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 2024-01-16.

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	16% 6% 2 5% 5% 3 4 4 5% 7% 44%	75022
	Night	14% 5% 2 4 4 2 3 3 5% 7% 50%	74817
Transparency	Day	41% 5% 10% 21% 19% 5%	74746
	Night	35% 4 9% 22% 22% 7%	74613
Seeing	Day	18% 11% 25% 26% 17% 4	14173
	Night	17% 10% 26% 29% 16% 2	24121

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	18% 9% 2 4 4 3 3 3 4 5% 47%	1707
	11th to 20th	Day	17% 6% 3 6% 4 3 4 4 4 4 46%	1670
	21st to end	Day	18% 7% 3 5% 3 2 3 4 2 5% 49%	1782
February	1st to 10th	Day	25% 7% 3 5% 4 2 4 3 4 4 40%	1620
	11th to 20th	Day	22% 6% 4 6% 5% 2 4 3 4 5% 40%	1636
	21st to end	Day	29% 8% 3 4 3 2 2 3 3 4 38%	1628
March	1st to 10th	Day	22% 7% 2 5% 3 2 3 3 3 5% 45%	1980
	11th to 20th	Day	24% 8% 2 6% 4 2 3 3 4 5% 38%	1980
	21st to end	Day	23% 7% 3 5% 5% 2 4 4 4 5% 38%	2178
April	1st to 10th	Day	19% 6% 2 5% 5% 2 5% 4 4 6% 43%	2060
	11th to 20th	Day	21% 4 2 4 5% 3 4 4 4 6% 42%	2134
	21st to end	Day	21% 7% 3 5% 5% 3 4 4 4 5% 40%	2160
May	1st to 10th	Day	20% 6% 3 4 4 3 4 4 5% 7% 40%	2272
	11th to 20th	Day	15% 6% 2 5% 6% 3 6% 5% 6% 8% 38%	2302
	21st to end	Day	15% 7% 3 6% 6% 5% 5% 6% 6% 8% 34%	2531
June	1st to 10th	Day	10% 6% 3 6% 5% 3 5% 5% 5% 9% 42%	2340
	11th to 20th	Day	9% 6% 3 6% 5% 4 6% 6% 6% 8% 41%	2340
	21st to end	Day	7% 4 2 4 5% 4 5% 5% 7% 8% 48%	2340
July	1st to 10th	Day	10% 5% 3 6% 8% 5% 7% 7% 8% 10% 33%	2340
	11th to 20th	Day	6% 3 2 4 5% 4 6% 5% 8% 10% 46%	2340
	21st to end	Day	7% 3 3 4 6% 4 7% 6% 8% 11% 42%	2546
August	1st to 10th	Day	5% 3 2 4 6% 4 6% 8% 8% 11% 42%	2328
	11th to 20th	Day	11% 4 2 5% 5% 3 5% 6% 6% 8% 44%	2238
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 7% 8% 42%	2376
September	1st to 10th	Day	12% 5% 2 5% 5% 3 4 4 5% 6% 49%	2160
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 5% 5% 7% 45%	1989
	21st to end	Day	11% 4 2 3 4 3 4 4 5% 7% 54%	1962
October	1st to 10th	Day	10% 5% 2 4 5% 3 4 4 4 6% 53%	1800
	11th to 20th	Day	17% 4 2 4 3 2 3 2 3 4 55%	1800
	21st to end	Day	22% 5% 1 4 3 2 3 2 4 4 49%	1980
November	1st to 10th	Day	17% 5% 2 4 4 2 3 3 4 6% 51%	1858
	11th to 20th	Day	14% 5% 1 3 4 2 3 4 4 5% 54%	1900
	21st to end	Day	22% 7% 3 3 3 1 3 2 3 4 48%	1739
December	1st to 10th	Day	21% 6% 2 3 3 1 3 4 4 5% 48%	1710
	11th to 20th	Day	18% 9% 3 5% 4 3 3 4 4 5% 42%	2161
	21st to end	Day	21% 9% 3 5% 5% 3 4 4 4 5% 38%	3135
January	1st to 10th	Night	17% 6% 2 3 3 2 4 3 4 6% 50%	2458
	11th to 20th	Night	18% 7% 3 5% 3 2 3 2 4 6% 49%	2418
	21st to end	Night	17% 5% 2 3 3 2 3 3 3 5% 52%	2574
February	1st to 10th	Night	20% 7% 4 5% 3 2 3 4 4 5% 44%	2340
	11th to 20th	Night	20% 7% 3 6% 4 2 3 4 4 5% 42%	2223
	21st to end	Night	23% 7% 2 4 3 2 3 3 4 5% 45%	1628
March	1st to 10th	Night	21% 6% 2 5% 4 2 3 4 4 6% 42%	1980
