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 2024-02-05.

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% 13% 5% 8% 7% 4 5% 5% 5% 6% 25%	77686
	Night	16% 14% 6% 10% 8% 4 5% 5% 5% 5% 22%	81205
Transparency	Day	60% 6% 10% 16% 7% 1	77371
	Night	64% 6% 10% 14% 6% 1	81055
Seeing	Day	24% 16% 31% 20% 7% 2	14255
	Night	24% 12% 31% 24% 8% 1	23521

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% 22% 8% 11% 11% 6% 7% 5% 5% 4 7%	2208
	11th to 20th	Day	7% 13% 8% 16% 12% 5% 7% 7% 6% 5% 15%	2210
	21st to end	Day	10% 22% 10% 13% 9% 5% 5% 6% 5% 4 11%	2754
February	1st to 10th	Day	8% 12% 6% 10% 12% 6% 10% 7% 6% 5% 18%	1656
	11th to 20th	Day	10% 19% 7% 10% 8% 3 7% 6% 6% 6% 18%	1573
	21st to end	Day	9% 17% 7% 10% 9% 5% 7% 6% 6% 5% 19%	1365
March	1st to 10th	Day	12% 12% 6% 10% 10% 7% 8% 5% 4 5% 22%	1882
	11th to 20th	Day	13% 17% 5% 9% 8% 4 5% 3 3 6% 26%	1980
	21st to end	Day	14% 15% 5% 9% 6% 4 5% 4 3 6% 29%	2178
April	1st to 10th	Day	19% 17% 6% 10% 6% 4 4 4 4 5% 21%	2178
	11th to 20th	Day	19% 13% 4 8% 6% 2 3 4 5% 6% 30%	2312
	21st to end	Day	18% 11% 4 8% 6% 4 5% 3 4 5% 32%	2340
May	1st to 10th	Day	19% 12% 6% 8% 6% 4 5% 5% 3 7% 27%	2435
	11th to 20th	Day	21% 9% 3 7% 5% 3 4 5% 5% 7% 30%	2503
	21st to end	Day	20% 9% 3 6% 5% 3 5% 5% 5% 7% 32%	2919
June	1st to 10th	Day	22% 8% 3 6% 6% 3 4 4 5% 7% 30%	2700
	11th to 20th	Day	18% 8% 3 5% 5% 3 5% 5% 6% 7% 35%	2700
	21st to end	Day	20% 8% 3 6% 6% 3 5% 5% 5% 7% 32%	2700
July	1st to 10th	Day	20% 7% 3 6% 5% 4 5% 6% 5% 8% 31%	2700
	11th to 20th	Day	16% 7% 4 6% 6% 4 6% 5% 6% 8% 31%	2700
	21st to end	Day	18% 7% 3 6% 6% 4 6% 5% 6% 8% 31%	2936
August	1st to 10th	Day	18% 8% 3 6% 7% 4 5% 6% 5% 7% 31%	2341
	11th to 20th	Day	19% 8% 3 5% 6% 3 5% 5% 6% 8% 32%	2303
	21st to end	Day	17% 8% 3 7% 6% 3 5% 4 6% 9% 32%	2574
September	1st to 10th	Day	20% 8% 3 6% 5% 3 5% 5% 5% 6% 34%	2192
	11th to 20th	Day	26% 10% 3 5% 4 3 5% 4 5% 7% 29%	2008
	21st to end	Day	26% 10% 3 6% 5% 3 3 3 4 6% 30%	1980
October	1st to 10th	Day	25% 14% 4 6% 4 2 4 3 4 5% 30%	1800
	11th to 20th	Day	29% 21% 5% 7% 5% 3 3 3 3 3 17%	1800
	21st to end	Day	25% 24% 6% 9% 7% 3 3 3 3 3 14%	1814
November	1st to 10th	Day	23% 21% 8% 9% 6% 2 5% 3 3 5% 16%	1486
	11th to 20th	Day	21% 20% 8% 13% 9% 4 4 3 3 4 11%	1520
	21st to end	Day	21% 21% 8% 12% 9% 4 5% 3 3 3 10%	1507
December	1st to 10th	Day	17% 18% 8% 16% 8% 3 5% 4 3 4 14%	1330
	11th to 20th	Day	14% 15% 6% 11% 10% 5% 7% 5% 6% 5% 17%	1671
	21st to end	Day	16% 21% 10% 16% 12% 4 4 3 4 2 8%	2431
January	1st to 10th	Night	11% 18% 8% 14% 11% 7% 8% 6% 5% 4 10%	4748
	11th to 20th	Night	9% 14% 8% 15% 12% 5% 7% 9% 6% 4 10%	4760
	21st to end	Night	11% 21% 10% 13% 8% 5% 6% 5% 5% 4 12%	4909
February	1st to 10th	Night	8% 13% 5% 11% 10% 7% 10% 6% 6% 8% 17%	2399
	11th to 20th	Night	10% 16% 8% 11% 8% 5% 7% 4 6% 7% 19%	2273
