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 2024-02-25.

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	9% 8% 4 7% 6% 4 5% 6% 6% 8% 37%	77876
	Night	6% 9% 4 7% 6% 4 5% 6% 6% 8% 38%	81383
Transparency	Day	43% 8% 14% 23% 10% 2	77583
	Night	42% 7% 14% 23% 11% 2	81256
Seeing	Day	15% 13% 31% 26% 12% 3	14280
	Night	11% 10% 33% 32% 12% 1	23585

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	2 6% 6% 11% 8% 4 5% 5% 7% 10% 36%	2219
	11th to 20th	Day	2 8% 4 9% 7% 5% 6% 6% 8% 7% 37%	2267
	21st to end	Day	3 8% 3 7% 6% 4 5% 6% 7% 10% 41%	2794
February	1st to 10th	Day	3 9% 4 9% 10% 4 6% 6% 7% 8% 33%	1520
	11th to 20th	Day	3 8% 5% 9% 9% 5% 7% 7% 6% 8% 33%	1631
	21st to end	Day	3 8% 7% 11% 9% 5% 7% 6% 6% 7% 32%	1497
March	1st to 10th	Day	3 9% 5% 9% 10% 5% 5% 7% 6% 8% 34%	1800
	11th to 20th	Day	4 8% 5% 7% 7% 4 6% 5% 7% 8% 38%	1938
	21st to end	Day	6% 15% 7% 10% 8% 4 6% 6% 6% 7% 27%	2231
April	1st to 10th	Day	8% 13% 6% 10% 7% 4 5% 6% 6% 8% 27%	2196
	11th to 20th	Day	11% 15% 5% 8% 6% 4 5% 4 6% 7% 29%	2310
	21st to end	Day	12% 12% 4 7% 6% 4 7% 5% 5% 7% 31%	2340
May	1st to 10th	Day	14% 12% 5% 7% 7% 4 4 6% 6% 7% 28%	2472
	11th to 20th	Day	14% 8% 3 6% 6% 3 6% 5% 7% 8% 33%	2654
	21st to end	Day	21% 9% 3 6% 7% 4 6% 5% 6% 6% 26%	2917
June	1st to 10th	Day	20% 8% 3 6% 6% 4 6% 7% 6% 7% 28%	2700
	11th to 20th	Day	23% 8% 3 7% 7% 4 6% 5% 6% 7% 24%	2700
	21st to end	Day	10% 5% 2 5% 6% 4 6% 6% 7% 9% 38%	2700
July	1st to 10th	Day	11% 5% 2 6% 5% 3 6% 6% 7% 8% 40%	2700
	11th to 20th	Day	12% 4 2 4 5% 3 5% 6% 7% 8% 43%	2700
	21st to end	Day	7% 3 2 4 4 3 5% 5% 7% 8% 51%	2856
August	1st to 10th	Day	12% 5% 2 4 5% 3 5% 6% 7% 8% 44%	2507
	11th to 20th	Day	9% 4 2 5% 5% 2 5% 5% 6% 8% 48%	2346
	21st to end	Day	11% 5% 2 5% 5% 4 4 5% 6% 9% 46%	2574
September	1st to 10th	Day	11% 6% 2 5% 4 2 4 5% 6% 7% 47%	2227
	11th to 20th	Day	14% 7% 2 5% 5% 3 4 4 5% 7% 44%	1982
	21st to end	Day	10% 7% 3 5% 5% 3 4 5% 6% 7% 45%	1980
October	1st to 10th	Day	12% 9% 3 6% 5% 3 5% 5% 6% 7% 39%	1918
	11th to 20th	Day	8% 9% 3 8% 6% 3 5% 5% 5% 9% 38%	1643
	21st to end	Day	7% 11% 4 7% 5% 3 5% 6% 7% 7% 37%	1782
November	1st to 10th	Day	6% 13% 5% 7% 6% 3 5% 6% 6% 7% 34%	1570
	11th to 20th	Day	4 9% 4 7% 7% 4 5% 6% 5% 6% 43%	1421
	21st to end	Day	3 10% 5% 8% 7% 3 4 5% 6% 9% 40%	1330
