Cochrane
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cochrane clear sky chart from 2005-11-06 to 2025-02-25.

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% 13% 5% 10% 6% 5% 5% 4 5% 6% 26%	108256
	Night	15% 15% 6% 12% 6% 6% 5% 4 5% 5% 22%	110569
Transparency	Day	59% 5% 11% 18% 7% 1	111730
	Night	63% 5% 10% 15% 6%	115258
Seeing	Day	24% 16% 32% 20% 7% 2	15635
	Night	24% 12% 31% 24% 8% 1	24892

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	9% 22% 10% 21% 9% 8% 5% 3 4 3 7%	3158
	11th to 20th	Day	5% 16% 9% 19% 9% 6% 7% 5% 9% 5% 11%	3133
	21st to end	Day	7% 17% 9% 15% 8% 8% 8% 6% 5% 5% 13%	3952
February	1st to 10th	Day	8% 14% 5% 12% 11% 8% 9% 6% 6% 5% 17%	1800
	11th to 20th	Day	8% 20% 6% 11% 9% 5% 7% 4 6% 6% 17%	1753
	21st to end	Day	10% 17% 6% 11% 8% 6% 7% 5% 6% 5% 19%	1484
March	1st to 10th	Day	13% 15% 4 10% 6% 6% 6% 4 5% 7% 23%	3272
	11th to 20th	Day	11% 20% 7% 15% 8% 4 3 2 3 5% 22%	3480
	21st to end	Day	12% 16% 5% 12% 6% 6% 5% 4 5% 6% 24%	3815
April	1st to 10th	Day	18% 18% 5% 11% 5% 5% 4 3 4 5% 23%	2300
	11th to 20th	Day	19% 15% 4 9% 5% 3 3 4 5% 6% 29%	2442
	21st to end	Day	18% 12% 4 9% 5% 5% 4 2 4 5% 32%	2470
May	1st to 10th	Day	17% 11% 4 8% 5% 5% 5% 4 5% 6% 31%	4287
	11th to 20th	Day	16% 11% 3 9% 6% 5% 5% 4 5% 7% 30%	4451
	21st to end	Day	21% 11% 2 6% 4 4 5% 3 5% 6% 34%	5185
June	1st to 10th	Day	22% 8% 3 7% 5% 4 4 4 6% 7% 29%	2850
	11th to 20th	Day	17% 9% 3 5% 4 4 5% 4 6% 7% 35%	2850
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 4 5% 7% 31%	2850
July	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 5% 6% 8% 28%	4760
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	4760
	21st to end	Day	16% 8% 3 6% 5% 5% 5% 4 5% 9% 35%	5145
August	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 4 6% 8% 37%	4118
	11th to 20th	Day	18% 8% 3 6% 5% 4 4 3 5% 7% 36%	4044
	21st to end	Day	16% 8% 2 7% 5% 4 5% 3 5% 8% 37%	4532
September	1st to 10th	Day	20% 8% 3 7% 4 5% 4 4 5% 6% 34%	2313
	11th to 20th	Day	24% 10% 3 6% 4 4 4 3 5% 7% 31%	2119
	21st to end	Day	25% 10% 3 7% 4 4 3 2 4 5% 32%	2090
October	1st to 10th	Day	24% 14% 4 7% 3 3 4 2 4 6% 29%	1900
	11th to 20th	Day	28% 21% 5% 8% 4 4 3 3 3 3 18%	1900
	21st to end	Day	24% 24% 6% 11% 6% 4 3 2 3 3 14%	1914
November	1st to 10th	Day	23% 22% 7% 10% 5% 4 4 3 3 5% 15%	1566
	11th to 20th	Day	20% 23% 7% 15% 7% 5% 3 2 3 4 11%	1600
	21st to end	Day	21% 23% 9% 14% 7% 5% 4 2 3 3 9%	1586
December	1st to 10th	Day	13% 21% 11% 24% 9% 4 4 2 2 2 8%	2280
	11th to 20th	Day	11% 18% 6% 13% 9% 10% 5% 3 5% 5% 16%	2621
	21st to end	Day	18% 22% 8% 14% 7% 4 4 3 5% 4 10%	3476
January	1st to 10th	Night	8% 19% 10% 21% 9% 7% 6% 4 4 3 9%	6808
	11th to 20th	Night	9% 15% 8% 19% 10% 6% 7% 7% 9% 3 7%	6772
	21st to end	Night	8% 18% 10% 15% 7% 6% 7% 5% 5% 5% 16%	7021
February	1st to 10th	Night	8% 14% 5% 12% 9% 9% 8% 4 6% 8% 17%	2600