	21st to end	Night	9% 17% 7% 12% 7% 4 8% 6% 5% 5% 19%	1888
March	1st to 10th	Night	12% 11% 6% 12% 11% 4 7% 5% 4 5% 24%	2065
	11th to 20th	Night	16% 14% 6% 9% 9% 5% 5% 4 4 5% 24%	1979
	21st to end	Night	14% 13% 4 8% 5% 4 6% 4 5% 5% 31%	2178
April	1st to 10th	Night	19% 14% 5% 9% 7% 3 4 4 5% 6% 24%	1798
	11th to 20th	Night	17% 11% 4 10% 5% 3 5% 5% 4 6% 30%	1597
	21st to end	Night	19% 11% 6% 6% 4 3 3 4 3 5% 36%	1620
May	1st to 10th	Night	17% 11% 4 7% 5% 2 4 5% 5% 7% 33%	1523
	11th to 20th	Night	20% 8% 5% 5% 5% 3 4 4 6% 5% 35%	1390
	21st to end	Night	19% 7% 2 6% 5% 3 4 4 5% 7% 38%	1357
June	1st to 10th	Night	21% 9% 3 6% 5% 3 5% 4 4 7% 33%	1260
	11th to 20th	Night	16% 8% 3 5% 6% 3 5% 4 5% 8% 38%	1260
	21st to end	Night	19% 7% 3 5% 4 3 4 5% 6% 8% 35%	1260
July	1st to 10th	Night	17% 7% 3 5% 5% 3 4 5% 4 8% 38%	1260
	11th to 20th	Night	14% 6% 3 5% 6% 4 5% 5% 6% 9% 37%	1260
	21st to end	Night	14% 7% 3 6% 7% 4 4 5% 6% 8% 35%	1378
August	1st to 10th	Night	15% 6% 3 7% 7% 4 4 5% 5% 8% 35%	1595
	11th to 20th	Night	16% 6% 2 5% 5% 3 5% 5% 5% 8% 40%	1598
	21st to end	Night	16% 7% 3 6% 6% 2 4 5% 5% 7% 39%	1782
September	1st to 10th	Night	20% 6% 3 6% 5% 3 4 4 5% 7% 39%	1749
	11th to 20th	Night	25% 9% 3 5% 5% 2 3 4 5% 6% 32%	1949
	21st to end	Night	24% 9% 3 5% 5% 3 4 3 4 6% 34%	1980
October	1st to 10th	Night	22% 12% 3 6% 5% 3 4 4 5% 6% 30%	2159
	11th to 20th	Night	29% 19% 5% 7% 6% 3 3 4 4 5% 17%	2160
	21st to end	Night	22% 22% 6% 8% 6% 3 4 3 3 6% 18%	2538
November	1st to 10th	Night	20% 18% 8% 10% 7% 3 5% 3 4 4 17%	2592
	11th to 20th	Night	22% 19% 9% 10% 8% 4 4 3 3 3 15%	2660
	21st to end	Night	20% 20% 7% 13% 9% 4 5% 3 3 4 11%	2669
December	1st to 10th	Night	13% 21% 8% 15% 11% 5% 4 4 4 4 12%	2850
	11th to 20th	Night	15% 18% 8% 11% 12% 6% 6% 5% 4 3 11%	3526
	21st to end	Night	18% 21% 8% 14% 11% 4 5% 4 3 3 8%	5236

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	78% 5% 8% 7% 2	2209
	11th to 20th	Day	69% 4 8% 14% 4	2210
	21st to end	Day	75% 4 5% 11% 4	2754
February	1st to 10th	Day	64% 6% 8% 16% 6%	1656
	11th to 20th	Day	65% 5% 8% 14% 7%	1582
	21st to end	Day	66% 5% 7% 16% 7% 1	1365
March	1st to 10th	Day	64% 5% 8% 14% 9% 1	1885
	11th to 20th	Day	61% 4 8% 16% 11% 1	1980
	21st to end	Day	57% 4 6% 18% 13% 2	2178
April	1st to 10th	Day	66% 4 7% 14% 9%	2178
	11th to 20th	Day	56% 4 9% 20% 10% 1	2312
	21st to end	Day	56% 4 9% 16% 13% 3	2340
May	1st to 10th	Day	58% 5% 9% 15% 11% 2	2435
	11th to 20th	Day	53% 6% 10% 21% 10% 1	2507
	21st to end	Day	51% 7% 10% 20% 10% 1	2919
June	1st to 10th	Day	53% 6% 11% 19% 10% 1	2700
	11th to 20th	Day	46% 7% 11% 22% 14%	2700
	21st to end	Day	52% 7% 11% 21% 8% 1	2700
July	1st to 10th	Day	48% 9% 12% 24% 7%	2626
	11th to 20th	Day	47% 9% 16% 22% 7%	2619
	21st to end	Day	50% 9% 16% 21% 3	2773
