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 2025-04-26.

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% 9% 3 8% 5% 5% 5% 4 7% 8% 38%	117102
	Night	6% 9% 4 9% 6% 5% 5% 4 6% 8% 39%	120241
Transparency	Day	42% 7% 14% 24% 11% 2	116849
	Night	42% 7% 14% 24% 12% 2	120140
Seeing	Day	15% 13% 31% 26% 12% 3	15836
	Night	11% 10% 33% 32% 12% 1	25097

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 7% 6% 12% 8% 8% 5% 4 6% 8% 35%	3179
	11th to 20th	Day	2 10% 3 9% 6% 6% 5% 4 7% 7% 40%	3269
	21st to end	Day	2 6% 2 7% 4 5% 5% 4 6% 10% 47%	4004
February	1st to 10th	Day	3 10% 3 12% 9% 6% 6% 5% 7% 7% 33%	1600
	11th to 20th	Day	2 9% 4 11% 8% 7% 7% 6% 6% 8% 32%	1720
	21st to end	Day	2 10% 5% 12% 7% 6% 7% 4 7% 7% 33%	1625
March	1st to 10th	Day	2 8% 4 10% 5% 5% 5% 4 6% 11% 39%	4518
	11th to 20th	Day	5% 8% 5% 9% 5% 4 4 4 6% 7% 42%	4906
	21st to end	Day	7% 21% 8% 13% 6% 4 5% 4 5% 6% 21%	5616
April	1st to 10th	Day	7% 15% 5% 11% 6% 5% 5% 4 6% 8% 28%	2440
	11th to 20th	Day	10% 15% 5% 9% 5% 5% 4 3 6% 7% 29%	2570
	21st to end	Day	11% 13% 4 8% 5% 6% 6% 4 5% 7% 31%	2537
May	1st to 10th	Day	17% 16% 3 8% 5% 5% 4 4 5% 5% 28%	4361
	11th to 20th	Day	11% 9% 3 9% 6% 6% 6% 5% 8% 9% 29%	4694
	21st to end	Day	17% 11% 3 10% 7% 6% 6% 4 6% 6% 23%	5161
June	1st to 10th	Day	19% 8% 3 7% 5% 5% 6% 6% 6% 7% 29%	2850
	11th to 20th	Day	22% 9% 3 8% 6% 6% 5% 4 6% 7% 24%	2850
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 6% 7% 9% 38%	2850
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	4740
	11th to 20th	Day	8% 4 1 4 4 4 5% 5% 8% 9% 47%	4740
	21st to end	Day	5% 4 2 4 3 4 4 4 7% 9% 54%	4989
August	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 8% 9% 42%	4401
	11th to 20th	Day	8% 4 2 6% 4 5% 6% 5% 8% 10% 42%	4099
	21st to end	Day	10% 4 2 4 3 4 4 3 6% 8% 51%	4521
September	1st to 10th	Day	11% 6% 2 6% 4 3 3 4 6% 6% 48%	2350
	11th to 20th	Day	14% 7% 2 5% 4 4 4 4 6% 7% 44%	2092
	21st to end	Day	9% 7% 3 6% 4 4 4 4 6% 7% 45%	2090
October	1st to 10th	Day	7% 6% 2 6% 4 4 5% 4 7% 10% 45%	3372
	11th to 20th	Day	5% 7% 3 8% 4 4 5% 5% 8% 12% 40%	2878
	21st to end	Day	7% 8% 3 6% 5% 5% 5% 4 6% 6% 46%	3101
November	1st to 10th	Day	6% 14% 5% 8% 6% 5% 5% 5% 6% 7% 34%	1654
	11th to 20th	Day	3 10% 4 8% 6% 5% 5% 5% 6% 6% 42%	1495
	21st to end	Day	3 11% 4 10% 6% 4 4 5% 6% 8% 38%	1400
December	1st to 10th	Day	3 9% 5% 10% 6% 5% 6% 5% 8% 9% 33%	2290
	11th to 20th	Day	2 8% 5% 10% 7% 7% 7% 5% 7% 9% 34%	2642
	21st to end	Day	2 10% 4 9% 5% 4 5% 4 6% 7% 44%	3498
January	1st to 10th	Night	3 12% 5% 13% 9% 7% 7% 5% 6% 7% 27%	6766
	11th to 20th	Night	2 8% 4 12% 7% 5% 6% 6% 6% 8% 35%	6670
	21st to end	Night	2 9% 4 9% 5% 5% 5% 4 6% 8% 42%	6952
February	1st to 10th	Night	4 10% 4 9% 7% 7% 7% 5% 7% 8% 32%	2800
	11th to 20th	Night	3 11% 4 11% 7% 6% 7% 5% 7% 9% 31%	2567
	21st to end	Night	2 10% 4 11% 9% 6% 5% 4 6% 8% 36%	2004