December	1st to 10th	Day	4 8% 5% 10% 8% 4 6% 5% 7% 9% 35%	1330
	11th to 20th	Day	3 9% 6% 11% 8% 5% 7% 6% 6% 8% 32%	1682
	21st to end	Day	2 9% 4 6% 6% 3 4 6% 6% 8% 45%	2442
January	1st to 10th	Night	3 11% 5% 11% 9% 4 7% 7% 7% 9% 28%	4726
	11th to 20th	Night	3 6% 5% 12% 8% 4 6% 8% 6% 8% 34%	4658
	21st to end	Night	3 10% 4 9% 7% 4 5% 5% 6% 8% 37%	4862
February	1st to 10th	Night	4 9% 5% 8% 8% 5% 7% 6% 6% 9% 33%	2660
	11th to 20th	Night	4 11% 5% 9% 8% 4 6% 6% 6% 9% 31%	2436
	21st to end	Night	2 8% 5% 9% 10% 5% 6% 5% 6% 8% 36%	1852
March	1st to 10th	Night	3 9% 5% 9% 8% 6% 6% 6% 7% 8% 33%	2160
	11th to 20th	Night	3 9% 5% 7% 5% 4 7% 6% 8% 10% 38%	2021
	21st to end	Night	5% 12% 7% 8% 7% 4 6% 6% 7% 8% 31%	2124
April	1st to 10th	Night	6% 15% 5% 9% 7% 4 4 5% 5% 8% 32%	1764
	11th to 20th	Night	10% 13% 5% 9% 6% 3 4 4 6% 7% 32%	1601
	21st to end	Night	10% 12% 3 8% 6% 3 4 4 6% 9% 34%	1620
May	1st to 10th	Night	13% 11% 4 7% 6% 3 6% 6% 6% 7% 32%	1486
	11th to 20th	Night	12% 7% 3 6% 5% 3 4 5% 6% 8% 41%	1239
	21st to end	Night	19% 8% 3 6% 5% 3 5% 6% 6% 7% 33%	1358
June	1st to 10th	Night	17% 6% 4 7% 5% 3 4 7% 5% 9% 33%	1260
	11th to 20th	Night	21% 6% 3 5% 4 3 5% 6% 7% 8% 31%	1260
	21st to end	Night	8% 4 2 5% 6% 5% 5% 6% 6% 11% 42%	1260
July	1st to 10th	Night	9% 4 2 4 5% 3 5% 5% 6% 8% 49%	1260
	11th to 20th	Night	10% 5% 2 3 5% 3 4 7% 5% 8% 46%	1260
	21st to end	Night	6% 3 2 4 4 3 4 4 5% 8% 59%	1459
August	1st to 10th	Night	11% 4 1 3 5% 3 3 5% 8% 8% 49%	1432
	11th to 20th	Night	10% 4 2 3 3 2 3 5% 7% 10% 51%	1554
	21st to end	Night	10% 4 2 3 4 3 4 4 6% 9% 52%	1782
September	1st to 10th	Night	11% 6% 2 4 4 3 3 4 6% 8% 49%	1732
	11th to 20th	Night	14% 7% 3 4 4 2 3 4 5% 7% 47%	1977
	21st to end	Night	10% 8% 3 5% 4 2 4 4 6% 8% 47%	1980
October	1st to 10th	Night	10% 9% 3 6% 5% 4 5% 5% 5% 8% 41%	2042
	11th to 20th	Night	8% 8% 3 5% 6% 3 5% 5% 7% 9% 42%	2316
	21st to end	Night	7% 9% 4 6% 6% 3 5% 5% 6% 9% 39%	2574
November	1st to 10th	Night	6% 13% 5% 7% 5% 3 5% 6% 5% 8% 37%	2489
	11th to 20th	Night	4 7% 4 7% 7% 4 5% 6% 6% 8% 42%	2755
	21st to end	Night	3 9% 5% 8% 6% 4 5% 5% 7% 9% 39%	2850
December	1st to 10th	Night	4 10% 5% 8% 7% 4 7% 5% 6% 8% 35%	2850
	11th to 20th	Night	2 9% 4 9% 9% 4 6% 6% 7% 9% 33%	3510