	11th to 20th	Night	19% 8% 3 5% 4 2 3 3 4 6% 43%	1980
	21st to end	Night	18% 6% 3 4 5% 2 4 2 3 6% 45%	2178
April	1st to 10th	Night	14% 5% 2 5% 5% 2 3 4 5% 6% 48%	1899
	11th to 20th	Night	14% 6% 2 4 4 3 4 4 4 6% 48%	1775
	21st to end	Night	16% 5% 2 5% 4 2 3 3 4 7% 49%	1800
May	1st to 10th	Night	15% 5% 2 5% 4 3 4 4 4 7% 47%	1684
	11th to 20th	Night	14% 5% 2 4 4 3 4 5% 5% 8% 46%	1593
	21st to end	Night	11% 5% 3 4 5% 3 4 5% 6% 9% 45%	1745
June	1st to 10th	Night	7% 4 2 4 4 2 3 3 5% 8% 56%	1620
	11th to 20th	Night	7% 3 2 3 3 2 4 4 6% 8% 57%	1620
	21st to end	Night	6% 2 1 3 3 2 4 4 5% 8% 62%	1620
July	1st to 10th	Night	5% 3 2 4 5% 3 6% 6% 9% 12% 46%	1620
	11th to 20th	Night	3 2 1 3 3 2 4 3 7% 11% 61%	1620
	21st to end	Night	4 2 2 4 5% 3 5% 5% 7% 11% 53%	1772
August	1st to 10th	Night	4 3 1 3 4 2 4 5% 8% 11% 55%	1611
	11th to 20th	Night	7% 4 1 3 4 3 5% 5% 6% 10% 51%	1662
	21st to end	Night	6% 3 2 4 5% 3 5% 4 7% 10% 50%	1980
September	1st to 10th	Night	6% 4 2 4 5% 2 3 4 5% 8% 58%	1800
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 7% 51%	1967
	21st to end	Night	9% 3 2 4 4 2 4 3 4 6% 59%	1980
October	1st to 10th	Night	9% 4 1 4 3 2 3 3 5% 7% 58%	2160
	11th to 20th	Night	15% 4 2 3 3 2 2 3 4 4 58%	2160
	21st to end	Night	18% 6% 2 4 3 1 2 2 3 4 55%	2376
November	1st to 10th	Night	14% 4 2 3 3 2 3 2 4 4 57%	2220
	11th to 20th	Night	12% 5% 2 4 4 2 3 3 4 6% 57%	2280
	21st to end	Night	19% 6% 2 4 3 2 3 3 3 5% 49%	2436
December	1st to 10th	Night	20% 6% 2 4 3 2 3 3 4 6% 48%	2470
	11th to 20th	Night	19% 5% 3 5% 5% 3 3 3 4 5% 46%	3036
	21st to end	Night	19% 8% 2 5% 4 2 3 3 3 5% 45%	4532

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% 4 7% 22% 19% 7%	1708
	11th to 20th	Day	43% 3 6% 23% 21% 4	1670
	21st to end	Day	40% 3 4 22% 25% 6%	1782
February	1st to 10th	Day	49% 3 8% 17% 17% 5%	1620
	11th to 20th	Day	49% 3 5% 19% 19% 5%	1645
	21st to end	Day	52% 2 5% 12% 20% 8%	1628
March	1st to 10th	Day	44% 3 7% 16% 20% 10%	1980
	11th to 20th	Day	50% 4 7% 17% 17% 5%	1980
	21st to end	Day	50% 4 8% 20% 16% 3	2178
April	1st to 10th	Day	44% 4 8% 21% 18% 5%	2060
	11th to 20th	Day	44% 4 8% 20% 19% 5%	2134
	21st to end	Day	47% 4 8% 19% 19% 2	2160
May	1st to 10th	Day	45% 6% 8% 20% 18% 3	2272
	11th to 20th	Day	42% 8% 12% 22% 14% 1	2306
	21st to end	Day	47% 8% 11% 20% 13%	2531
June	1st to 10th	Day	37% 8% 14% 26% 13% 2	2340
	11th to 20th	Day	41% 6% 12% 22% 17% 1	2340
	21st to end	Day	32% 7% 12% 28% 18% 2	2340
July	1st to 10th	Day	42% 10% 17% 20% 11% 1	2276
	11th to 20th	Day	31% 7% 14% 27% 20% 1	2271
	21st to end	Day	33% 11% 16% 29% 10% 1	2405
August	1st to 10th	Day	32% 10% 15% 28% 15% 1	2314
	11th to 20th	Day	36% 8% 13% 26% 15% 2	2238
	21st to end	Day	37% 8% 14% 25% 13% 3	2376
September	1st to 10th	Day	37% 5% 11% 20% 19% 9%	2160