	11th to 20th	Night	9% 16% 8% 14% 7% 8% 6% 3 6% 6% 18%	2533
	21st to end	Night	9% 18% 6% 13% 6% 6% 7% 5% 5% 5% 18%	2062
March	1st to 10th	Night	17% 15% 4 10% 7% 5% 5% 3 4 4 26%	3568
	11th to 20th	Night	12% 18% 6% 16% 7% 5% 4 2 4 4 20%	3479
	21st to end	Night	13% 13% 5% 15% 5% 6% 5% 3 4 4 26%	3817
April	1st to 10th	Night	18% 15% 5% 11% 5% 4 4 3 5% 6% 25%	1897
	11th to 20th	Night	17% 12% 4 10% 4 5% 5% 4 4 6% 30%	1687
	21st to end	Night	19% 12% 5% 7% 3 4 3 4 4 5% 35%	1710
May	1st to 10th	Night	18% 12% 3 8% 5% 4 4 3 4 5% 34%	2652
	11th to 20th	Night	15% 10% 4 7% 5% 5% 5% 4 6% 6% 33%	2444
	21st to end	Night	18% 8% 2 6% 4 5% 4 4 7% 7% 36%	2402
June	1st to 10th	Night	21% 10% 2 7% 4 5% 5% 3 4 7% 32%	1330
	11th to 20th	Night	15% 8% 3 6% 6% 4 4 4 5% 7% 38%	1330
	21st to end	Night	19% 8% 2 5% 4 5% 4 4 7% 7% 34%	1330
July	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 5% 8% 39%	2210
	11th to 20th	Night	16% 9% 3 7% 5% 6% 5% 4 5% 7% 34%	2210
	21st to end	Night	13% 8% 3 8% 6% 5% 4 4 6% 7% 36%	2415
August	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 4 6% 8% 42%	2809
	11th to 20th	Night	17% 9% 2 7% 4 4 4 3 5% 7% 38%	2785
	21st to end	Night	14% 7% 2 6% 4 2 4 3 4 6% 48%	3124
September	1st to 10th	Night	20% 7% 3 7% 4 4 3 3 5% 7% 39%	1847
	11th to 20th	Night	23% 9% 2 6% 4 3 3 3 5% 7% 34%	2058
	21st to end	Night	24% 9% 3 6% 4 4 3 2 4 6% 35%	2090
October	1st to 10th	Night	22% 12% 3 8% 4 4 4 3 5% 6% 30%	2279
	11th to 20th	Night	27% 20% 4 8% 5% 4 3 3 4 5% 18%	2280
	21st to end	Night	21% 23% 6% 9% 5% 4 4 3 3 6% 18%	2680
November	1st to 10th	Night	20% 19% 6% 11% 6% 4 4 3 4 4 17%	2732
	11th to 20th	Night	21% 21% 7% 12% 7% 5% 4 3 3 3 15%	2800
	21st to end	Night	20% 22% 7% 15% 7% 6% 3 3 3 3 10%	2810
December	1st to 10th	Night	10% 27% 10% 19% 10% 8% 3 2 2 2 7%	4910
	11th to 20th	Night	10% 17% 7% 16% 10% 9% 6% 4 4 3 12%	5586
	21st to end	Night	19% 23% 6% 13% 7% 5% 4 4 4 4 11%	7502

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	83% 3 6% 5% 3 1	3159
	11th to 20th	Day	71% 4 7% 13% 6%	3160
	21st to end	Day	72% 4 5% 14% 4 1	3952
February	1st to 10th	Day	65% 6% 8% 15% 5%	1800
	11th to 20th	Day	66% 5% 9% 13% 7%	1762
	21st to end	Day	65% 4 7% 16% 7% 1	1484
March	1st to 10th	Day	60% 7% 10% 18% 5%	3275
	11th to 20th	Day	66% 4 8% 14% 6%	3480
	21st to end	Day	60% 5% 7% 14% 12% 1	3815
April	1st to 10th	Day	65% 4 7% 15% 9%	2300
	11th to 20th	Day	57% 4 9% 19% 10% 1	2442
	21st to end	Day	56% 4 9% 16% 13% 3	2470
May	1st to 10th	Day	52% 6% 10% 19% 11% 2	4299
	11th to 20th	Day	50% 8% 12% 20% 9% 1	4455
	21st to end	Day	50% 7% 10% 19% 13% 2	5185
June	1st to 10th	Day	53% 6% 11% 19% 10% 1	2850
	11th to 20th	Day	46% 7% 11% 22% 14%	2850
	21st to end	Day	52% 6% 11% 21% 8% 1	2850
July	1st to 10th	Day	51% 8% 12% 23% 5%	4686
	11th to 20th	Day	45% 7% 18% 20% 9% 1	4679
