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 2026-09-14.

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	8% 8% 3 8% 5% 5% 5% 4 7% 8% 40%	154125
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 40%	145711
Transparency	Day	40% 6% 14% 25% 12% 2	153829
	Night	41% 6% 15% 24% 12% 2	145590
Seeing	Day	15% 13% 31% 27% 11% 3	17789
	Night	11% 10% 33% 33% 12% 1	26710

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	1 6% 5% 11% 8% 8% 6% 4 7% 10% 36%	4125
	11th to 20th	Day	1 8% 3 8% 6% 6% 5% 4 6% 7% 46%	4271
	21st to end	Day	2 6% 3 9% 4 5% 4 4 7% 11% 46%	5186
February	1st to 10th	Day	3 10% 3 11% 9% 6% 6% 5% 7% 8% 33%	1680
	11th to 20th	Day	2 9% 4 10% 8% 7% 7% 5% 6% 8% 33%	1808
	21st to end	Day	2 9% 5% 12% 7% 6% 7% 4 7% 7% 34%	1704
March	1st to 10th	Day	2 8% 4 10% 5% 5% 5% 5% 6% 11% 39%	4618
	11th to 20th	Day	5% 8% 5% 9% 5% 4 4 4 7% 8% 42%	5014
	21st to end	Day	7% 21% 7% 13% 6% 4 5% 4 5% 6% 21%	5740
April	1st to 10th	Day	7% 15% 5% 11% 6% 5% 5% 4 6% 8% 28%	2562
	11th to 20th	Day	10% 15% 5% 9% 5% 5% 4 3 7% 7% 29%	2700
	21st to end	Day	11% 13% 4 9% 5% 6% 6% 4 5% 7% 30%	2730
May	1st to 10th	Day	11% 11% 3 7% 4 5% 5% 4 6% 7% 37%	8059
	11th to 20th	Day	12% 10% 3 10% 6% 6% 6% 5% 7% 8% 26%	8755
	21st to end	Day	13% 8% 2 8% 5% 5% 6% 4 7% 8% 33%	9649
June	1st to 10th	Day	18% 8% 3 7% 5% 5% 6% 5% 6% 7% 30%	3150
	11th to 20th	Day	20% 9% 3 8% 6% 6% 5% 4 7% 7% 26%	3150
	21st to end	Day	10% 6% 2 6% 6% 6% 5% 5% 7% 9% 38%	3150
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 9% 48%	8820
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 7% 8% 48%	8820
	21st to end	Day	8% 4 2 5% 3 3 4 3 6% 8% 53%	9280
August	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 7% 9% 47%	6441
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 4 7% 9% 39%	6038
	21st to end	Day	8% 3 1 4 3 4 4 3 6% 9% 54%	6570
September	1st to 10th	Day	11% 6% 2 6% 4 4 3 4 6% 7% 47%	2597
	11th to 20th	Day	13% 7% 2 5% 4 4 4 4 5% 7% 45%	2236
	21st to end	Day	9% 6% 3 6% 5% 4 4 4 6% 7% 46%	2200
October	1st to 10th	Day	7% 6% 2 6% 4 4 5% 4 7% 10% 45%	3478
	11th to 20th	Day	5% 7% 3 7% 4 4 5% 5% 8% 11% 41%	2969
	21st to end	Day	7% 7% 3 6% 5% 5% 5% 4 6% 7% 47%	3200
November	1st to 10th	Day	6% 14% 5% 8% 5% 5% 5% 5% 6% 7% 35%	1738
	11th to 20th	Day	4 10% 4 8% 6% 6% 5% 4 6% 6% 42%	1570
	21st to end	Day	3 11% 5% 11% 6% 4 4 4 6% 8% 37%	1470
December	1st to 10th	Day	3 9% 5% 11% 6% 5% 6% 5% 8% 9% 32%	2360
	11th to 20th	Day	2 8% 5% 10% 7% 7% 7% 5% 7% 10% 34%	2712
	21st to end	Day	2 10% 4 9% 5% 4 5% 4 6% 7% 44%	3575
January	1st to 10th	Night	3 11% 5% 12% 8% 7% 6% 5% 7% 8% 29%	8768
	11th to 20th	Night	2 7% 4 11% 6% 5% 5% 5% 6% 9% 40%	8682
	21st to end	Night	2 7% 4 10% 6% 6% 5% 5% 6% 8% 40%	8994
February	1st to 10th	Night	4 10% 4 9% 7% 7% 7% 5% 7% 9% 32%	2940
	11th to 20th	Night	3 11% 4 11% 7% 6% 7% 5% 7% 9% 30%	2698