August	1st to 10th	Day	52% 8% 16% 20% 3	2327
	11th to 20th	Day	49% 7% 17% 19% 8% 1	2303
	21st to end	Day	49% 8% 16% 22% 6%	2574
September	1st to 10th	Day	49% 7% 15% 20% 9%	2192
	11th to 20th	Day	55% 7% 17% 18% 4	2008
	21st to end	Day	57% 5% 10% 21% 7%	1980
October	1st to 10th	Day	58% 5% 14% 18% 6%	1800
	11th to 20th	Day	73% 4 8% 12% 3	1800
	21st to end	Day	77% 4 7% 8% 3	1814
November	1st to 10th	Day	72% 6% 11% 8% 3	1486
	11th to 20th	Day	79% 4 8% 8% 2	1520
	21st to end	Day	81% 4 7% 7% 1	1507
December	1st to 10th	Day	76% 4 8% 8% 3	1330
	11th to 20th	Day	68% 5% 9% 14% 4	1671
	21st to end	Day	84% 3 5% 6% 2	2431
January	1st to 10th	Night	77% 5% 10% 7% 1	4748
	11th to 20th	Night	73% 6% 9% 10% 3	4760
	21st to end	Night	74% 4 7% 11% 3	4909
February	1st to 10th	Night	64% 5% 11% 13% 6%	2399
	11th to 20th	Night	65% 3 10% 15% 7%	2275
	21st to end	Night	65% 5% 10% 14% 6% 1	1888
March	1st to 10th	Night	63% 4 8% 14% 10% 1	2075
	11th to 20th	Night	63% 4 8% 16% 8% 1	1980
	21st to end	Night	54% 4 9% 16% 14% 3	2178
April	1st to 10th	Night	61% 4 9% 14% 11% 1	1798
	11th to 20th	Night	55% 5% 8% 18% 11% 3	1597
	21st to end	Night	52% 5% 8% 15% 18% 2	1620
May	1st to 10th	Night	50% 6% 10% 21% 10% 2	1523
	11th to 20th	Night	50% 6% 9% 20% 14% 2	1391
	21st to end	Night	46% 7% 13% 19% 14% 1	1358
June	1st to 10th	Night	51% 7% 9% 19% 12% 2	1260
	11th to 20th	Night	44% 7% 10% 22% 16% 1	1260
	21st to end	Night	45% 8% 14% 19% 12% 2	1260
July	1st to 10th	Night	43% 7% 14% 23% 11% 1	1225
	11th to 20th	Night	42% 8% 16% 25% 9%	1218
	21st to end	Night	46% 10% 15% 26% 3	1301
August	1st to 10th	Night	46% 8% 17% 24% 3	1585
	11th to 20th	Night	41% 9% 18% 25% 7%	1598
	21st to end	Night	44% 8% 16% 24% 8%	1782
September	1st to 10th	Night	46% 7% 14% 25% 8%	1749
	11th to 20th	Night	53% 7% 16% 20% 4	1949
	21st to end	Night	52% 7% 15% 23% 4	1980
October	1st to 10th	Night	55% 6% 14% 18% 6%	2159
	11th to 20th	Night	71% 5% 9% 12% 2	2160
	21st to end	Night	69% 6% 10% 10% 4	2538
November	1st to 10th	Night	71% 6% 10% 10% 3	2592
	11th to 20th	Night	76% 5% 9% 7% 2	2660
	21st to end	Night	80% 5% 8% 6% 1	2669
December	1st to 10th	Night	78% 4 8% 7% 3	2850
	11th to 20th	Night	76% 5% 8% 9% 2	3525
	21st to end	Night	81% 5% 7% 5% 2	5236

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	26% 19% 33% 14% 7% 1	180
	11th to 20th	Day	17% 19% 38% 21% 4 1	180
	21st to end	Day	18% 15% 35% 28% 3 1	288
February	1st to 10th	Day	17% 25% 38% 15% 3 2	252
	11th to 20th	Day	21% 18% 38% 18% 3 3	240
	21st to end	Day	26% 27% 34% 12% 1	198
March	1st to 10th	Day	18% 22% 38% 18% 4	325
	11th to 20th	Day	25% 17% 34% 18% 5% 1	420
	21st to end	Day	24% 19% 34% 16% 6%	462
April	1st to 10th	Day	23% 20% 33% 18% 5% 1	420
	11th to 20th	Day	26% 20% 29% 17% 6% 3	412
	21st to end	Day	26% 19% 27% 20% 7% 2	420