	21st to end	Day	46% 7% 15% 27% 5%	4982
August	1st to 10th	Day	44% 7% 17% 26% 7%	4104
	11th to 20th	Day	47% 5% 14% 27% 5%	4083
	21st to end	Day	45% 6% 15% 27% 6% 1	4531
September	1st to 10th	Day	49% 7% 15% 20% 9%	2313
	11th to 20th	Day	52% 7% 18% 19% 5%	2119
	21st to end	Day	56% 5% 10% 21% 8%	2090
October	1st to 10th	Day	61% 4 14% 16% 5%	3160
	11th to 20th	Day	64% 3 10% 19% 4	3140
	21st to end	Day	74% 3 10% 6% 6%	3126
November	1st to 10th	Day	72% 6% 10% 9% 3	1566
	11th to 20th	Day	79% 4 7% 8% 2	1600
	21st to end	Day	82% 4 7% 6% 1	1586
December	1st to 10th	Day	85% 3 5% 5% 2	2280
	11th to 20th	Day	71% 4 10% 12% 2	2621
	21st to end	Day	78% 3 5% 12% 2	3476
January	1st to 10th	Night	80% 4 8% 6% 2	6808
	11th to 20th	Night	75% 4 7% 9% 5%	6820
	21st to end	Night	70% 3 6% 15% 5%	7021
February	1st to 10th	Night	64% 5% 11% 12% 7%	2600
	11th to 20th	Night	67% 3 9% 14% 6%	2535
	21st to end	Night	65% 5% 10% 13% 6% 1	2062
March	1st to 10th	Night	62% 4 9% 16% 9% 1	3578
	11th to 20th	Night	67% 5% 7% 14% 6% 1	3480
	21st to end	Night	60% 5% 8% 12% 13% 2	3817
April	1st to 10th	Night	60% 4 9% 15% 11% 1	1897
	11th to 20th	Night	56% 5% 9% 17% 11% 2	1687
	21st to end	Night	52% 5% 8% 16% 17% 2	1710
May	1st to 10th	Night	50% 6% 11% 18% 13% 2	2659
	11th to 20th	Night	47% 9% 12% 22% 9% 1	2445
	21st to end	Night	44% 8% 12% 19% 16% 1	2403
June	1st to 10th	Night	51% 7% 10% 19% 12% 2	1330
	11th to 20th	Night	44% 7% 10% 22% 16% 1	1330
	21st to end	Night	46% 8% 14% 19% 12% 2	1330
July	1st to 10th	Night	43% 6% 16% 26% 8%	2175
	11th to 20th	Night	48% 6% 15% 21% 10% 1	2168
	21st to end	Night	47% 8% 13% 27% 6%	2338
August	1st to 10th	Night	38% 7% 17% 33% 5%	2799
	11th to 20th	Night	45% 7% 13% 30% 4	2812
	21st to end	Night	38% 6% 13% 32% 11% 1	3124
September	1st to 10th	Night	46% 7% 14% 25% 8%	1847
	11th to 20th	Night	51% 7% 17% 21% 5%	2058
	21st to end	Night	51% 6% 15% 23% 5%	2090
October	1st to 10th	Night	56% 5% 12% 22% 5%	3799
	11th to 20th	Night	63% 5% 10% 18% 4	3779
	21st to end	Night	67% 5% 13% 10% 5%	4418
November	1st to 10th	Night	71% 6% 10% 10% 2	2732
	11th to 20th	Night	76% 4 9% 8% 2	2800
	21st to end	Night	81% 5% 7% 6% 1	2810
December	1st to 10th	Night	86% 3 5% 4 2	4910
	11th to 20th	Night	76% 5% 7% 8% 4	5585
	21st to end	Night	76% 4 7% 10% 3	7502

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	25% 19% 34% 15% 7% 1	200
	11th to 20th	Day	17% 19% 37% 24% 5% 1	200
	21st to end	Day	18% 17% 34% 28% 4 1	320
February	1st to 10th	Day	17% 26% 37% 14% 3 2	300
	11th to 20th	Day	19% 17% 42% 17% 3 2	300
	21st to end	Day	27% 27% 32% 13% 2	237
March	1st to 10th	Day	18% 21% 40% 18% 4	360
	11th to 20th	Day	24% 17% 36% 17% 5% 1	460
	21st to end	Day	24% 20% 34% 16% 6%	506
April	1st to 10th	Day	23% 20% 33% 17% 6% 1	460
	11th to 20th	Day	27% 21% 28% 16% 5% 2	452
	21st to end	Day	28% 19% 26% 19% 7% 2	460
