Clarke Priscu Observatory.
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clarke Priscu Observatory. clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	126697
	Night	17% 11% 4 9% 5% 5% 5% 4 5% 6% 31%	122336
Transparency	Day	51% 6% 11% 21% 10% 1	130175
	Night	53% 5% 11% 19% 10% 1	126945
Seeing	Day	21% 15% 30% 20% 10% 3	15566
	Night	22% 12% 29% 27% 10% 1	24607

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% 16% 5% 14% 6% 6% 5% 4 5% 5% 21%	3597
	11th to 20th	Day	13% 15% 6% 13% 8% 5% 5% 4 4 5% 23%	3570
	21st to end	Day	11% 16% 7% 14% 6% 7% 5% 4 5% 5% 21%	4162
February	1st to 10th	Day	14% 15% 5% 11% 7% 6% 5% 4 6% 5% 24%	1800
	11th to 20th	Day	13% 14% 4 10% 6% 6% 5% 3 4 6% 29%	1753
	21st to end	Day	19% 13% 3 9% 6% 4 4 6% 5% 5% 25%	1650
March	1st to 10th	Day	19% 12% 4 9% 7% 5% 4 4 4 5% 26%	4494
	11th to 20th	Day	11% 12% 4 8% 6% 5% 6% 4 6% 8% 30%	4980
	21st to end	Day	17% 11% 3 7% 5% 4 3 3 4 4 38%	5758
April	1st to 10th	Day	21% 11% 3 6% 4 4 4 3 4 6% 32%	2400
	11th to 20th	Day	22% 9% 3 7% 4 4 4 3 4 4 37%	2374
	21st to end	Day	22% 10% 3 8% 5% 4 5% 3 5% 5% 32%	2537
May	1st to 10th	Day	23% 9% 3 8% 5% 4 4 4 5% 6% 27%	6346
	11th to 20th	Day	17% 8% 3 8% 6% 5% 5% 4 5% 7% 31%	6318
	21st to end	Day	21% 8% 3 6% 4 5% 5% 3 5% 6% 34%	6949
June	1st to 10th	Day	20% 7% 3 7% 6% 5% 5% 5% 6% 6% 31%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 4 6% 4 6% 8% 40%	2800
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 5% 6% 7% 33%	2800
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	10% 4 2 6% 4 5% 5% 4 7% 8% 44%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 10% 46%	6926
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 7% 38%	4475
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 3 6% 8% 39%	4123
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 40%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 5% 8% 44%	2400
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 47%	2376
	21st to end	Day	17% 5% 2 6% 5% 5% 4 4 6% 8% 39%	2025
October	1st to 10th	Day	20% 7% 2 6% 5% 4 5% 4 5% 7% 36%	1900
	11th to 20th	Day	22% 8% 3 7% 5% 5% 5% 4 5% 6% 30%	1900
	21st to end	Day	24% 9% 4 7% 4 4 5% 4 4 6% 29%	1943
November	1st to 10th	Day	15% 9% 3 6% 6% 5% 4 4 6% 7% 34%	1761
	11th to 20th	Day	18% 11% 3 8% 6% 6% 5% 4 5% 5% 30%	1602
	21st to end	Day	21% 13% 3 9% 7% 6% 6% 4 5% 6% 20%	1600
December	1st to 10th	Day	20% 19% 4 12% 6% 5% 5% 4 5% 5% 15%	2600
	11th to 20th	Day	18% 14% 4 11% 7% 5% 5% 3 4 4 25%	2986
	21st to end	Day	22% 15% 5% 12% 5% 5% 4 3 4 4 21%	3960
January	1st to 10th	Night	13% 19% 8% 15% 6% 4 3 3 4 5% 20%	6349
	11th to 20th	Night	11% 14% 6% 14% 8% 7% 5% 4 4 5% 22%	6315