May	1st to 10th	Day	26% 15% 31% 20% 7% 1	423
	11th to 20th	Day	23% 18% 33% 18% 6% 3	472
	21st to end	Day	25% 15% 31% 23% 5% 1	679
June	1st to 10th	Day	26% 14% 33% 21% 5% 1	630
	11th to 20th	Day	25% 14% 34% 19% 7% 1	630
	21st to end	Day	21% 13% 29% 26% 10% 1	630
July	1st to 10th	Day	21% 10% 35% 24% 7% 2	621
	11th to 20th	Day	17% 12% 33% 28% 8% 1	618
	21st to end	Day	18% 10% 31% 27% 11% 3	684
August	1st to 10th	Day	20% 10% 30% 25% 12% 3	460
	11th to 20th	Day	22% 8% 26% 26% 13% 5%	439
	21st to end	Day	22% 10% 29% 22% 11% 5%	495
September	1st to 10th	Day	21% 16% 28% 20% 12% 2	450
	11th to 20th	Day	31% 11% 28% 20% 8% 2	450
	21st to end	Day	28% 13% 30% 19% 8% 2	450
October	1st to 10th	Day	31% 14% 28% 18% 7% 2	450
	11th to 20th	Day	39% 19% 27% 12% 1 2	450
	21st to end	Day	34% 19% 31% 13% 3	329
November	1st to 10th	Day	37% 22% 29% 12% 1	180
	11th to 20th	Day	29% 26% 28% 13% 4	180
	21st to end	Day	30% 28% 29% 7% 5%	180
December	1st to 10th	Day	27% 17% 31% 16% 7% 2	180
	11th to 20th	Day	22% 22% 35% 16% 4 1	180
	21st to end	Day	27% 18% 32% 18% 3 3	198
January	1st to 10th	Night	22% 14% 40% 18% 6% 1	950
	11th to 20th	Night	21% 13% 31% 29% 7%	950
	21st to end	Night	17% 11% 31% 29% 11% 1	1038
February	1st to 10th	Night	13% 13% 38% 25% 9% 1	739
	11th to 20th	Night	19% 13% 32% 28% 8%	703
	21st to end	Night	19% 12% 39% 22% 7% 1	592
March	1st to 10th	Night	18% 14% 36% 25% 6% 1	720
	11th to 20th	Night	25% 17% 29% 23% 5% 1	720
	21st to end	Night	20% 13% 35% 23% 8% 1	792
April	1st to 10th	Night	26% 16% 29% 23% 6%	666
	11th to 20th	Night	22% 16% 27% 26% 8% 1	533
	21st to end	Night	24% 10% 32% 25% 9% 1	540
May	1st to 10th	Night	21% 10% 34% 27% 7%	443
	11th to 20th	Night	23% 12% 31% 25% 8% 1	354
	21st to end	Night	21% 8% 34% 31% 4 1	389
June	1st to 10th	Night	27% 9% 26% 29% 8%	360
	11th to 20th	Night	20% 9% 31% 31% 9% 1	360
	21st to end	Night	20% 11% 24% 35% 11% 1	360
July	1st to 10th	Night	20% 7% 29% 31% 10% 2	350
	11th to 20th	Night	17% 7% 32% 32% 12%	348
	21st to end	Night	16% 8% 25% 37% 14% 1	394
August	1st to 10th	Night	17% 8% 25% 33% 14% 2	537
	11th to 20th	Night	18% 7% 22% 32% 19% 1	534
	21st to end	Night	20% 8% 27% 27% 16% 1	594
September	1st to 10th	Night	25% 8% 28% 26% 12% 1	669
	11th to 20th	Night	28% 10% 30% 20% 10% 1	720
	21st to end	Night	27% 10% 26% 25% 10% 2	720
October	1st to 10th	Night	30% 10% 28% 22% 9% 1	720
	11th to 20th	Night	38% 14% 27% 15% 4 2	720
	21st to end	Night	33% 12% 30% 19% 6% 1	792
November	1st to 10th	Night	31% 14% 34% 17% 2 1	740
	11th to 20th	Night	34% 15% 30% 16% 5%	760
	21st to end	Night	31% 16% 34% 15% 4	769
December	1st to 10th	Night	26% 16% 30% 20% 7% 1	950
	11th to 20th	Night	24% 15% 34% 20% 7% 1	950
	21st to end	Night	28% 14% 30% 19% 7% 2	1045

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