May	1st to 10th	Day	25% 15% 32% 20% 7% 1	463
	11th to 20th	Day	22% 18% 33% 18% 6% 2	514
	21st to end	Day	25% 16% 31% 23% 5% 1	734
June	1st to 10th	Day	26% 15% 34% 20% 5% 1	680
	11th to 20th	Day	25% 14% 35% 18% 7% 1	680
	21st to end	Day	21% 13% 30% 25% 10% 1	680
July	1st to 10th	Day	20% 11% 35% 24% 7% 3	671
	11th to 20th	Day	17% 12% 33% 29% 8% 1	668
	21st to end	Day	18% 10% 32% 27% 10% 3	739
August	1st to 10th	Day	20% 11% 31% 25% 11% 3	500
	11th to 20th	Day	22% 8% 26% 25% 13% 6%	479
	21st to end	Day	22% 10% 29% 22% 11% 6%	539
September	1st to 10th	Day	22% 17% 28% 19% 12% 2	490
	11th to 20th	Day	29% 10% 28% 21% 10% 2	490
	21st to end	Day	27% 12% 30% 20% 9% 2	490
October	1st to 10th	Day	30% 16% 27% 18% 7% 2	490
	11th to 20th	Day	38% 19% 27% 12% 2 1	490
	21st to end	Day	34% 21% 30% 12% 2	363
November	1st to 10th	Day	37% 22% 28% 13% 1	200
	11th to 20th	Day	31% 24% 30% 13% 4	200
	21st to end	Day	30% 27% 29% 9% 6%	200
December	1st to 10th	Day	27% 18% 30% 17% 7% 2	200
	11th to 20th	Day	21% 21% 38% 16% 4 1	200
	21st to end	Day	29% 16% 31% 17% 3 3	220
January	1st to 10th	Night	22% 14% 40% 18% 6% 1	1000
	11th to 20th	Night	21% 13% 31% 28% 7%	1000
	21st to end	Night	17% 11% 31% 29% 10% 1	1092
February	1st to 10th	Night	13% 14% 38% 25% 9% 1	800
	11th to 20th	Night	18% 14% 34% 27% 7%	783
	21st to end	Night	19% 13% 38% 21% 7% 1	647
March	1st to 10th	Night	18% 14% 37% 24% 6% 1	760
	11th to 20th	Night	25% 17% 30% 23% 4 1	760
	21st to end	Night	20% 14% 36% 23% 7% 1	836
April	1st to 10th	Night	26% 16% 29% 22% 6%	703
	11th to 20th	Night	23% 16% 28% 25% 8% 1	563
	21st to end	Night	24% 11% 32% 24% 8% 1	570
May	1st to 10th	Night	21% 11% 34% 26% 7%	467
	11th to 20th	Night	22% 12% 31% 25% 8% 1	374
	21st to end	Night	22% 9% 34% 31% 4	411
June	1st to 10th	Night	28% 9% 27% 28% 7%	380
	11th to 20th	Night	20% 10% 32% 30% 8% 1	380
	21st to end	Night	21% 10% 24% 35% 10% 1	380
July	1st to 10th	Night	19% 8% 29% 32% 11% 2	370
	11th to 20th	Night	17% 7% 31% 33% 12%	368
	21st to end	Night	15% 8% 26% 36% 14% 1	417
August	1st to 10th	Night	16% 8% 26% 34% 13% 2	567
	11th to 20th	Night	18% 7% 22% 32% 19% 1	564
	21st to end	Night	20% 9% 26% 27% 17% 1	627
September	1st to 10th	Night	25% 8% 27% 26% 12% 1	707
	11th to 20th	Night	27% 9% 30% 21% 11% 1	760
	21st to end	Night	27% 10% 25% 26% 11% 1	760
October	1st to 10th	Night	30% 10% 28% 22% 8% 1	760
	11th to 20th	Night	38% 14% 28% 15% 4 2	760
	21st to end	Night	32% 13% 31% 18% 6% 1	836
November	1st to 10th	Night	32% 14% 33% 17% 3 1	780
	11th to 20th	Night	34% 15% 30% 16% 5%	800
	21st to end	Night	31% 16% 34% 15% 3	810
December	1st to 10th	Night	25% 16% 31% 20% 7% 1	1000
	11th to 20th	Night	23% 15% 34% 20% 7% 1	1000
	21st to end	Night	28% 13% 30% 19% 7% 2	1100

Page generated at 2025-02-22 06:51:33. Version ac07

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