Cossatot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cossatot Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	15% 6% 2 5% 4 4 4 3 5% 6% 45%	131691
	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	130426
Transparency	Day	38% 5% 10% 22% 19% 6%	135301
	Night	34% 4 9% 22% 22% 8%	134893
Seeing	Day	18% 11% 26% 26% 15% 4	17117
	Night	17% 10% 26% 29% 16% 2	27068

By Month

Percentage Cloud Cover SE (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	16% 7% 2 5% 3 3 3 3 4 6% 47%	5289
	11th to 20th	Day	7% 6% 2 5% 4 3 3 3 4 5% 58%	5310
	21st to end	Day	22% 6% 2 5% 3 3 3 3 3 4 47%	5812
February	1st to 10th	Day	24% 7% 3 5% 4 3 3 3 4 4 40%	1890
	11th to 20th	Day	21% 7% 3 6% 4 3 4 3 4 5% 41%	1917
	21st to end	Day	25% 8% 3 4 3 3 2 2 4 5% 42%	1903
March	1st to 10th	Day	24% 9% 3 6% 3 3 3 3 4 5% 40%	5046
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 6% 48%	5090
	21st to end	Day	19% 8% 3 6% 4 4 3 3 4 6% 40%	5583
April	1st to 10th	Day	20% 6% 2 6% 4 2 4 3 4 6% 43%	2301
	11th to 20th	Day	21% 5% 2 5% 4 4 4 3 4 6% 42%	2374
	21st to end	Day	21% 8% 2 6% 4 4 4 3 4 5% 39%	2400
May	1st to 10th	Day	23% 8% 2 5% 4 3 4 3 5% 6% 37%	5698
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	5862
	21st to end	Day	20% 8% 3 7% 5% 4 4 4 5% 6% 35%	6434
June	1st to 10th	Day	10% 6% 2 7% 4 5% 5% 4 6% 9% 42%	2600
	11th to 20th	Day	9% 7% 3 7% 5% 5% 6% 5% 6% 8% 41%	2600
	21st to end	Day	6% 5% 2 5% 4 5% 5% 5% 7% 9% 48%	2600
July	1st to 10th	Day	9% 5% 2 7% 6% 6% 6% 5% 7% 11% 36%	5900
	11th to 20th	Day	6% 3 1 4 3 4 3 3 5% 8% 59%	5900
	21st to end	Day	7% 6% 2 7% 5% 6% 6% 5% 7% 8% 40%	6422
August	1st to 10th	Day	3 2 1 4 4 4 4 5% 6% 9% 58%	4234
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 5% 7% 9% 47%	4048
	21st to end	Day	6% 4 2 5% 5% 5% 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 3 5% 6% 49%	2400
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 6% 7% 45%	2209
	21st to end	Day	11% 4 2 4 3 4 4 3 5% 7% 53%	2180
October	1st to 10th	Day	9% 5% 2 5% 4 4 3 3 4 6% 57%	2000
	11th to 20th	Day	16% 4 2 4 2 2 2 2 3 5% 58%	2000
	21st to end	Day	21% 6% 1 6% 3 3 3 2 4 4 49%	2200
November	1st to 10th	Day	18% 5% 2 4 3 3 2 2 4 6% 51%	2058
	11th to 20th	Day	15% 6% 2 3 3 3 4 3 4 5% 54%	2100
	21st to end	Day	21% 7% 2 4 3 2 2 2 3 4 49%	1921
December	1st to 10th	Day	20% 8% 2 4 3 2 3 3 4 4 49%	3030
	11th to 20th	Day	24% 9% 2 4 3 2 3 2 3 5% 43%	3481
	21st to end	Day	24% 9% 2 6% 3 3 3 3 4 4 39%	4587
January	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 4 5% 49%	7634
	11th to 20th	Night	9% 5% 2 3 1 2 2 2 3 4 68%	7680
	21st to end	Night	24% 8% 2 4 3 3 3 2 4 4 44%	8402
February	1st to 10th	Night	20% 7% 3 5% 3 3 4 3 4 4 44%	2730
	11th to 20th	Night	21% 7% 3 6% 3 3 3 3 4 5% 42%	2605
	21st to end	Night	19% 7% 2 4 3 3 2 2 4 5% 49%	1903
