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 2026-07-01.

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%	138157
	Night	14% 15% 6% 12% 6% 6% 5% 4 5% 5% 22%	134424
Transparency	Day	58% 5% 10% 18% 8% 1	141581
	Night	63% 5% 9% 16% 7% 1	139088
Seeing	Day	24% 16% 32% 20% 7% 2	17492
	Night	24% 12% 31% 24% 8% 1	26538

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	10% 22% 8% 18% 8% 7% 8% 5% 4 3 6%	4094
	11th to 20th	Day	5% 17% 9% 20% 9% 6% 6% 5% 8% 5% 9%	4083
	21st to end	Day	5% 13% 7% 15% 10% 8% 9% 7% 6% 5% 14%	5125
February	1st to 10th	Day	8% 14% 5% 12% 11% 8% 9% 6% 6% 5% 16%	1890
	11th to 20th	Day	8% 20% 6% 12% 8% 5% 7% 4 6% 6% 17%	1843
	21st to end	Day	10% 16% 6% 11% 9% 6% 7% 5% 6% 5% 19%	1596
March	1st to 10th	Day	14% 17% 5% 10% 7% 6% 6% 5% 6% 5% 21%	4807
	11th to 20th	Day	15% 18% 6% 13% 8% 6% 4 3 3 5% 19%	5090
	21st to end	Day	12% 18% 5% 12% 6% 7% 6% 3 4 4 23%	5586
April	1st to 10th	Day	18% 18% 5% 12% 5% 4 4 3 4 4 22%	2542
	11th to 20th	Day	19% 15% 4 9% 5% 3 3 3 5% 6% 29%	2702
	21st to end	Day	18% 12% 4 9% 5% 5% 4 2 4 5% 32%	2730
May	1st to 10th	Day	15% 16% 5% 9% 5% 4 4 3 4 4 30%	7973
	11th to 20th	Day	15% 11% 5% 9% 5% 5% 4 4 5% 7% 32%	8316
	21st to end	Day	17% 10% 3 6% 4 3 4 3 5% 7% 37%	9716
June	1st to 10th	Day	21% 8% 3 7% 5% 4 4 4 6% 7% 31%	3150
	11th to 20th	Day	17% 9% 3 6% 5% 4 5% 4 6% 7% 34%	3150
	21st to end	Day	19% 9% 3 7% 6% 5% 5% 4 6% 7% 31%	3150
July	1st to 10th	Day	15% 7% 3 9% 5% 5% 6% 5% 7% 9% 28%	6820
	11th to 20th	Day	16% 6% 3 6% 5% 6% 5% 5% 7% 9% 32%	6820
	21st to end	Day	17% 8% 3 6% 5% 5% 5% 4 6% 10% 33%	7411
August	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 4 6% 8% 37%	4249
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 3 5% 7% 35%	4174
	21st to end	Day	16% 8% 2 7% 5% 4 5% 3 5% 8% 37%	4675
September	1st to 10th	Day	20% 9% 3 7% 5% 5% 4 4 5% 6% 33%	2435
	11th to 20th	Day	23% 10% 3 6% 3 4 4 3 5% 8% 31%	2230
	21st to end	Day	24% 10% 3 7% 4 4 3 2 4 6% 32%	2200
October	1st to 10th	Day	23% 14% 4 7% 3 3 4 2 4 6% 30%	2000
	11th to 20th	Day	28% 21% 5% 8% 4 4 3 3 3 4 18%	2000
	21st to end	Day	23% 24% 6% 11% 6% 4 3 2 3 3 15%	2015
November	1st to 10th	Day	22% 22% 7% 10% 5% 4 4 3 3 5% 16%	1646
	11th to 20th	Day	20% 23% 8% 15% 7% 5% 3 2 3 4 11%	1680
	21st to end	Day	21% 23% 9% 14% 7% 5% 4 2 3 3 9%	1665
December	1st to 10th	Day	13% 21% 10% 24% 9% 4 4 2 2 2 8%	2350
	11th to 20th	Day	11% 18% 6% 13% 9% 10% 5% 3 5% 5% 15%	2691
	21st to end	Day	18% 22% 8% 14% 7% 4 4 3 5% 4 10%	3553
January	1st to 10th	Night	8% 19% 9% 21% 9% 7% 7% 4 4 3 8%	8831
	11th to 20th	Night	7% 17% 9% 19% 10% 6% 7% 7% 7% 3 7%	8832
	21st to end	Night	6% 14% 8% 15% 7% 8% 9% 6% 8% 5% 14%	9083
February	1st to 10th	Night	8% 14% 5% 12% 9% 9% 8% 5% 6% 7% 16%	2730
	11th to 20th	Night	9% 17% 8% 14% 7% 8% 6% 3 6% 6% 17%	2663
