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 2024-12-09.

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% 44%	101497
	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	98381
Transparency	Day	39% 5% 10% 22% 19% 6%	105146
	Night	34% 4 10% 22% 22% 8%	102872
Seeing	Day	18% 11% 25% 26% 16% 4	15418
	Night	17% 10% 26% 29% 16% 2	25259

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% 3 5% 3 3 3 2 3 4 45%	2847
	11th to 20th	Day	12% 8% 2 6% 3 3 3 3 3 4 53%	2850
	21st to end	Day	24% 6% 2 4 3 3 2 3 3 4 47%	3135
February	1st to 10th	Day	24% 8% 3 5% 4 3 3 3 4 4 39%	1710
	11th to 20th	Day	21% 7% 3 6% 4 3 3 3 4 5% 41%	1729
	21st to end	Day	28% 8% 3 4 3 3 2 2 4 4 39%	1727
March	1st to 10th	Day	23% 9% 3 6% 3 3 3 2 3 4 40%	3436
	11th to 20th	Day	19% 8% 2 7% 4 4 4 3 5% 6% 38%	3480
	21st to end	Day	21% 8% 3 7% 4 3 3 3 4 5% 39%	3814
April	1st to 10th	Day	19% 6% 2 6% 4 2 4 3 4 6% 43%	2175
	11th to 20th	Day	22% 5% 2 5% 4 4 4 3 4 6% 41%	2254
	21st to end	Day	21% 7% 2 6% 4 4 4 3 4 5% 39%	2280
May	1st to 10th	Day	27% 7% 3 5% 4 3 3 3 5% 6% 35%	3974
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 6% 7% 42%	4082
	21st to end	Day	22% 9% 3 7% 5% 5% 4 4 5% 6% 31%	4477
June	1st to 10th	Day	10% 6% 2 7% 4 5% 4 4 6% 9% 42%	2470
	11th to 20th	Day	9% 7% 3 7% 4 5% 6% 5% 6% 8% 42%	2470
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 5% 7% 9% 47%	2470
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 9% 41%	4120
	11th to 20th	Day	6% 3 2 4 4 5% 4 3 6% 8% 55%	4120
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 8% 8% 32%	4465
August	1st to 10th	Day	3 2 1 4 4 5% 4 5% 6% 9% 58%	4104
	11th to 20th	Day	8% 4 2 5% 4 4 5% 5% 7% 9% 47%	3922
	21st to end	Day	5% 4 1 5% 5% 5% 5% 4 7% 9% 48%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 4 4 3 5% 6% 49%	2280
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 5% 7% 45%	2099
	21st to end	Day	11% 4 2 4 3 4 4 3 5% 7% 53%	2071
October	1st to 10th	Day	10% 5% 2 5% 4 4 3 3 4 6% 56%	1900
	11th to 20th	Day	16% 4 2 4 2 2 2 2 3 4 57%	1900
	21st to end	Day	21% 6% 1 5% 3 3 3 2 4 4 49%	2090
November	1st to 10th	Day	18% 5% 2 4 3 3 3 2 4 6% 50%	1958
	11th to 20th	Day	15% 5% 2 3 3 3 4 3 4 5% 55%	2000
	21st to end	Day	21% 7% 2 4 3 2 2 2 3 4 49%	1830
December	1st to 10th	Day	21% 6% 2 3 3 2 3 3 4 5% 48%	1782
	11th to 20th	Day	17% 10% 3 5% 4 3 4 3 4 5% 42%	2161
	21st to end	Day	20% 10% 3 6% 4 4 4 3 4 4 37%	3135
January	1st to 10th	Night	14% 9% 2 6% 3 3 4 2 4 5% 47%	4108
	11th to 20th	Night	16% 8% 2 4 2 2 2 2 3 4 56%	4120
	21st to end	Night	24% 7% 2 3 2 2 3 2 4 4 46%	4532
February	1st to 10th	Night	20% 8% 3 5% 3 3 4 3 4 5% 43%	2470
	11th to 20th	Night	20% 8% 3 6% 3 3 3 3 4 5% 43%	2351