March	1st to 10th	Night	23% 8% 2 5% 3 3 3 3 3 5% 42%	5029
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 6% 51%	5090
	21st to end	Night	17% 6% 2 5% 4 3 3 3 5% 7% 45%	5588
April	1st to 10th	Night	16% 6% 2 5% 4 3 3 4 4 5% 48%	2120
	11th to 20th	Night	15% 7% 2 5% 4 3 4 3 4 6% 48%	1975
	21st to end	Night	17% 6% 2 6% 3 3 2 3 5% 7% 48%	2000
May	1st to 10th	Night	24% 7% 2 5% 4 3 4 3 4 7% 38%	4193
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 40%	4053
	21st to end	Night	18% 6% 2 6% 4 4 4 3 5% 7% 42%	4443
June	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 9% 54%	1800
	11th to 20th	Night	9% 5% 1 4 4 2 3 3 6% 8% 55%	1800
	21st to end	Night	5% 3 1 3 3 3 3 3 6% 8% 61%	1800
July	1st to 10th	Night	7% 4 2 6% 5% 4 5% 5% 8% 12% 43%	4078
	11th to 20th	Night	5% 3 1 3 2 2 3 2 5% 9% 64%	4077
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 6% 8% 49%	4449
August	1st to 10th	Night	3 2 1 2 2 2 3 3 6% 8% 68%	2926
	11th to 20th	Night	5% 2 1 3 3 4 4 4 5% 10% 59%	3006
	21st to end	Night	4 3 2 4 4 4 4 3 6% 9% 58%	3586
September	1st to 10th	Night	6% 4 2 5% 4 3 3 3 4 7% 57%	2000
	11th to 20th	Night	9% 5% 2 5% 3 5% 4 3 6% 7% 51%	2187
	21st to end	Night	9% 3 2 4 3 3 4 3 4 6% 58%	2200
October	1st to 10th	Night	8% 4 1 4 3 2 3 2 5% 7% 61%	2400
	11th to 20th	Night	13% 4 2 4 3 2 2 2 4 4 61%	2400
	21st to end	Night	18% 6% 2 5% 3 2 2 1 3 4 54%	2640
November	1st to 10th	Night	15% 5% 2 4 3 3 3 2 4 4 57%	2460
	11th to 20th	Night	13% 5% 2 4 3 2 2 2 4 6% 57%	2520
	21st to end	Night	18% 6% 2 4 3 2 3 2 3 5% 50%	2693
December	1st to 10th	Night	15% 8% 2 5% 3 3 3 3 4 5% 50%	4380
	11th to 20th	Night	22% 7% 2 6% 4 3 2 2 3 3 45%	4946
	21st to end	Night	20% 10% 3 6% 3 3 3 2 3 4 43%	6633

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	40% 3 9% 27% 19% 3	5290
	11th to 20th	Day	29% 3 8% 22% 31% 7%	5310
	21st to end	Day	43% 2 4 17% 22% 11%	5812
February	1st to 10th	Day	49% 4 8% 18% 18% 4	1890
	11th to 20th	Day	47% 4 6% 20% 19% 5%	1926
	21st to end	Day	48% 2 6% 13% 22% 9%	1903
March	1st to 10th	Day	48% 4 5% 13% 17% 13%	5046
	11th to 20th	Day	37% 5% 7% 14% 19% 18%	5090
	21st to end	Day	46% 4 8% 22% 18% 1	5585
April	1st to 10th	Day	44% 4 8% 20% 18% 6%	2301
	11th to 20th	Day	45% 4 8% 19% 19% 5%	2374
	21st to end	Day	48% 5% 8% 19% 18% 3	2400
May	1st to 10th	Day	48% 5% 9% 23% 13% 1	5734
	11th to 20th	Day	44% 7% 11% 19% 17% 2	5866
	21st to end	Day	49% 7% 11% 17% 13% 3	6447
June	1st to 10th	Day	37% 8% 13% 26% 14% 2	2600
	11th to 20th	Day	41% 6% 12% 22% 18% 1	2600
	21st to end	Day	31% 8% 14% 29% 17% 2	2600
July	1st to 10th	Day	38% 9% 19% 24% 9% 1	5836
	11th to 20th	Day	24% 5% 14% 38% 18% 1	5831
	21st to end	Day	37% 9% 14% 29% 10% 1	6240
August	1st to 10th	Day	22% 7% 13% 26% 28% 5%	4220
	11th to 20th	Day	32% 7% 17% 33% 10% 1	4087
	21st to end	Day	31% 8% 14% 30% 15% 2	4311