March	1st to 10th	Night	2 8% 3 12% 7% 6% 7% 4 6% 10% 35%	5376
	11th to 20th	Night	3 7% 5% 10% 6% 4 6% 5% 8% 9% 38%	5093
	21st to end	Night	9% 18% 6% 12% 6% 5% 4 4 5% 6% 24%	5358
April	1st to 10th	Night	7% 16% 4 10% 5% 4 4 4 6% 7% 33%	1960
	11th to 20th	Night	10% 14% 5% 10% 5% 5% 4 3 7% 6% 33%	1781
	21st to end	Night	10% 12% 3 9% 6% 4 5% 4 7% 8% 33%	1759
May	1st to 10th	Night	18% 16% 3 7% 5% 4 4 3 5% 6% 30%	2600
	11th to 20th	Night	11% 9% 3 8% 6% 5% 5% 4 7% 8% 35%	2199
	21st to end	Night	15% 10% 4 10% 5% 5% 6% 5% 5% 7% 28%	2414
June	1st to 10th	Night	17% 7% 3 8% 5% 4 5% 5% 6% 8% 34%	1330
	11th to 20th	Night	21% 7% 2 6% 4 4 5% 5% 8% 8% 31%	1330
	21st to end	Night	8% 5% 2 5% 6% 6% 6% 5% 6% 10% 42%	1330
July	1st to 10th	Night	9% 4 2 6% 5% 5% 4 3 5% 6% 50%	2220
	11th to 20th	Night	7% 3 2 4 4 4 6% 5% 7% 8% 52%	2220
	21st to end	Night	4 4 2 5% 4 4 4 3 4 8% 60%	2561
August	1st to 10th	Night	11% 5% 2 5% 5% 4 4 4 8% 9% 43%	2529
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 5% 8% 11% 45%	2737
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 9% 54%	3146
September	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 6% 7% 50%	1829
	11th to 20th	Night	14% 7% 3 5% 3 3 4 3 6% 7% 47%	2087
	21st to end	Night	10% 8% 3 5% 3 3 4 3 6% 8% 48%	2090
October	1st to 10th	Night	8% 7% 2 6% 3 4 5% 3 5% 7% 50%	3604
	11th to 20th	Night	5% 6% 2 6% 5% 4 5% 3 6% 8% 50%	4049
	21st to end	Night	5% 9% 3 7% 5% 4 4 4 6% 8% 46%	4457
November	1st to 10th	Night	5% 13% 4 8% 5% 4 5% 4 5% 8% 37%	2624
	11th to 20th	Night	4 8% 4 9% 6% 6% 5% 4 6% 8% 41%	2901
	21st to end	Night	3 10% 4 9% 6% 5% 5% 4 6% 9% 37%	3000
December	1st to 10th	Night	2 9% 4 8% 6% 6% 6% 5% 7% 9% 37%	4890
	11th to 20th	Night	2 10% 4 12% 8% 7% 6% 5% 7% 9% 31%	5550
	21st to end	Night	3 11% 4 9% 5% 5% 5% 4 6% 8% 39%	7458

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	47% 5% 10% 25% 11% 3	3179
	11th to 20th	Day	40% 7% 13% 18% 19% 3	3290
	21st to end	Day	30% 6% 16% 32% 15% 1	4004
February	1st to 10th	Day	48% 7% 12% 19% 10% 3	1600
	11th to 20th	Day	46% 7% 13% 23% 9% 2	1729
	21st to end	Day	48% 5% 12% 21% 12% 2	1625
March	1st to 10th	Day	39% 5% 14% 23% 17% 1	4518
	11th to 20th	Day	39% 5% 14% 25% 16% 1	4907
	21st to end	Day	62% 7% 10% 13% 7% 1	5616
April	1st to 10th	Day	51% 8% 13% 19% 8%	2440
	11th to 20th	Day	52% 6% 11% 20% 10% 1	2570
	21st to end	Day	51% 6% 12% 22% 8% 1	2537
May	1st to 10th	Day	56% 6% 9% 21% 7%	4361
	11th to 20th	Day	46% 10% 13% 23% 9%	4698
	21st to end	Day	58% 9% 11% 14% 8% 1	5162
June	1st to 10th	Day	52% 8% 14% 17% 8% 1	2850
	11th to 20th	Day	56% 8% 12% 15% 7% 2	2850
	21st to end	Day	39% 9% 17% 24% 10% 2	2850
July	1st to 10th	Day	33% 7% 13% 25% 18% 4	4666
	11th to 20th	Day	30% 7% 17% 33% 11% 2	4659
	21st to end	Day	25% 6% 11% 28% 23% 7%	4831
August	1st to 10th	Day	36% 7% 15% 35% 6% 1	4384
	11th to 20th	Day	34% 7% 25% 27% 6% 1	4139