	21st to end	Night	9% 18% 6% 13% 6% 6% 7% 5% 5% 5% 19%	2207
March	1st to 10th	Night	18% 17% 5% 11% 7% 5% 6% 3 4 4 21%	5253
	11th to 20th	Night	16% 19% 6% 15% 7% 6% 4 2 4 4 17%	5089
	21st to end	Night	13% 14% 6% 14% 6% 6% 5% 3 4 5% 23%	5588
April	1st to 10th	Night	18% 14% 5% 11% 5% 5% 4 3 4 6% 25%	2097
	11th to 20th	Night	17% 13% 4 10% 4 5% 5% 3 4 6% 29%	1867
	21st to end	Night	19% 12% 5% 9% 4 3 3 4 4 5% 33%	1890
May	1st to 10th	Night	15% 15% 4 10% 5% 4 4 3 4 5% 28%	4924
	11th to 20th	Night	13% 11% 4 7% 5% 4 5% 4 5% 6% 36%	4536
	21st to end	Night	16% 9% 2 7% 4 4 4 3 6% 6% 40%	4492
June	1st to 10th	Night	21% 10% 2 6% 4 5% 4 3 5% 7% 32%	1470
	11th to 20th	Night	15% 8% 3 6% 6% 4 5% 3 5% 7% 38%	1470
	21st to end	Night	19% 8% 2 5% 4 5% 4 4 7% 8% 34%	1470
July	1st to 10th	Night	17% 8% 3 7% 4 3 4 4 5% 7% 39%	3160
	11th to 20th	Night	17% 8% 3 7% 4 5% 4 4 5% 7% 36%	3160
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 7% 8% 37%	3459
August	1st to 10th	Night	9% 5% 3 7% 5% 5% 5% 4 6% 8% 42%	2898
	11th to 20th	Night	16% 9% 2 7% 4 4 4 3 5% 7% 38%	2875
	21st to end	Night	14% 7% 2 6% 4 3 4 3 4 6% 47%	3223
September	1st to 10th	Night	20% 7% 3 7% 4 4 4 3 5% 7% 38%	1944
	11th to 20th	Night	23% 9% 2 6% 4 3 3 3 5% 7% 35%	2167
	21st to end	Night	23% 10% 3 6% 4 4 3 2 4 6% 36%	2200
October	1st to 10th	Night	21% 12% 3 7% 4 4 4 3 5% 6% 31%	2399
	11th to 20th	Night	27% 19% 4 7% 5% 4 3 3 4 5% 18%	2400
	21st to end	Night	20% 23% 6% 8% 4 4 3 3 3 6% 19%	2821
November	1st to 10th	Night	20% 19% 7% 12% 6% 4 4 3 4 4 16%	2872
	11th to 20th	Night	21% 22% 7% 13% 7% 5% 3 3 3 3 14%	2940
	21st to end	Night	21% 23% 7% 15% 7% 6% 3 3 3 3 10%	2951
December	1st to 10th	Night	10% 27% 10% 19% 10% 8% 3 2 2 2 7%	5060
	11th to 20th	Night	10% 18% 7% 16% 10% 9% 6% 4 4 3 11%	5736
	21st to end	Night	19% 23% 6% 13% 7% 5% 4 4 4 4 11%	7667

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	82% 3 6% 5% 2 1	4095
	11th to 20th	Day	74% 3 6% 12% 6%	4110
	21st to end	Day	69% 4 6% 17% 4 1	5125
February	1st to 10th	Day	66% 6% 9% 15% 5%	1890
	11th to 20th	Day	66% 5% 8% 14% 7%	1852
	21st to end	Day	66% 4 7% 15% 7% 1	1596
March	1st to 10th	Day	63% 5% 8% 14% 9% 1	4810
	11th to 20th	Day	69% 4 9% 12% 7%	5090
	21st to end	Day	64% 4 6% 16% 9% 1	5586
April	1st to 10th	Day	65% 4 6% 15% 9%	2542
	11th to 20th	Day	57% 4 9% 19% 10% 1	2702
	21st to end	Day	57% 3 9% 16% 12% 2	2730
May	1st to 10th	Day	56% 5% 7% 15% 16% 2	7982
	11th to 20th	Day	51% 6% 11% 20% 8% 3	8320
	21st to end	Day	46% 5% 10% 22% 15% 3	9717
June	1st to 10th	Day	52% 6% 11% 19% 11% 1	3150
	11th to 20th	Day	47% 6% 11% 21% 14% 1	3150
	21st to end	Day	52% 6% 11% 21% 9% 1	3150
July	1st to 10th	Day	49% 7% 13% 25% 6%	6747
	11th to 20th	Day	45% 7% 18% 21% 7% 1	6739
	21st to end	Day	46% 7% 16% 27% 5%	7199
August	1st to 10th	Day	43% 7% 17% 26% 7%	4235