September	1st to 10th	Day	37% 5% 10% 19% 19% 9%	2400
	11th to 20th	Day	36% 6% 11% 23% 17% 6%	2209
	21st to end	Day	31% 5% 10% 19% 25% 10%	2180
October	1st to 10th	Day	18% 2 5% 11% 30% 33%	3260
	11th to 20th	Day	21% 2 4 19% 35% 20%	3240
	21st to end	Day	34% 4 4 27% 24% 7%	3538
November	1st to 10th	Day	36% 3 7% 22% 25% 8%	2058
	11th to 20th	Day	33% 3 8% 20% 26% 10%	2100
	21st to end	Day	41% 3 5% 18% 25% 7%	1921
December	1st to 10th	Day	40% 3 7% 26% 20% 4	3030
	11th to 20th	Day	46% 2 4 19% 23% 6%	3479
	21st to end	Day	50% 3 6% 20% 18% 4	4587
January	1st to 10th	Night	37% 2 9% 27% 22% 2	7635
	11th to 20th	Night	23% 2 5% 20% 37% 13%	7680
	21st to end	Night	45% 3 6% 15% 22% 10%	8402
February	1st to 10th	Night	44% 3 8% 19% 19% 7%	2730
	11th to 20th	Night	46% 3 7% 19% 20% 6%	2606
	21st to end	Night	39% 4 8% 16% 23% 11%	1903
March	1st to 10th	Night	46% 4 5% 12% 19% 14%	5029
	11th to 20th	Night	34% 4 6% 20% 19% 17%	5090
	21st to end	Night	40% 4 10% 22% 21% 4	5588
April	1st to 10th	Night	40% 4 8% 20% 19% 10%	2120
	11th to 20th	Night	38% 4 8% 19% 23% 9%	1975
	21st to end	Night	38% 5% 9% 23% 22% 4	2000
May	1st to 10th	Night	47% 5% 12% 22% 11% 3	4222
	11th to 20th	Night	43% 6% 14% 21% 15% 1	4054
	21st to end	Night	41% 7% 15% 18% 13% 6%	4452
June	1st to 10th	Night	29% 6% 13% 30% 19% 3	1800
	11th to 20th	Night	27% 5% 12% 32% 22% 3	1800
	21st to end	Night	22% 6% 12% 30% 26% 4	1800
July	1st to 10th	Night	31% 8% 22% 26% 11% 1	4035
	11th to 20th	Night	18% 5% 13% 43% 19% 1	4026
	21st to end	Night	32% 7% 15% 31% 14% 1	4320
August	1st to 10th	Night	14% 5% 14% 30% 30% 7%	2916
	11th to 20th	Night	22% 5% 19% 36% 16% 2	3033
	21st to end	Night	23% 6% 15% 30% 24% 2	3586
September	1st to 10th	Night	27% 6% 11% 23% 24% 10%	2000
	11th to 20th	Night	31% 7% 11% 23% 21% 8%	2187
	21st to end	Night	29% 4 8% 21% 27% 11%	2200
October	1st to 10th	Night	15% 3 6% 14% 26% 36%	3920
	11th to 20th	Night	19% 2 4 16% 35% 23%	3899
	21st to end	Night	30% 3 5% 24% 28% 11%	4252
November	1st to 10th	Night	33% 3 7% 20% 26% 11%	2460
	11th to 20th	Night	30% 3 8% 20% 26% 12%	2520
	21st to end	Night	39% 3 6% 19% 27% 8%	2693
December	1st to 10th	Night	38% 3 7% 26% 21% 5%	4380
	11th to 20th	Night	46% 3 4 17% 20% 11%	4947
	21st to end	Night	46% 3 7% 20% 21% 4	6633

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	29% 20% 33% 16% 2 1	330
	11th to 20th	Day	28% 22% 37% 10% 2 1	330
	21st to end	Day	25% 28% 35% 10% 3	363
February	1st to 10th	Day	31% 22% 31% 13% 2	330
	11th to 20th	Day	38% 27% 28% 6% 1	330
	21st to end	Day	40% 21% 28% 11% 1	273
March	1st to 10th	Day	35% 18% 34% 12% 2	330
	11th to 20th	Day	37% 22% 32% 8% 2	330
	21st to end	Day	29% 24% 37% 10% 1	363
April	1st to 10th	Day	32% 19% 32% 12% 5%	393
	11th to 20th	Day	33% 16% 32% 13% 5% 1	492
	21st to end	Day	26% 19% 29% 18% 5% 2	500
