30% 39% 10% 16% 5%	4092
	11th to 20th	Day	31% 40% 12% 13% 4	4084
	21st to end	Day	16% 40% 11% 20% 14%	4358
September	1st to 10th	Day	11% 61% 6% 13% 8%	2223
	11th to 20th	Day	16% 60% 4 11% 8% 1	2090
	21st to end	Day	15% 60% 4 13% 7%	2090
October	1st to 10th	Day	30% 39% 7% 15% 9% 1	3480
	11th to 20th	Day	17% 37% 6% 21% 19% 1	3028
	21st to end	Day	20% 37% 8% 18% 14% 2	3081
November	1st to 10th	Day	18% 57% 6% 12% 6% 1	1761
	11th to 20th	Day	17% 56% 4 9% 11% 3	1800
	21st to end	Day	19% 57% 7% 10% 7% 1	1775
December	1st to 10th	Day	23% 36% 11% 22% 8%	2379
	11th to 20th	Day	26% 32% 8% 13% 13% 8%	2684
	21st to end	Day	38% 29% 5% 13% 12% 3	3960
January	1st to 10th	Night	30% 28% 8% 23% 9% 2	6349
	11th to 20th	Night	36% 29% 9% 15% 9% 2	6360
	21st to end	Night	34% 30% 5% 13% 13% 5%	6996
February	1st to 10th	Night	19% 62% 5% 7% 6% 2	2608
	11th to 20th	Night	18% 61% 4 7% 9% 1	2345
	21st to end	Night	18% 61% 4 8% 6% 2	1980
March	1st to 10th	Night	43% 29% 6% 7% 10% 6%	5374
	11th to 20th	Night	35% 31% 12% 13% 7% 1	5036
	21st to end	Night	41% 30% 3 8% 14% 4	5361
April	1st to 10th	Night	21% 62% 2 7% 7% 1	2000
	11th to 20th	Night	20% 63% 3 7% 6% 1	1862
	21st to end	Night	21% 62% 3 6% 6% 1	1800
May	1st to 10th	Night	34% 44% 11% 8% 3	2930
	11th to 20th	Night	32% 44% 11% 10% 3	2821
	21st to end	Night	28% 45% 9% 10% 8% 1	2748
June	1st to 10th	Night	18% 66% 3 8% 5%	1520
	11th to 20th	Night	14% 66% 7% 8% 5%	1520
	21st to end	Night	18% 64% 4 9% 4	1520
July	1st to 10th	Night	31% 39% 8% 12% 10%	2480
	11th to 20th	Night	29% 38% 8% 16% 9%	2472
	21st to end	Night	29% 37% 8% 17% 8%	2651
August	1st to 10th	Night	38% 38% 9% 10% 4	2800
	11th to 20th	Night	33% 40% 9% 12% 5%	2809
	21st to end	Night	19% 40% 11% 17% 13%	3282
September	1st to 10th	Night	13% 61% 6% 12% 7% 1	1957
	11th to 20th	Night	18% 60% 4 11% 7% 1	2090
	21st to end	Night	15% 60% 4 15% 5% 1	2090
October	1st to 10th	Night	28% 38% 10% 15% 9% 1	3480
	11th to 20th	Night	16% 37% 6% 20% 20% 1	3886
	21st to end	Night	21% 37% 7% 20% 13% 2	4467
November	1st to 10th	Night	19% 56% 4 13% 7% 1	2537
	11th to 20th	Night	18% 57% 4 9% 11% 2	2600
	21st to end	Night	18% 58% 6% 10% 7% 1	2620
December	1st to 10th	Night	29% 36% 5% 22% 9%	4808
	11th to 20th	Night	32% 32% 7% 11% 15% 3	5518
	21st to end	Night	39% 27% 7% 12% 14% 1	6996