	21st to end	Night	3 8% 4 7% 6% 4 6% 6% 5% 7% 43%	5214

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	41% 7% 12% 27% 11% 3	2219
	11th to 20th	Day	42% 8% 9% 18% 19% 4	2288
	21st to end	Day	36% 8% 15% 31% 9% 1	2794
February	1st to 10th	Day	48% 8% 12% 20% 9% 3	1520
	11th to 20th	Day	46% 7% 14% 22% 9% 2	1640
	21st to end	Day	49% 6% 11% 20% 12% 1	1497
March	1st to 10th	Day	46% 9% 15% 20% 9% 1	1800
	11th to 20th	Day	42% 7% 13% 25% 12% 2	1939
	21st to end	Day	55% 8% 13% 17% 7% 1	2231
April	1st to 10th	Day	53% 8% 13% 18% 7%	2196
	11th to 20th	Day	54% 6% 10% 19% 10% 1	2310
	21st to end	Day	52% 5% 13% 20% 8% 1	2340
May	1st to 10th	Day	54% 7% 13% 19% 7%	2472
	11th to 20th	Day	46% 8% 13% 23% 10% 1	2658
	21st to end	Day	55% 7% 12% 20% 6% 1	2918
June	1st to 10th	Day	53% 8% 14% 17% 7% 1	2700
	11th to 20th	Day	57% 9% 12% 15% 6% 2	2700
	21st to end	Day	39% 9% 17% 24% 9% 2	2700
July	1st to 10th	Day	39% 9% 17% 22% 11% 2	2626
	11th to 20th	Day	36% 9% 16% 26% 10% 2	2619
	21st to end	Day	28% 8% 16% 32% 14% 2	2697
August	1st to 10th	Day	36% 8% 18% 28% 9% 1	2490
	11th to 20th	Day	31% 6% 23% 29% 9% 2	2346
	21st to end	Day	33% 9% 16% 29% 11% 2	2574
September	1st to 10th	Day	35% 7% 15% 26% 14% 4	2227
	11th to 20th	Day	39% 7% 15% 25% 13% 2	1982
	21st to end	Day	39% 7% 14% 26% 12% 2	1980
October	1st to 10th	Day	43% 8% 13% 24% 11% 1	1918
	11th to 20th	Day	42% 6% 15% 23% 10% 3	1643
	21st to end	Day	43% 8% 13% 22% 13% 1	1782
November	1st to 10th	Day	48% 7% 12% 20% 12% 1	1570
	11th to 20th	Day	40% 7% 15% 25% 11% 1	1421
	21st to end	Day	41% 7% 16% 24% 10% 3	1330
December	1st to 10th	Day	45% 7% 13% 20% 13% 3	1330
	11th to 20th	Day	45% 10% 15% 19% 10% 1	1684
	21st to end	Day	36% 6% 10% 24% 20% 5%	2442
January	1st to 10th	Night	48% 9% 13% 20% 8% 2	4726
	11th to 20th	Night	46% 7% 10% 18% 14% 6%	4699
	21st to end	Night	42% 7% 15% 24% 11% 1	4862
February	1st to 10th	Night	46% 8% 11% 21% 12% 3	2660
	11th to 20th	Night	47% 8% 14% 18% 13% 1	2437
	21st to end	Night	46% 6% 9% 24% 13% 2	1852
March	1st to 10th	Night	46% 7% 13% 25% 8% 1	2160
	11th to 20th	Night	39% 8% 16% 22% 13% 1	2021
	21st to end	Night	49% 7% 15% 19% 9% 2	2124
April	1st to 10th	Night	50% 7% 12% 19% 11% 1	1764
	11th to 20th	Night	51% 6% 12% 21% 9% 1	1601
	21st to end	Night	47% 8% 14% 21% 8% 2	1620