May	1st to 10th	Day	21% 17% 36% 21% 6%	628
	11th to 20th	Day	18% 11% 34% 26% 9% 2	725
	21st to end	Day	16% 9% 33% 26% 13% 3	789
June	1st to 10th	Day	12% 6% 22% 37% 19% 5%	730
	11th to 20th	Day	10% 6% 24% 34% 22% 4	730
	21st to end	Day	5% 4 24% 37% 22% 7%	730
July	1st to 10th	Day	9% 3 16% 36% 29% 8%	721
	11th to 20th	Day	6% 2 13% 36% 34% 9%	718
	21st to end	Day	7% 2 10% 36% 34% 11%	794
August	1st to 10th	Day	7% 2 13% 41% 31% 6%	726
	11th to 20th	Day	11% 2 11% 37% 30% 9%	644
	21st to end	Day	11% 2 7% 34% 33% 13%	583
September	1st to 10th	Day	10% 2 13% 34% 34% 7%	530
	11th to 20th	Day	7% 3 20% 36% 28% 6%	339
	21st to end	Day	14% 8% 22% 31% 23% 2	330
October	1st to 10th	Day	6% 6% 29% 41% 14% 3	330
	11th to 20th	Day	14% 14% 35% 27% 9% 1	330
	21st to end	Day	28% 15% 33% 19% 4 1	363
November	1st to 10th	Day	23% 14% 32% 24% 5% 2	330
	11th to 20th	Day	20% 16% 40% 20% 3 1	330
	21st to end	Day	27% 21% 30% 18% 3 1	330
December	1st to 10th	Day	27% 19% 33% 17% 3 1	330
	11th to 20th	Day	31% 19% 33% 13% 3 1	330
	21st to end	Day	27% 23% 35% 11% 2 1	363
January	1st to 10th	Night	23% 15% 33% 26% 4	1050
	11th to 20th	Night	23% 19% 32% 21% 4	1050
	21st to end	Night	22% 17% 33% 25% 3	1155
February	1st to 10th	Night	25% 18% 29% 24% 4	1050
	11th to 20th	Night	28% 20% 32% 18% 2	971
	21st to end	Night	27% 15% 35% 20% 3	519
March	1st to 10th	Night	27% 13% 32% 24% 4	630
	11th to 20th	Night	25% 14% 32% 25% 4	630
	21st to end	Night	21% 15% 36% 24% 4 1	693
April	1st to 10th	Night	20% 17% 35% 23% 5%	603
	11th to 20th	Night	19% 17% 30% 24% 10%	593
	21st to end	Night	18% 14% 34% 25% 8% 1	600
May	1st to 10th	Night	17% 8% 35% 31% 9% 1	600
	11th to 20th	Night	16% 9% 30% 29% 14% 2	591
	21st to end	Night	12% 6% 29% 31% 19% 2	650
June	1st to 10th	Night	7% 5% 16% 39% 30% 4	600
	11th to 20th	Night	10% 1 18% 41% 27% 2	600
	21st to end	Night	4 2 15% 35% 39% 5%	600
July	1st to 10th	Night	6% 1 10% 34% 44% 5%	585
	11th to 20th	Night	4 1 7% 36% 43% 8%	582
	21st to end	Night	3 1 8% 34% 45% 9%	657
August	1st to 10th	Night	4 1 6% 40% 42% 7%	597
	11th to 20th	Night	9% 1 9% 37% 38% 7%	594
	21st to end	Night	7% 1 10% 36% 39% 6%	660
September	1st to 10th	Night	8% 2 12% 39% 36% 3	600
	11th to 20th	Night	11% 2 12% 36% 36% 4	600
	21st to end	Night	10% 4 18% 34% 30% 4	600
October	1st to 10th	Night	10% 3 20% 40% 24% 3	800
	11th to 20th	Night	16% 8% 27% 33% 15% 1	800
	21st to end	Night	21% 11% 28% 25% 15% 1	880
November	1st to 10th	Night	17% 9% 30% 33% 10% 1	820
	11th to 20th	Night	17% 12% 31% 33% 8%	840
	21st to end	Night	24% 13% 29% 24% 9%	1013
December	1st to 10th	Night	23% 12% 33% 28% 5%	1050
	11th to 20th	Night	27% 13% 30% 25% 5%	1050
	21st to end	Night	24% 14% 31% 24% 6%	1155

Page generated at 2026-03-30 01:18:41. Version ac07

Copyright © 2026 Attilla Danko.