	21st to end	Night	2 10% 4 10% 9% 6% 5% 5% 5% 8% 36%	2101
March	1st to 10th	Night	2 8% 3 12% 7% 6% 7% 4 6% 10% 35%	5496
	11th to 20th	Night	3 7% 4 10% 6% 4 6% 5% 8% 9% 38%	5205
	21st to end	Night	9% 18% 6% 12% 6% 5% 4 4 5% 7% 24%	5476
April	1st to 10th	Night	7% 16% 4 9% 5% 4 4 4 5% 7% 33%	2058
	11th to 20th	Night	10% 14% 4 10% 5% 5% 4 3 6% 6% 33%	1871
	21st to end	Night	10% 12% 3 9% 6% 4 5% 4 7% 8% 32%	1890
May	1st to 10th	Night	14% 12% 3 6% 4 3 4 3 5% 7% 40%	4869
	11th to 20th	Night	12% 11% 3 8% 5% 4 5% 4 7% 9% 31%	4105
	21st to end	Night	10% 7% 3 8% 5% 6% 6% 5% 7% 8% 37%	4522
June	1st to 10th	Night	16% 7% 3 7% 5% 4 5% 5% 6% 8% 34%	1470
	11th to 20th	Night	19% 7% 2 6% 4 4 5% 5% 8% 8% 31%	1470
	21st to end	Night	9% 5% 1 6% 5% 6% 6% 5% 7% 10% 41%	1470
July	1st to 10th	Night	7% 4 2 6% 4 4 4 3 6% 8% 51%	4140
	11th to 20th	Night	8% 4 2 4 4 3 4 4 5% 8% 53%	4140
	21st to end	Night	9% 4 2 4 3 3 3 3 4 8% 56%	4772
August	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 7% 8% 47%	3709
	11th to 20th	Night	12% 5% 2 5% 4 5% 5% 5% 7% 11% 40%	4022
	21st to end	Night	7% 3 1 4 3 3 4 4 6% 10% 54%	4578
September	1st to 10th	Night	10% 6% 2 5% 3 4 3 3 6% 7% 50%	2021
	11th to 20th	Night	13% 7% 3 5% 3 3 4 3 6% 7% 47%	2230
	21st to end	Night	10% 8% 3 5% 3 3 4 3 6% 8% 48%	2200
October	1st to 10th	Night	8% 6% 2 6% 3 4 5% 3 5% 7% 50%	3718
	11th to 20th	Night	5% 6% 2 6% 5% 4 4 3 6% 8% 50%	4178
	21st to end	Night	4 9% 3 7% 5% 4 4 4 6% 8% 47%	4600
November	1st to 10th	Night	5% 13% 4 8% 5% 4 5% 5% 6% 8% 37%	2759
	11th to 20th	Night	4 8% 4 9% 6% 6% 5% 5% 6% 8% 40%	3046
	21st to end	Night	3 10% 4 9% 6% 5% 5% 4 6% 9% 38%	3150
December	1st to 10th	Night	2 10% 4 8% 6% 6% 6% 5% 7% 9% 36%	5040
	11th to 20th	Night	2 10% 4 12% 8% 7% 6% 5% 7% 9% 31%	5700
	21st to end	Night	3 11% 4 9% 5% 5% 5% 5% 6% 8% 39%	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	44% 5% 11% 24% 14% 3	4125
	11th to 20th	Day	36% 6% 14% 24% 17% 3	4292
	21st to end	Day	32% 7% 16% 31% 14%	5187
February	1st to 10th	Day	47% 7% 12% 19% 11% 3	1680
	11th to 20th	Day	45% 7% 13% 24% 10% 2	1817
	21st to end	Day	47% 5% 13% 21% 12% 2	1704
March	1st to 10th	Day	39% 5% 14% 24% 17% 1	4618
	11th to 20th	Day	39% 5% 14% 25% 16% 1	5015
	21st to end	Day	62% 7% 10% 13% 7% 1	5740
April	1st to 10th	Day	51% 8% 14% 19% 8%	2562
	11th to 20th	Day	52% 6% 12% 20% 10% 1	2700
	21st to end	Day	52% 6% 12% 21% 8% 1	2730
May	1st to 10th	Day	45% 6% 9% 26% 14% 1	8059
	11th to 20th	Day	51% 8% 11% 21% 8% 2	8759
	21st to end	Day	46% 7% 15% 23% 8% 1	9650
June	1st to 10th	Day	51% 8% 13% 17% 9% 1	3150
	11th to 20th	Day	55% 8% 11% 17% 7% 2	3150
	21st to end	Day	39% 9% 17% 24% 9% 2	3150
July	1st to 10th	Day	30% 6% 16% 28% 16% 3	8746
	11th to 20th	Day	32% 6% 16% 29% 12% 5%	8739
	21st to end	Day	28% 5% 13% 29% 20% 6%	9078
August	1st to 10th	Day	32% 6% 16% 34% 9% 3	6424
	11th to 20th	Day	39% 6% 23% 25% 5% 1	6078