	21st to end	Day	30% 6% 20% 32% 10% 2	4520
September	1st to 10th	Day	34% 7% 15% 26% 14% 4	2350
	11th to 20th	Day	39% 7% 15% 25% 12% 2	2092
	21st to end	Day	38% 7% 14% 26% 13% 3	2090
October	1st to 10th	Day	34% 6% 18% 33% 8% 1	3372
	11th to 20th	Day	35% 7% 21% 23% 12% 3	2878
	21st to end	Day	37% 6% 11% 34% 12% 1	3101
November	1st to 10th	Day	48% 7% 12% 20% 12% 1	1654
	11th to 20th	Day	41% 8% 15% 24% 11% 2	1495
	21st to end	Day	43% 7% 15% 23% 10% 3	1400
December	1st to 10th	Day	46% 6% 16% 22% 8% 2	2290
	11th to 20th	Day	43% 8% 19% 20% 8% 1	2644
	21st to end	Day	39% 5% 9% 25% 18% 4	3498
January	1st to 10th	Night	53% 7% 11% 21% 6% 2	6766
	11th to 20th	Night	45% 6% 11% 20% 14% 5%	6711
	21st to end	Night	38% 6% 15% 27% 13% 1	6952
February	1st to 10th	Night	46% 8% 11% 20% 13% 3	2800
	11th to 20th	Night	47% 8% 13% 18% 13% 1	2568
	21st to end	Night	46% 6% 10% 24% 13% 2	2004
March	1st to 10th	Night	43% 7% 15% 20% 13% 1	5376
	11th to 20th	Night	40% 8% 14% 24% 14% 1	5093
	21st to end	Night	60% 6% 10% 14% 9% 1	5358
April	1st to 10th	Night	49% 7% 12% 20% 11% 1	1960
	11th to 20th	Night	51% 6% 11% 21% 10% 1	1781
	21st to end	Night	47% 8% 14% 22% 8% 2	1759
May	1st to 10th	Night	55% 6% 10% 23% 6%	2600
	11th to 20th	Night	41% 8% 12% 32% 7% 1	2199
	21st to end	Night	49% 13% 12% 18% 7% 1	2414
June	1st to 10th	Night	46% 8% 14% 22% 9% 1	1330
	11th to 20th	Night	49% 8% 15% 18% 9% 2	1330
	21st to end	Night	35% 9% 19% 27% 9% 2	1330
July	1st to 10th	Night	34% 7% 14% 25% 17% 3	2185
	11th to 20th	Night	28% 8% 23% 28% 9% 4	2178
	21st to end	Night	25% 5% 10% 33% 23% 4	2480
August	1st to 10th	Night	36% 7% 15% 30% 11% 1	2521
	11th to 20th	Night	30% 8% 30% 23% 8% 1	2763
	21st to end	Night	28% 6% 21% 32% 12% 1	3146
September	1st to 10th	Night	33% 6% 17% 27% 14% 4	1829
	11th to 20th	Night	37% 6% 15% 27% 12% 4	2087
	21st to end	Night	34% 6% 17% 26% 14% 3	2090
October	1st to 10th	Night	34% 5% 17% 26% 17% 1	3604
	11th to 20th	Night	33% 5% 20% 29% 12% 2	4049
	21st to end	Night	36% 5% 12% 35% 12%	4457
November	1st to 10th	Night	45% 6% 13% 22% 12% 1	2624
	11th to 20th	Night	40% 7% 15% 22% 14% 2	2901
	21st to end	Night	42% 7% 15% 22% 11% 3	3000
December	1st to 10th	Night	43% 8% 19% 21% 7% 2	4890
	11th to 20th	Night	48% 8% 17% 17% 9%	5547
	21st to end	Night	41% 6% 10% 25% 14% 4	7458

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% 37% 26% 13% 6%	200
	11th to 20th	Day	9% 11% 36% 30% 15% 1	200
	21st to end	Day	10% 8% 40% 32% 7% 3	220
February	1st to 10th	Day	11% 15% 41% 22% 8% 4	200
	11th to 20th	Day	9% 14% 37% 32% 6% 1	283
	21st to end	Day	8% 17% 35% 27% 10% 2	389
March	1st to 10th	Day	13% 18% 35% 24% 9% 2	500
	11th to 20th	Day	10% 17% 38% 25% 7% 2	500
	21st to end	Day	16% 21% 32% 21% 7% 3	550
April	1st to 10th	Day	17% 19% 38% 20% 6% 1	500
	11th to 20th	Day	18% 21% 32% 20% 7% 2	492
	21st to end	Day	21% 20% 28% 21% 8% 2	481