	21st to end	Night	21% 7% 2 4 3 3 2 2 4 5% 47%	1727
March	1st to 10th	Night	25% 8% 3 6% 3 3 3 3 4 5% 38%	3419
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 6% 46%	3480
	21st to end	Night	17% 7% 3 6% 4 4 3 2 4 7% 43%	3817
April	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 5% 5% 48%	2004
	11th to 20th	Night	15% 7% 2 5% 4 3 4 3 4 6% 47%	1875
	21st to end	Night	17% 6% 2 6% 3 3 2 3 4 7% 47%	1900
May	1st to 10th	Night	25% 8% 2 5% 4 3 4 3 4 6% 37%	2924
	11th to 20th	Night	16% 6% 2 6% 3 3 4 4 5% 7% 44%	2823
	21st to end	Night	18% 6% 3 5% 4 4 3 3 5% 8% 41%	3090
June	1st to 10th	Night	7% 5% 1 5% 4 4 3 2 5% 9% 55%	1710
	11th to 20th	Night	7% 4 1 4 3 2 3 3 6% 7% 57%	1710
	21st to end	Night	5% 3 1 4 3 3 3 3 6% 8% 61%	1710
July	1st to 10th	Night	8% 3 1 4 4 4 4 5% 7% 10% 49%	2850
	11th to 20th	Night	4 2 1 3 2 2 3 2 6% 10% 65%	2850
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	3098
August	1st to 10th	Night	3 2 1 2 2 2 3 3 6% 8% 69%	2836
	11th to 20th	Night	5% 3 1 3 3 4 4 4 5% 10% 59%	2912
	21st to end	Night	4 3 1 3 3 4 4 3 6% 9% 59%	3476
September	1st to 10th	Night	6% 4 2 5% 4 3 3 3 5% 7% 57%	1900
	11th to 20th	Night	9% 5% 2 5% 3 5% 4 3 6% 7% 51%	2077
	21st to end	Night	9% 3 2 5% 3 3 3 3 4 6% 58%	2090
October	1st to 10th	Night	9% 4 1 4 3 2 3 2 5% 7% 60%	2280
	11th to 20th	Night	14% 4 2 4 2 2 2 2 4 4 60%	2280
	21st to end	Night	18% 6% 2 4 3 2 2 1 3 4 56%	2508
November	1st to 10th	Night	15% 5% 2 4 3 3 3 2 4 4 56%	2340
	11th to 20th	Night	12% 5% 2 4 3 2 2 2 4 6% 57%	2400
	21st to end	Night	18% 6% 2 4 3 2 3 3 3 5% 50%	2565
December	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 4 5% 48%	2581
	11th to 20th	Night	19% 6% 2 6% 4 4 3 2 4 5% 46%	3036
	21st to end	Night	19% 8% 2 6% 3 3 3 2 3 5% 44%	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	44% 3 5% 23% 21% 4	2848
	11th to 20th	Day	37% 4 10% 25% 21% 4	2850
	21st to end	Day	44% 3 5% 19% 21% 9%	3135
February	1st to 10th	Day	50% 4 8% 17% 17% 5%	1710
	11th to 20th	Day	47% 4 5% 20% 20% 5%	1738
	21st to end	Day	51% 2 6% 13% 20% 8%	1727
March	1st to 10th	Day	49% 3 6% 13% 20% 9%	3436
	11th to 20th	Day	46% 6% 10% 16% 16% 6%	3480
	21st to end	Day	48% 4 8% 21% 17% 2	3814
April	1st to 10th	Day	43% 4 8% 20% 18% 6%	2175
	11th to 20th	Day	45% 4 8% 19% 19% 5%	2254
	21st to end	Day	47% 5% 8% 19% 18% 3	2280
May	1st to 10th	Day	50% 5% 9% 21% 13% 2	4010
	11th to 20th	Day	39% 7% 11% 23% 18% 1	4086
	21st to end	Day	52% 8% 10% 16% 10% 3	4489
June	1st to 10th	Day	37% 8% 14% 27% 13% 2	2470
	11th to 20th	Day	40% 6% 12% 23% 18% 1	2470
	21st to end	Day	31% 8% 13% 29% 17% 2	2470
July	1st to 10th	Day	36% 8% 20% 23% 11% 1	4056
	11th to 20th	Day	26% 5% 11% 34% 24% 1	4051
	21st to end	Day	45% 10% 14% 23% 8% 1	4325