	11th to 20th	Day	48% 5% 14% 27% 5%	4213
	21st to end	Day	45% 6% 15% 27% 6% 1	4674
September	1st to 10th	Day	50% 7% 15% 20% 8%	2435
	11th to 20th	Day	51% 7% 18% 20% 5%	2230
	21st to end	Day	55% 5% 10% 22% 8%	2200
October	1st to 10th	Day	60% 4 14% 17% 5%	3260
	11th to 20th	Day	64% 3 9% 19% 4	3240
	21st to end	Day	73% 3 10% 7% 6%	3227
November	1st to 10th	Day	72% 5% 10% 9% 4	1646
	11th to 20th	Day	80% 4 7% 7% 1	1680
	21st to end	Day	83% 4 6% 6% 1	1665
December	1st to 10th	Day	85% 3 5% 5% 2	2350
	11th to 20th	Day	72% 4 10% 11% 2	2691
	21st to end	Day	79% 3 4 12% 2	3553
January	1st to 10th	Night	81% 3 7% 7% 2	8831
	11th to 20th	Night	77% 4 6% 9% 5%	8880
	21st to end	Night	69% 3 6% 18% 4	9083
February	1st to 10th	Night	65% 5% 11% 12% 7%	2730
	11th to 20th	Night	67% 3 9% 14% 6%	2665
	21st to end	Night	65% 4 9% 14% 7% 1	2207
March	1st to 10th	Night	68% 3 6% 11% 9% 3	5263
	11th to 20th	Night	71% 4 6% 12% 6% 1	5090
	21st to end	Night	63% 4 7% 13% 11% 2	5588
April	1st to 10th	Night	61% 4 9% 15% 10% 1	2097
	11th to 20th	Night	56% 4 9% 17% 12% 2	1867
	21st to end	Night	53% 5% 8% 15% 16% 2	1890
May	1st to 10th	Night	56% 5% 8% 17% 12% 2	4931
	11th to 20th	Night	46% 7% 12% 21% 12% 1	4537
	21st to end	Night	44% 6% 11% 21% 17% 2	4493
June	1st to 10th	Night	50% 6% 10% 19% 13% 2	1470
	11th to 20th	Night	45% 7% 10% 23% 15% 1	1470
	21st to end	Night	45% 7% 14% 20% 12% 2	1470
July	1st to 10th	Night	45% 5% 13% 27% 9%	3125
	11th to 20th	Night	47% 6% 14% 24% 8%	3118
	21st to end	Night	44% 7% 15% 29% 5%	3357
August	1st to 10th	Night	38% 7% 17% 33% 5%	2888
	11th to 20th	Night	45% 7% 13% 30% 4	2902
	21st to end	Night	39% 5% 13% 31% 11% 1	3223
September	1st to 10th	Night	47% 7% 14% 25% 8%	1944
	11th to 20th	Night	50% 7% 16% 23% 5%	2167
	21st to end	Night	50% 6% 14% 24% 5%	2200
October	1st to 10th	Night	55% 5% 12% 23% 5%	3919
	11th to 20th	Night	63% 5% 10% 18% 5%	3899
	21st to end	Night	66% 5% 13% 11% 5%	4559
November	1st to 10th	Night	71% 5% 10% 10% 2	2872
	11th to 20th	Night	77% 4 9% 7% 2	2940
	21st to end	Night	81% 5% 7% 5% 1	2951
December	1st to 10th	Night	86% 3 5% 4 2	5060
	11th to 20th	Night	76% 4 7% 8% 4	5735
	21st to end	Night	76% 4 7% 10% 3	7667

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% 18% 35% 14% 6% 1	220
	11th to 20th	Day	16% 19% 36% 23% 5%	220
	21st to end	Day	16% 15% 38% 27% 4 1	352
February	1st to 10th	Day	17% 25% 36% 16% 4 2	330
	11th to 20th	Day	19% 17% 42% 17% 3 2	330
	21st to end	Day	26% 26% 32% 15% 1	273
March	1st to 10th	Day	21% 21% 39% 16% 3	430
	11th to 20th	Day	25% 16% 36% 17% 5% 1	540
	21st to end	Day	24% 21% 34% 15% 5%	594
April	1st to 10th	Day	24% 20% 32% 16% 6% 1	540
	11th to 20th	Day	27% 20% 29% 16% 5% 3	532
	21st to end	Day	29% 18% 26% 19% 7% 2	540
May	1st to 10th	Day	26% 16% 31% 20% 7% 1	543