May	1st to 10th	Night	50% 6% 14% 21% 9% 1	1486
	11th to 20th	Night	39% 7% 14% 30% 9% 1	1239
	21st to end	Night	49% 8% 14% 22% 6% 1	1358
June	1st to 10th	Night	47% 8% 14% 21% 9%	1260
	11th to 20th	Night	49% 8% 15% 18% 8% 2	1260
	21st to end	Night	35% 9% 19% 27% 8% 2	1260
July	1st to 10th	Night	31% 10% 21% 26% 11% 1	1225
	11th to 20th	Night	35% 9% 19% 25% 11% 1	1218
	21st to end	Night	25% 7% 16% 32% 18% 2	1378
August	1st to 10th	Night	32% 8% 18% 31% 9% 2	1424
	11th to 20th	Night	26% 9% 22% 30% 12% 1	1554
	21st to end	Night	28% 7% 15% 32% 16% 2	1782
September	1st to 10th	Night	34% 6% 17% 27% 14% 3	1732
	11th to 20th	Night	36% 6% 15% 27% 12% 4	1977
	21st to end	Night	34% 6% 17% 25% 14% 3	1980
October	1st to 10th	Night	41% 6% 14% 24% 12% 2	2042
	11th to 20th	Night	37% 7% 16% 25% 12% 3	2316
	21st to end	Night	41% 7% 15% 25% 11% 1	2574
November	1st to 10th	Night	45% 6% 13% 22% 12% 1	2489
	11th to 20th	Night	39% 7% 15% 23% 15% 2	2755
	21st to end	Night	41% 7% 15% 22% 11% 4	2850
December	1st to 10th	Night	45% 8% 12% 21% 11% 3	2850
	11th to 20th	Night	43% 10% 16% 22% 8% 1	3507
	21st to end	Night	38% 6% 12% 25% 13% 5%	5214

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	6% 13% 37% 26% 14% 6%	180
	11th to 20th	Day	9% 11% 34% 29% 16% 1	180
	21st to end	Day	10% 8% 37% 34% 8% 4	198
February	1st to 10th	Day	12% 16% 40% 21% 7% 4	180
	11th to 20th	Day	10% 16% 36% 31% 6% 1	254
	21st to end	Day	8% 15% 35% 29% 11% 2	339
March	1st to 10th	Day	13% 18% 34% 25% 8% 3	420
	11th to 20th	Day	11% 18% 38% 24% 7% 3	420
	21st to end	Day	15% 22% 32% 20% 7% 3	462
April	1st to 10th	Day	17% 21% 35% 20% 6%	420
	11th to 20th	Day	19% 20% 30% 21% 8% 2	412
	21st to end	Day	21% 20% 29% 21% 8% 1	420
May	1st to 10th	Day	20% 17% 25% 24% 11% 2	445
	11th to 20th	Day	17% 12% 25% 29% 13% 4	625
	21st to end	Day	22% 10% 22% 27% 16% 2	679
June	1st to 10th	Day	20% 12% 30% 25% 11% 2	630
	11th to 20th	Day	26% 12% 28% 23% 9% 2	630
	21st to end	Day	12% 9% 27% 30% 18% 4	630
July	1st to 10th	Day	13% 8% 28% 28% 19% 4	621
	11th to 20th	Day	14% 7% 29% 32% 15% 4	618
	21st to end	Day	9% 8% 30% 33% 15% 4	602
August	1st to 10th	Day	14% 8% 32% 30% 12% 4	447
	11th to 20th	Day	9% 9% 30% 32% 16% 4	437
	21st to end	Day	12% 8% 32% 26% 15% 6%	495
September	1st to 10th	Day	14% 12% 37% 24% 12% 2	450