August	1st to 10th	Day	22% 7% 13% 26% 28% 5%	4090
	11th to 20th	Day	32% 7% 17% 33% 10% 1	3961
	21st to end	Day	30% 8% 15% 30% 15% 2	4179
September	1st to 10th	Day	37% 5% 11% 19% 18% 10%	2280
	11th to 20th	Day	36% 6% 11% 22% 18% 6%	2099
	21st to end	Day	31% 5% 10% 19% 25% 10%	2071
October	1st to 10th	Day	18% 2 5% 11% 29% 34%	3160
	11th to 20th	Day	21% 2 4 18% 35% 20%	3140
	21st to end	Day	33% 4 4 27% 24% 7%	3428
November	1st to 10th	Day	37% 4 7% 22% 24% 8%	1958
	11th to 20th	Day	34% 3 7% 19% 27% 11%	2000
	21st to end	Day	41% 3 5% 17% 26% 8%	1830
December	1st to 10th	Day	40% 4 8% 23% 19% 7%	1782
	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	40% 3 7% 17% 28% 5%	4108
	11th to 20th	Night	34% 3 9% 22% 26% 6%	4120
	21st to end	Night	43% 3 8% 16% 22% 8%	4532
February	1st to 10th	Night	45% 3 9% 18% 19% 7%	2470
	11th to 20th	Night	45% 3 6% 19% 21% 7%	2352
	21st to end	Night	41% 3 8% 16% 22% 10%	1727
March	1st to 10th	Night	48% 4 7% 13% 17% 11%	3419
	11th to 20th	Night	40% 5% 8% 22% 16% 9%	3480
	21st to end	Night	43% 4 8% 21% 21% 2	3817
April	1st to 10th	Night	39% 4 8% 19% 19% 11%	2004
	11th to 20th	Night	38% 4 8% 19% 22% 9%	1875
	21st to end	Night	38% 5% 8% 23% 22% 3	1900
May	1st to 10th	Night	49% 6% 9% 20% 12% 4	2953
	11th to 20th	Night	39% 6% 13% 24% 17% 2	2824
	21st to end	Night	41% 8% 15% 20% 12% 4	3099
June	1st to 10th	Night	29% 5% 12% 31% 19% 3	1710
	11th to 20th	Night	26% 5% 12% 33% 22% 3	1710
	21st to end	Night	22% 6% 11% 30% 27% 4	1710
July	1st to 10th	Night	28% 8% 22% 25% 15% 2	2805
	11th to 20th	Night	16% 4 13% 41% 23% 2	2796
	21st to end	Night	38% 7% 16% 26% 12%	2999
August	1st to 10th	Night	14% 5% 15% 30% 30% 7%	2826
	11th to 20th	Night	22% 5% 19% 35% 17% 2	2939
	21st to end	Night	22% 6% 15% 30% 25% 2	3476
September	1st to 10th	Night	27% 6% 11% 23% 24% 10%	1900
	11th to 20th	Night	31% 7% 11% 21% 22% 8%	2077
	21st to end	Night	28% 4 8% 21% 27% 11%	2090
October	1st to 10th	Night	16% 3 6% 14% 25% 36%	3800
	11th to 20th	Night	19% 2 4 16% 36% 24%	3779
	21st to end	Night	29% 3 5% 25% 28% 11%	4120
November	1st to 10th	Night	34% 3 7% 20% 25% 11%	2340
	11th to 20th	Night	30% 2 8% 19% 27% 13%	2400
	21st to end	Night	38% 3 6% 18% 27% 8%	2565
December	1st to 10th	Night	39% 3 8% 19% 22% 8%	2581
	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	30% 23% 34% 10% 2 1	270
	21st to end	Day	24% 28% 35% 9% 4	297
February	1st to 10th	Day	31% 23% 30% 12% 3	270
	11th to 20th	Day	38% 27% 27% 7% 1	270
	21st to end	Day	45% 20% 26% 7% 1	225
March	1st to 10th	Day	33% 17% 34% 14% 1	270
	11th to 20th	Day	36% 24% 29% 9% 2	270
	21st to end	Day	33% 25% 33% 9% 1	297
April	1st to 10th	Day	31% 21% 32% 12% 6%	355
	11th to 20th	Day	33% 15% 32% 13% 6% 1	452