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% 22% 30% 21% 5%	200
	11th to 20th	Day	24% 17% 30% 24% 5% 1	200
	21st to end	Day	16% 15% 40% 22% 7%	220
February	1st to 10th	Day	23% 15% 38% 21% 3	390
	11th to 20th	Day	20% 17% 43% 18% 1	492
	21st to end	Day	19% 15% 38% 22% 6%	414
March	1st to 10th	Day	23% 15% 36% 23% 3	500
	11th to 20th	Day	18% 15% 41% 21% 5%	500
	21st to end	Day	25% 14% 35% 18% 7% 3	550
April	1st to 10th	Day	24% 15% 33% 20% 6% 1	500
	11th to 20th	Day	24% 10% 36% 22% 8% 1	492
	21st to end	Day	26% 8% 33% 25% 7% 1	500
May	1st to 10th	Day	28% 6% 32% 25% 8% 2	503
	11th to 20th	Day	19% 5% 36% 29% 10% 2	487
	21st to end	Day	20% 6% 31% 32% 10% 2	529
June	1st to 10th	Day	21% 5% 26% 35% 11% 1	490
	11th to 20th	Day	17% 5% 29% 34% 12% 3	490
	21st to end	Day	22% 4 27% 30% 14% 2	490
July	1st to 10th	Day	18% 3 20% 39% 17% 3	485
	11th to 20th	Day	15% 2 20% 39% 20% 3	484
	21st to end	Day	14% 2 18% 42% 20% 4	532
August	1st to 10th	Day	15% 3 20% 37% 22% 3	486
	11th to 20th	Day	17% 2 19% 35% 20% 6%	479
	21st to end	Day	12% 3 20% 38% 22% 5%	539
September	1st to 10th	Day	13% 5% 25% 31% 22% 4	490
	11th to 20th	Day	14% 7% 26% 29% 18% 7%	490
	21st to end	Day	18% 6% 25% 29% 17% 4	490
October	1st to 10th	Day	20% 8% 22% 30% 17% 4	490
	11th to 20th	Day	21% 8% 29% 25% 14% 3	390
	21st to end	Day	28% 10% 28% 25% 9%	429
November	1st to 10th	Day	17% 15% 30% 26% 10% 2	395
	11th to 20th	Day	23% 13% 37% 20% 8% 1	400
	21st to end	Day	25% 16% 32% 20% 7% 1	400
December	1st to 10th	Day	25% 12% 37% 16% 10%	260
	11th to 20th	Day	25% 18% 35% 18% 5% 1	200
	21st to end	Day	24% 19% 30% 20% 6% 1	220
January	1st to 10th	Night	21% 19% 37% 19% 4	800
	11th to 20th	Night	23% 19% 35% 18% 4 1	800
	21st to end	Night	20% 18% 39% 20% 4	880
February	1st to 10th	Night	22% 19% 34% 21% 4	800
	11th to 20th	Night	20% 19% 40% 19% 2	783
	21st to end	Night	19% 15% 38% 23% 5%	660
March	1st to 10th	Night	24% 16% 37% 21% 3	800
	11th to 20th	Night	22% 11% 34% 24% 8% 1	800
	21st to end	Night	24% 12% 32% 25% 6% 1	880
April	1st to 10th	Night	28% 11% 32% 25% 6%	800
	11th to 20th	Night	22% 7% 34% 29% 8% 1	678
	21st to end	Night	26% 7% 28% 28% 10% 1	600
May	1st to 10th	Night	25% 3 26% 32% 12% 2	600
	11th to 20th	Night	23% 5% 27% 32% 12% 2	565
	21st to end	Night	27% 3 20% 33% 15% 2	420
June	1st to 10th	Night	25% 4 18% 34% 15% 3	380
	11th to 20th	Night	19% 4 18% 39% 17% 3	380
	21st to end	Night	25% 3 16% 40% 15% 1	380
July	1st to 10th	Night	21% 2 11% 44% 21% 2	370
	11th to 20th	Night	20% 3 14% 39% 23% 2	368
	21st to end	Night	17% 2 13% 41% 25% 2	416
August	1st to 10th	Night	16% 3 14% 43% 23% 2	554
	11th to 20th	Night	16% 2 14% 41% 23% 4	562
	21st to end	Night	15% 3 15% 36% 27% 4	720
September	1st to 10th	Night	15% 2 19% 37% 25% 3	760
	11th to 20th	Night	19% 3 22% 28% 23% 5%	760
	21st to end	Night	18% 4 23% 33% 19% 3	760
October	1st to 10th	Night	22% 6% 21% 29% 19% 2	760
	11th to 20th	Night	19% 8% 30% 29% 11% 3	760
	21st to end	Night	28% 10% 28% 26% 8%	836
November	1st to 10th	Night	20% 8% 34% 28% 8% 2	780
	11th to 20th	Night	20% 15% 35% 22% 7% 1	800
	21st to end	Night	20% 18% 34% 22% 6%	810
December	1st to 10th	Night	27% 12% 37% 17% 7%	900
	11th to 20th	Night	24% 18% 36% 19% 4	883
	21st to end	Night	28% 14% 34% 19% 4	880

Page generated at 2025-05-08 09:34:00. Version ac07

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