May	1st to 10th	Day	21% 16% 25% 24% 11% 2	487
	11th to 20th	Day	17% 12% 25% 29% 13% 3	675
	21st to end	Day	22% 11% 22% 27% 16% 2	734
June	1st to 10th	Day	21% 12% 30% 25% 10% 2	680
	11th to 20th	Day	26% 13% 28% 23% 9% 2	680
	21st to end	Day	12% 10% 28% 30% 17% 4	680
July	1st to 10th	Day	13% 8% 27% 28% 20% 4	671
	11th to 20th	Day	13% 7% 28% 32% 17% 4	668
	21st to end	Day	9% 9% 30% 34% 15% 4	652
August	1st to 10th	Day	14% 8% 31% 30% 12% 5%	487
	11th to 20th	Day	8% 9% 29% 33% 17% 4	477
	21st to end	Day	13% 9% 32% 26% 15% 6%	539
September	1st to 10th	Day	13% 12% 36% 25% 13% 2	490
	11th to 20th	Day	18% 10% 30% 27% 13% 2	490
	21st to end	Day	16% 19% 30% 25% 9% 2	490
October	1st to 10th	Day	15% 15% 29% 26% 12% 3	424
	11th to 20th	Day	14% 12% 36% 25% 11% 3	300
	21st to end	Day	18% 14% 36% 22% 9% 1	330
November	1st to 10th	Day	16% 14% 37% 26% 6% 2	300
	11th to 20th	Day	15% 14% 38% 27% 6%	247
	21st to end	Day	7% 13% 40% 23% 13% 5%	200
December	1st to 10th	Day	8% 11% 40% 32% 9% 1	200
	11th to 20th	Day	8% 16% 43% 23% 9% 3	200
	21st to end	Day	8% 9% 39% 31% 11% 2	220
January	1st to 10th	Night	10% 13% 41% 27% 8% 1	1000
	11th to 20th	Night	9% 12% 43% 29% 6% 1	970
	21st to end	Night	10% 15% 39% 27% 8% 1	880
February	1st to 10th	Night	8% 13% 33% 32% 12% 2	800
	11th to 20th	Night	9% 11% 35% 33% 10% 1	784
	21st to end	Night	7% 10% 40% 27% 15% 2	660
March	1st to 10th	Night	7% 10% 39% 32% 10% 2	800
	11th to 20th	Night	7% 11% 37% 34% 10% 1	800
	21st to end	Night	13% 9% 33% 33% 10% 2	880
April	1st to 10th	Night	13% 12% 32% 30% 13% 1	760
	11th to 20th	Night	17% 8% 36% 28% 10%	593
	21st to end	Night	16% 12% 30% 30% 12%	587
May	1st to 10th	Night	20% 12% 22% 33% 12%	451
	11th to 20th	Night	16% 6% 28% 34% 15% 1	374
	21st to end	Night	20% 7% 20% 36% 16% 1	411
June	1st to 10th	Night	22% 8% 23% 34% 11% 2	380
	11th to 20th	Night	25% 4 25% 30% 15% 1	380
	21st to end	Night	11% 4 20% 42% 22% 2	380
July	1st to 10th	Night	11% 4 20% 39% 25% 2	370
	11th to 20th	Night	11% 3 24% 42% 17% 2	368
	21st to end	Night	6% 4 19% 44% 25% 2	416
August	1st to 10th	Night	13% 3 12% 41% 29% 2	378
	11th to 20th	Night	11% 4 20% 40% 23% 2	514
	21st to end	Night	11% 6% 19% 41% 21% 1	627
September	1st to 10th	Night	15% 4 23% 36% 19% 3	689
	11th to 20th	Night	18% 5% 20% 36% 19% 2	760
	21st to end	Night	12% 8% 30% 35% 12% 2	760
October	1st to 10th	Night	14% 7% 29% 34% 14% 1	760
	11th to 20th	Night	11% 8% 33% 34% 12% 2	760
	21st to end	Night	12% 11% 33% 36% 7% 1	836
November	1st to 10th	Night	14% 13% 35% 29% 8% 1	869
	11th to 20th	Night	9% 15% 44% 27% 5% 1	1000
	21st to end	Night	8% 12% 41% 30% 9% 1	1000
December	1st to 10th	Night	9% 13% 39% 28% 10% 1	1000
	11th to 20th	Night	6% 13% 38% 34% 8% 1	1000
	21st to end	Night	8% 10% 41% 29% 10% 2	1100

Page generated at 2025-04-23 07:25:02. Version ac07

Copyright © 2025 Attilla Danko.