	11th to 20th	Day	22% 18% 35% 17% 5% 2	597
	21st to end	Day	25% 15% 31% 23% 5% 1	844
June	1st to 10th	Day	25% 15% 32% 21% 5% 1	780
	11th to 20th	Day	25% 15% 34% 19% 7% 1	780
	21st to end	Day	22% 13% 29% 24% 10% 2	780
July	1st to 10th	Day	19% 12% 35% 24% 7% 3	722
	11th to 20th	Day	18% 13% 33% 27% 7% 1	718
	21st to end	Day	18% 10% 32% 27% 10% 3	794
August	1st to 10th	Day	19% 10% 32% 24% 12% 3	541
	11th to 20th	Day	21% 9% 26% 25% 13% 6%	519
	21st to end	Day	22% 10% 30% 22% 11% 5%	583
September	1st to 10th	Day	23% 17% 27% 20% 11% 2	530
	11th to 20th	Day	27% 10% 27% 22% 11% 3	530
	21st to end	Day	26% 13% 30% 21% 9% 2	530
October	1st to 10th	Day	29% 16% 29% 18% 7% 2	530
	11th to 20th	Day	37% 19% 28% 12% 2 1	530
	21st to end	Day	33% 20% 28% 15% 4	398
November	1st to 10th	Day	35% 22% 29% 13% 1	220
	11th to 20th	Day	30% 23% 30% 13% 3	220
	21st to end	Day	31% 27% 28% 9% 5%	220
December	1st to 10th	Day	25% 18% 33% 16% 6% 2	220
	11th to 20th	Day	23% 20% 37% 16% 4 1	220
	21st to end	Day	30% 16% 32% 17% 3 2	242
January	1st to 10th	Night	21% 13% 41% 18% 5% 1	1050
	11th to 20th	Night	21% 12% 31% 28% 7%	1050
	21st to end	Night	16% 11% 32% 29% 11% 1	1146
February	1st to 10th	Night	13% 14% 38% 25% 9% 1	840
	11th to 20th	Night	18% 13% 34% 27% 7%	823
	21st to end	Night	19% 14% 38% 21% 7% 1	692
March	1st to 10th	Night	20% 14% 36% 23% 5% 1	840
	11th to 20th	Night	25% 17% 31% 22% 4 1	840
	21st to end	Night	20% 13% 36% 23% 7% 1	924
April	1st to 10th	Night	26% 17% 30% 21% 6%	777
	11th to 20th	Night	23% 16% 28% 26% 8% 1	623
	21st to end	Night	24% 12% 32% 24% 8% 1	630
May	1st to 10th	Night	21% 11% 34% 27% 6% 1	516
	11th to 20th	Night	21% 12% 33% 25% 8% 1	414
	21st to end	Night	22% 9% 34% 30% 5%	455
June	1st to 10th	Night	27% 9% 28% 29% 7%	420
	11th to 20th	Night	20% 11% 31% 31% 7%	420
	21st to end	Night	21% 10% 25% 33% 11% 1	420
July	1st to 10th	Night	19% 7% 29% 32% 11% 2	390
	11th to 20th	Night	17% 8% 31% 33% 11%	388
	21st to end	Night	15% 8% 27% 36% 13% 1	439
August	1st to 10th	Night	16% 8% 26% 34% 14% 2	597
	11th to 20th	Night	18% 8% 22% 32% 18% 1	594
	21st to end	Night	20% 9% 26% 28% 17% 1	660
September	1st to 10th	Night	25% 9% 27% 26% 11% 1	744
	11th to 20th	Night	27% 9% 29% 23% 12% 1	800
	21st to end	Night	26% 10% 26% 26% 11% 1	800
October	1st to 10th	Night	29% 10% 30% 23% 8% 1	800
	11th to 20th	Night	38% 13% 29% 15% 4 2	800
	21st to end	Night	32% 12% 30% 19% 6% 1	880
November	1st to 10th	Night	32% 14% 33% 17% 2 1	820
	11th to 20th	Night	33% 15% 31% 17% 5%	840
	21st to end	Night	32% 16% 33% 15% 3	851
December	1st to 10th	Night	24% 17% 31% 20% 7% 1	1050
	11th to 20th	Night	24% 14% 35% 20% 7% 1	1050
	21st to end	Night	29% 14% 29% 19% 7% 2	1155

Page generated at 2026-06-28 02:10:49. Version ac07

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