	21st to end	Day	27% 5% 18% 37% 11% 1	6569
September	1st to 10th	Day	35% 6% 15% 26% 13% 4	2597
	11th to 20th	Day	38% 7% 15% 26% 13% 1	2236
	21st to end	Day	37% 7% 14% 28% 12% 2	2200
October	1st to 10th	Day	34% 5% 18% 33% 8% 1	3478
	11th to 20th	Day	34% 7% 20% 23% 13% 3	2969
	21st to end	Day	37% 6% 11% 34% 12% 1	3200
November	1st to 10th	Day	47% 7% 12% 20% 12% 2	1738
	11th to 20th	Day	41% 7% 15% 25% 11% 1	1570
	21st to end	Day	44% 6% 15% 23% 9% 2	1470
December	1st to 10th	Day	47% 6% 15% 22% 8% 2	2360
	11th to 20th	Day	43% 8% 19% 20% 9% 1	2714
	21st to end	Day	39% 5% 9% 25% 18% 3	3575
January	1st to 10th	Night	50% 6% 12% 20% 10% 2	8769
	11th to 20th	Night	40% 5% 15% 21% 13% 6%	8723
	21st to end	Night	40% 5% 17% 26% 11% 1	8994
February	1st to 10th	Night	46% 8% 11% 20% 13% 3	2940
	11th to 20th	Night	47% 8% 13% 18% 14% 1	2699
	21st to end	Night	46% 6% 9% 24% 12% 2	2101
March	1st to 10th	Night	43% 7% 15% 21% 13% 1	5496
	11th to 20th	Night	40% 8% 14% 24% 14% 1	5205
	21st to end	Night	60% 6% 10% 15% 9% 1	5476
April	1st to 10th	Night	49% 7% 13% 21% 10% 1	2058
	11th to 20th	Night	51% 6% 11% 21% 10% 1	1871
	21st to end	Night	47% 8% 13% 22% 8% 2	1890
May	1st to 10th	Night	44% 5% 10% 26% 15% 1	4866
	11th to 20th	Night	45% 7% 12% 28% 7%	4105
	21st to end	Night	41% 9% 16% 27% 6% 1	4526
June	1st to 10th	Night	47% 8% 13% 22% 9% 1	1470
	11th to 20th	Night	48% 8% 14% 19% 9% 2	1470
	21st to end	Night	36% 9% 19% 27% 8% 1	1470
July	1st to 10th	Night	31% 7% 17% 32% 12% 2	4105
	11th to 20th	Night	29% 6% 20% 28% 12% 5%	4098
	21st to end	Night	28% 4 18% 29% 17% 4	4669
August	1st to 10th	Night	34% 6% 15% 29% 13% 2	3701
	11th to 20th	Night	36% 8% 29% 21% 6%	4048
	21st to end	Night	26% 5% 24% 34% 11% 1	4578
September	1st to 10th	Night	33% 5% 18% 28% 13% 4	2021
	11th to 20th	Night	36% 6% 14% 27% 12% 4	2230
	21st to end	Night	33% 6% 17% 27% 14% 3	2200
October	1st to 10th	Night	34% 5% 17% 26% 17% 1	3718
	11th to 20th	Night	33% 5% 19% 28% 13% 2	4178
	21st to end	Night	36% 5% 12% 35% 12% 1	4600
November	1st to 10th	Night	45% 6% 13% 22% 12% 1	2759
	11th to 20th	Night	40% 7% 15% 23% 14% 1	3046
	21st to end	Night	42% 7% 15% 23% 10% 3	3150
December	1st to 10th	Night	43% 8% 19% 21% 7% 2	5040
	11th to 20th	Night	48% 8% 17% 17% 10% 1	5697
	21st to end	Night	41% 6% 10% 25% 14% 4	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	5% 14% 38% 26% 12% 5%	220
	11th to 20th	Day	10% 12% 37% 28% 13%	220
	21st to end	Day	10% 8% 41% 31% 7% 3	242
February	1st to 10th	Day	11% 15% 40% 23% 8% 4	220
	11th to 20th	Day	9% 14% 37% 33% 6% 1	311
	21st to end	Day	10% 18% 36% 26% 10% 2	420
March	1st to 10th	Day	13% 18% 35% 24% 9% 2	540
	11th to 20th	Day	11% 17% 38% 25% 7% 2	540
	21st to end	Day	16% 22% 32% 20% 7% 3	594
April	1st to 10th	Day	17% 18% 37% 20% 6% 1	540
	11th to 20th	Day	18% 20% 32% 20% 8% 2	532
	21st to end	Day	21% 20% 28% 21% 8% 2	540