	11th to 20th	Day	36% 6% 11% 22% 18% 7%	1989
	21st to end	Day	31% 5% 10% 19% 25% 10%	1962
October	1st to 10th	Day	32% 4 10% 19% 24% 11%	1800
	11th to 20th	Day	34% 2 7% 15% 27% 14%	1800
	21st to end	Day	41% 3 6% 17% 23% 11%	1980
November	1st to 10th	Day	36% 4 7% 22% 24% 8%	1858
	11th to 20th	Day	34% 3 7% 19% 27% 9%	1900
	21st to end	Day	42% 3 5% 17% 25% 8%	1739
December	1st to 10th	Day	40% 4 8% 22% 19% 7%	1710
	11th to 20th	Day	46% 2 5% 17% 22% 7%	2159
	21st to end	Day	49% 3 8% 16% 19% 5%	3135
January	1st to 10th	Night	37% 3 7% 19% 27% 7%	2458
	11th to 20th	Night	40% 3 9% 19% 23% 6%	2418
	21st to end	Night	37% 3 4 19% 30% 8%	2574
February	1st to 10th	Night	44% 3 9% 18% 19% 7%	2340
	11th to 20th	Night	46% 3 6% 19% 20% 6%	2224
	21st to end	Night	43% 3 7% 15% 22% 10%	1628
March	1st to 10th	Night	44% 3 6% 14% 20% 12%	1980
	11th to 20th	Night	44% 4 8% 18% 20% 6%	1980
	21st to end	Night	43% 3 7% 22% 21% 4	2178
April	1st to 10th	Night	38% 4 8% 20% 20% 10%	1899
	11th to 20th	Night	38% 4 8% 19% 22% 9%	1775
	21st to end	Night	37% 5% 9% 23% 23% 3	1800
May	1st to 10th	Night	38% 6% 10% 22% 18% 6%	1684
	11th to 20th	Night	38% 5% 15% 25% 15% 2	1594
	21st to end	Night	36% 8% 13% 26% 15% 1	1746
June	1st to 10th	Night	28% 6% 13% 31% 19% 3	1620
	11th to 20th	Night	25% 5% 12% 34% 21% 3	1620
	21st to end	Night	22% 6% 10% 29% 28% 4	1620
July	1st to 10th	Night	27% 10% 21% 25% 16% 1	1575
	11th to 20th	Night	16% 6% 19% 34% 23% 2	1566
	21st to end	Night	23% 8% 16% 35% 16% 1	1673
August	1st to 10th	Night	21% 7% 20% 32% 19% 2	1601
	11th to 20th	Night	28% 7% 12% 31% 19% 3	1662
	21st to end	Night	28% 8% 14% 28% 19% 3	1980
September	1st to 10th	Night	25% 6% 11% 24% 25% 9%	1800
	11th to 20th	Night	31% 7% 11% 21% 21% 8%	1967
	21st to end	Night	27% 4 8% 22% 28% 11%	1980
October	1st to 10th	Night	26% 5% 10% 24% 22% 14%	2160
	11th to 20th	Night	30% 3 6% 15% 28% 17%	2160
	21st to end	Night	37% 2 4 20% 26% 11%	2376
November	1st to 10th	Night	32% 3 7% 21% 26% 11%	2220
	11th to 20th	Night	31% 3 9% 20% 27% 12%	2280
	21st to end	Night	40% 3 5% 18% 27% 8%	2436
December	1st to 10th	Night	40% 3 8% 18% 22% 9%	2470
	11th to 20th	Night	43% 3 5% 15% 24% 10%	3037
	21st to end	Night	43% 3 7% 21% 21% 5%	4532

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	28% 21% 31% 17% 1 1	270
	11th to 20th	Day	31% 21% 35% 10% 2 1	257
	21st to end	Day	23% 28% 35% 9% 5%	264
February	1st to 10th	Day	31% 23% 31% 12% 3	240
	11th to 20th	Day	38% 29% 28% 4 2	240
	21st to end	Day	45% 19% 27% 8% 1	198
March	1st to 10th	Day	33% 18% 35% 13% 1	240
	11th to 20th	Day	38% 22% 29% 9% 3	240
	21st to end	Day	32% 25% 33% 9% 1	264
April	1st to 10th	Day	28% 21% 33% 13% 6%	320
	11th to 20th	Day	33% 14% 32% 14% 7% 1	412
	21st to end	Day	26% 19% 30% 17% 6% 2	420
May	1st to 10th	Day	20% 16% 36% 21% 7%	535