	21st to end	Day	27% 19% 30% 17% 6% 2	460
May	1st to 10th	Day	21% 17% 35% 21% 6%	582
	11th to 20th	Day	16% 10% 35% 27% 10% 2	675
	21st to end	Day	16% 8% 32% 26% 14% 3	734
June	1st to 10th	Day	12% 5% 20% 38% 20% 5%	680
	11th to 20th	Day	9% 5% 24% 35% 23% 4	680
	21st to end	Day	5% 5% 25% 35% 23% 7%	680
July	1st to 10th	Day	8% 3 17% 35% 30% 8%	671
	11th to 20th	Day	6% 2 14% 36% 34% 9%	668
	21st to end	Day	7% 2 11% 36% 34% 11%	739
August	1st to 10th	Day	6% 2 13% 41% 31% 7%	676
	11th to 20th	Day	12% 2 12% 37% 29% 8%	598
	21st to end	Day	9% 2 7% 34% 34% 14%	539
September	1st to 10th	Day	10% 2 12% 34% 35% 7%	490
	11th to 20th	Day	7% 3 21% 36% 27% 5%	309
	21st to end	Day	13% 8% 23% 32% 22% 2	300
October	1st to 10th	Day	7% 7% 31% 39% 14% 3	300
	11th to 20th	Day	14% 14% 36% 26% 9% 1	300
	21st to end	Day	28% 14% 33% 20% 4 1	330
November	1st to 10th	Day	24% 15% 32% 22% 5% 2	300
	11th to 20th	Day	20% 17% 40% 20% 3 1	300
	21st to end	Day	27% 22% 29% 18% 3 1	300
December	1st to 10th	Day	27% 19% 33% 16% 3 1	294
	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	950
	21st to end	Night	22% 17% 33% 25% 4	1045
February	1st to 10th	Night	25% 19% 28% 24% 4	950
	11th to 20th	Night	26% 20% 33% 19% 3	877
	21st to end	Night	30% 15% 35% 17% 3	471
March	1st to 10th	Night	25% 13% 32% 26% 5%	570
	11th to 20th	Night	25% 15% 32% 24% 4 1	570
	21st to end	Night	22% 15% 37% 22% 4 1	627
April	1st to 10th	Night	18% 18% 36% 23% 5%	570
	11th to 20th	Night	19% 16% 30% 25% 10%	563
	21st to end	Night	18% 14% 35% 23% 8% 1	570
May	1st to 10th	Night	17% 8% 36% 30% 9% 1	570
	11th to 20th	Night	16% 9% 30% 29% 14% 2	561
	21st to end	Night	11% 6% 29% 30% 20% 2	617
June	1st to 10th	Night	7% 4 16% 39% 31% 4	570
	11th to 20th	Night	9% 1 18% 42% 28% 2	570
	21st to end	Night	4 2 16% 35% 38% 5%	570
July	1st to 10th	Night	6% 1 10% 34% 44% 5%	555
	11th to 20th	Night	4 1 8% 37% 43% 8%	552
	21st to end	Night	3 1 8% 35% 45% 8%	624
August	1st to 10th	Night	4 1 7% 38% 44% 7%	567
	11th to 20th	Night	9% 1 9% 38% 37% 6%	564
	21st to end	Night	7% 1 10% 36% 40% 7%	627
September	1st to 10th	Night	7% 2 12% 38% 37% 4	570
	11th to 20th	Night	11% 2 13% 37% 34% 4	570
	21st to end	Night	10% 3 19% 34% 31% 4	570
October	1st to 10th	Night	10% 3 20% 39% 24% 3	760
	11th to 20th	Night	17% 8% 27% 32% 15% 1	760
	21st to end	Night	20% 10% 28% 25% 15% 1	836
November	1st to 10th	Night	17% 9% 30% 33% 10% 1	780
	11th to 20th	Night	16% 12% 31% 33% 9%	800
	21st to end	Night	24% 13% 29% 24% 9%	965
December	1st to 10th	Night	24% 12% 32% 27% 5%	993
	11th to 20th	Night	27% 13% 30% 25% 5%	950
	21st to end	Night	23% 15% 32% 24% 6%	1045

Page generated at 2024-12-06 02:54:51. Version ac07

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