May	1st to 10th	Day	20% 17% 27% 24% 10% 2	569
	11th to 20th	Day	18% 13% 25% 29% 12% 3	775
	21st to end	Day	20% 11% 22% 28% 16% 2	844
June	1st to 10th	Day	21% 12% 31% 24% 10% 2	780
	11th to 20th	Day	24% 13% 28% 24% 9% 2	780
	21st to end	Day	12% 10% 28% 30% 17% 4	780
July	1st to 10th	Day	12% 8% 29% 28% 19% 4	771
	11th to 20th	Day	14% 8% 28% 32% 15% 3	768
	21st to end	Day	10% 9% 29% 34% 15% 4	752
August	1st to 10th	Day	14% 9% 30% 30% 13% 4	567
	11th to 20th	Day	10% 9% 29% 32% 16% 4	557
	21st to end	Day	12% 8% 31% 27% 16% 7%	627
September	1st to 10th	Day	14% 11% 32% 26% 14% 3	570
	11th to 20th	Day	17% 10% 28% 29% 13% 3	543
	21st to end	Day	14% 21% 29% 25% 9% 2	530
October	1st to 10th	Day	14% 14% 31% 26% 12% 3	460
	11th to 20th	Day	14% 12% 36% 25% 10% 2	330
	21st to end	Day	18% 15% 36% 21% 9% 1	363
November	1st to 10th	Day	17% 14% 37% 25% 5% 2	330
	11th to 20th	Day	15% 14% 36% 29% 6% 1	272
	21st to end	Day	7% 13% 39% 24% 13% 4	220
December	1st to 10th	Day	8% 13% 39% 31% 8% 1	220
	11th to 20th	Day	9% 15% 43% 22% 8% 2	220
	21st to end	Day	8% 10% 38% 32% 10% 2	242
January	1st to 10th	Night	10% 13% 41% 27% 8% 1	1050
	11th to 20th	Night	9% 12% 44% 28% 5% 1	1018
	21st to end	Night	10% 15% 40% 28% 7% 1	924
February	1st to 10th	Night	8% 13% 34% 31% 11% 2	840
	11th to 20th	Night	9% 11% 35% 33% 10% 1	824
	21st to end	Night	7% 9% 41% 26% 14% 2	692
March	1st to 10th	Night	7% 10% 40% 32% 10% 2	840
	11th to 20th	Night	7% 11% 38% 33% 10% 1	840
	21st to end	Night	13% 9% 34% 33% 9% 2	924
April	1st to 10th	Night	13% 11% 32% 30% 13% 1	798
	11th to 20th	Night	17% 8% 36% 28% 10%	623
	21st to end	Night	16% 13% 30% 30% 11%	630
May	1st to 10th	Night	19% 12% 23% 34% 11%	499
	11th to 20th	Night	16% 6% 28% 34% 15% 1	414
	21st to end	Night	19% 7% 20% 37% 16% 1	455
June	1st to 10th	Night	21% 9% 23% 34% 11% 1	420
	11th to 20th	Night	23% 5% 25% 31% 14% 1	420
	21st to end	Night	11% 4 20% 41% 22% 2	420
July	1st to 10th	Night	10% 4 18% 41% 25% 1	410
	11th to 20th	Night	12% 3 24% 42% 17% 2	408
	21st to end	Night	7% 4 19% 45% 24% 2	460
August	1st to 10th	Night	13% 3 12% 42% 28% 2	418
	11th to 20th	Night	11% 4 19% 41% 23% 2	568
	21st to end	Night	11% 6% 18% 42% 22% 1	693
September	1st to 10th	Night	14% 4 23% 37% 19% 3	761
	11th to 20th	Night	17% 4 20% 36% 20% 1	812
	21st to end	Night	12% 8% 30% 36% 11% 2	800
October	1st to 10th	Night	14% 7% 29% 35% 14% 1	800
	11th to 20th	Night	11% 9% 33% 34% 11% 2	800
	21st to end	Night	12% 12% 34% 35% 6% 1	880
November	1st to 10th	Night	14% 13% 36% 30% 7% 1	914
	11th to 20th	Night	9% 14% 43% 27% 5% 1	1050
	21st to end	Night	8% 12% 41% 30% 9% 1	1050
December	1st to 10th	Night	10% 13% 40% 28% 9% 1	1050
	11th to 20th	Night	6% 13% 39% 34% 8%	1050
	21st to end	Night	8% 11% 42% 29% 10% 1	1155

Page generated at 2026-09-11 02:11:56. Version ac07

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