	11th to 20th	Day	17% 10% 35% 27% 10% 2	625
	21st to end	Day	16% 7% 32% 27% 15% 4	679
June	1st to 10th	Day	12% 5% 19% 37% 21% 6%	630
	11th to 20th	Day	10% 5% 24% 34% 23% 4	630
	21st to end	Day	6% 5% 27% 34% 22% 7%	630
July	1st to 10th	Day	8% 3 16% 34% 31% 8%	621
	11th to 20th	Day	6% 2 13% 35% 34% 10%	618
	21st to end	Day	7% 1 10% 36% 36% 11%	684
August	1st to 10th	Day	7% 2 13% 40% 32% 6%	626
	11th to 20th	Day	13% 2 11% 36% 29% 8%	552
	21st to end	Day	10% 2 7% 34% 35% 13%	495
September	1st to 10th	Day	10% 1 10% 34% 37% 7%	450
	11th to 20th	Day	6% 3 20% 35% 29% 6%	279
	21st to end	Day	13% 6% 23% 33% 23% 3	270
October	1st to 10th	Day	7% 8% 31% 36% 14% 3	270
	11th to 20th	Day	15% 16% 37% 24% 7% 1	270
	21st to end	Day	30% 14% 32% 19% 4 1	297
November	1st to 10th	Day	21% 15% 32% 24% 6% 2	270
	11th to 20th	Day	19% 17% 41% 18% 3 1	270
	21st to end	Day	29% 19% 28% 19% 4 1	270
December	1st to 10th	Day	26% 20% 31% 17% 3 1	270
	11th to 20th	Day	29% 20% 34% 14% 3 1	270
	21st to end	Day	25% 26% 38% 8% 3 1	297
January	1st to 10th	Night	23% 15% 33% 25% 4	950
	11th to 20th	Night	25% 19% 30% 21% 4	930
	21st to end	Night	21% 17% 33% 25% 4	990
February	1st to 10th	Night	25% 19% 28% 24% 4	900
	11th to 20th	Night	26% 20% 33% 18% 2	829
	21st to end	Night	30% 16% 35% 17% 3	444
March	1st to 10th	Night	25% 13% 32% 26% 5%	540
	11th to 20th	Night	25% 15% 32% 24% 5% 1	540
	21st to end	Night	21% 14% 38% 22% 4 1	594
April	1st to 10th	Night	17% 18% 37% 23% 5%	540
	11th to 20th	Night	18% 16% 30% 26% 11%	533
	21st to end	Night	18% 13% 36% 23% 8% 1	540
May	1st to 10th	Night	17% 7% 36% 30% 9% 1	540
	11th to 20th	Night	15% 9% 31% 28% 15% 3	531
	21st to end	Night	11% 5% 30% 30% 21% 3	584
June	1st to 10th	Night	6% 4 15% 39% 32% 4	540
	11th to 20th	Night	9% 1 19% 43% 27% 2	540
	21st to end	Night	4 2 16% 36% 37% 5%	540
July	1st to 10th	Night	5% 1 10% 34% 45% 6%	525
	11th to 20th	Night	4 1 8% 36% 43% 8%	522
	21st to end	Night	3 1 8% 35% 45% 9%	591
August	1st to 10th	Night	4 1 7% 37% 43% 7%	537
	11th to 20th	Night	9% 1 9% 39% 36% 6%	534
	21st to end	Night	7% 1 9% 36% 40% 7%	594
September	1st to 10th	Night	7% 2 12% 37% 38% 4	540
	11th to 20th	Night	11% 1 11% 38% 34% 4	540
	21st to end	Night	10% 3 18% 34% 31% 4	540
October	1st to 10th	Night	11% 3 21% 38% 24% 3	720
	11th to 20th	Night	17% 8% 28% 30% 15% 1	720
	21st to end	Night	21% 11% 29% 23% 15% 1	792
November	1st to 10th	Night	15% 9% 31% 33% 11% 1	740
	11th to 20th	Night	16% 12% 30% 33% 9%	760
	21st to end	Night	25% 13% 28% 24% 10%	916
December	1st to 10th	Night	24% 12% 32% 27% 6%	950
	11th to 20th	Night	27% 13% 30% 25% 5%	950
	21st to end	Night	23% 15% 32% 24% 6%	1045

Page generated at 2024-01-13 18:28:34. Version ac07

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