	11th to 20th	Day	17% 10% 30% 27% 13% 3	450
	21st to end	Day	16% 19% 29% 25% 10% 2	450
October	1st to 10th	Day	15% 15% 28% 26% 12% 3	388
	11th to 20th	Day	14% 11% 36% 25% 11% 2	270
	21st to end	Day	19% 15% 35% 21% 9% 1	297
November	1st to 10th	Day	16% 14% 37% 26% 5% 2	270
	11th to 20th	Day	15% 14% 40% 25% 5%	223
	21st to end	Day	7% 14% 38% 23% 13% 5%	180
December	1st to 10th	Day	7% 11% 42% 29% 10% 1	180
	11th to 20th	Day	8% 16% 43% 22% 8% 3	180
	21st to end	Day	8% 8% 41% 29% 11% 3	198
January	1st to 10th	Night	10% 13% 41% 28% 7% 1	950
	11th to 20th	Night	9% 11% 43% 30% 6% 1	922
	21st to end	Night	10% 16% 37% 28% 8% 1	836
February	1st to 10th	Night	8% 14% 34% 30% 12% 3	760
	11th to 20th	Night	10% 12% 35% 32% 10% 1	744
	21st to end	Night	6% 10% 39% 27% 16% 2	610
March	1st to 10th	Night	7% 10% 41% 31% 10% 2	720
	11th to 20th	Night	7% 12% 37% 32% 10% 2	720
	21st to end	Night	12% 9% 33% 33% 11% 2	792
April	1st to 10th	Night	12% 12% 31% 30% 13% 1	684
	11th to 20th	Night	18% 8% 35% 28% 11%	533
	21st to end	Night	16% 13% 31% 30% 11%	540
May	1st to 10th	Night	19% 12% 22% 34% 12%	428
	11th to 20th	Night	15% 6% 28% 34% 16% 1	354
	21st to end	Night	20% 6% 21% 36% 15% 1	389
June	1st to 10th	Night	22% 8% 23% 34% 12% 2	360
	11th to 20th	Night	26% 4 24% 31% 14% 1	360
	21st to end	Night	11% 3 19% 42% 22% 3	360
July	1st to 10th	Night	11% 3 20% 40% 25% 2	350
	11th to 20th	Night	12% 3 25% 42% 16% 2	348
	21st to end	Night	6% 4 19% 44% 25% 2	394
August	1st to 10th	Night	13% 3 13% 41% 28% 3	358
	11th to 20th	Night	11% 3 20% 40% 23% 2	486
	21st to end	Night	11% 6% 19% 41% 22% 1	594
September	1st to 10th	Night	15% 5% 23% 36% 18% 3	652
	11th to 20th	Night	18% 5% 21% 36% 19% 2	720
	21st to end	Night	13% 8% 29% 35% 13% 3	720
October	1st to 10th	Night	15% 6% 29% 33% 15% 1	720
	11th to 20th	Night	11% 8% 33% 34% 12% 2	720
	21st to end	Night	12% 12% 33% 36% 6% 1	792
November	1st to 10th	Night	15% 13% 35% 29% 7% 1	824
	11th to 20th	Night	9% 15% 44% 27% 4 1	950
	21st to end	Night	8% 12% 42% 28% 9% 1	950
December	1st to 10th	Night	9% 13% 39% 28% 10% 1	950
	11th to 20th	Night	6% 13% 39% 34% 8% 1	950
	21st to end	Night	8% 11% 41% 29% 10% 1	1045

Page generated at 2024-02-22 04:05:42. Version ac07

Copyright 2024 Attilla Danko