	21st to end	Night	14% 18% 6% 10% 6% 6% 5% 4 5% 5% 21%	6792
February	1st to 10th	Night	14% 15% 4 11% 7% 6% 5% 4 5% 6% 24%	2600
	11th to 20th	Night	13% 12% 4 12% 7% 5% 4 3 5% 4 29%	2532
	21st to end	Night	16% 13% 4 8% 7% 4 5% 3 5% 7% 28%	1980
March	1st to 10th	Night	22% 14% 4 9% 5% 4 4 2 3 4 29%	5330
	11th to 20th	Night	12% 12% 4 8% 5% 5% 5% 4 5% 8% 31%	4979
	21st to end	Night	19% 10% 3 7% 4 5% 4 3 5% 6% 34%	5174
April	1st to 10th	Night	20% 11% 3 8% 4 4 4 3 5% 6% 33%	2000
	11th to 20th	Night	19% 9% 2 8% 5% 4 4 2 5% 6% 36%	1975
	21st to end	Night	21% 8% 3 7% 5% 4 3 3 5% 7% 34%	1877
May	1st to 10th	Night	23% 8% 3 7% 5% 5% 5% 4 7% 7% 25%	3594
	11th to 20th	Night	15% 8% 3 7% 4 4 4 4 5% 7% 39%	3575
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	3926
June	1st to 10th	Night	15% 7% 2 7% 6% 5% 6% 4 6% 8% 33%	1600
	11th to 20th	Night	15% 5% 3 6% 4 4 3 3 6% 8% 43%	1600
	21st to end	Night	18% 7% 2 7% 5% 4 5% 3 5% 8% 36%	1600
July	1st to 10th	Night	14% 6% 3 8% 6% 5% 5% 4 6% 9% 34%	3600
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 4 6% 7% 44%	3600
	21st to end	Night	9% 6% 2 6% 5% 4 5% 4 6% 6% 47%	3926
August	1st to 10th	Night	11% 6% 3 8% 4 5% 5% 4 6% 8% 41%	2672
	11th to 20th	Night	13% 6% 2 7% 4 4 5% 4 5% 7% 43%	2893
	21st to end	Night	12% 6% 2 7% 5% 4 5% 4 6% 8% 41%	3586
September	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 4 5% 6% 48%	2000
	11th to 20th	Night	13% 4 2 5% 3 4 4 3 5% 7% 50%	1981
	21st to end	Night	16% 6% 2 6% 5% 4 3 3 5% 6% 44%	2150
October	1st to 10th	Night	17% 8% 2 6% 4 4 5% 4 5% 5% 39%	2280
	11th to 20th	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	2280
	21st to end	Night	22% 10% 3 7% 4 5% 4 3 5% 6% 29%	2654
November	1st to 10th	Night	19% 11% 3 7% 5% 4 4 3 4 6% 35%	2537
	11th to 20th	Night	17% 11% 3 8% 6% 5% 5% 4 5% 6% 31%	2795
	21st to end	Night	21% 14% 4 9% 6% 4 4 4 6% 6% 24%	2800
December	1st to 10th	Night	19% 16% 4 11% 7% 6% 5% 4 6% 6% 16%	4580
	11th to 20th	Night	22% 14% 5% 9% 5% 5% 3 3 4 5% 25%	5208
	21st to end	Night	26% 17% 4 10% 5% 3 4 3 4 4 20%	6996

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	66% 6% 9% 13% 5% 1	3598
	11th to 20th	Day	64% 5% 10% 13% 8%	3600
	21st to end	Day	65% 6% 10% 11% 7% 1	4162
February	1st to 10th	Day	61% 5% 10% 18% 5% 1	1800
	11th to 20th	Day	57% 5% 11% 17% 9% 1	1762
	21st to end	Day	59% 6% 10% 14% 10% 1	1650
March	1st to 10th	Day	59% 4 9% 13% 12% 3	4497
	11th to 20th	Day	52% 5% 16% 15% 10% 1	4980
	21st to end	Day	49% 4 6% 19% 19% 3	5758
April	1st to 10th	Day	54% 4 7% 19% 14% 3	2400
	11th to 20th	Day	51% 4 7% 21% 16% 2	2374
	21st to end	Day	54% 5% 8% 15% 14% 4	2537
May	1st to 10th	Day	57% 6% 9% 18% 8% 2	6357
	11th to 20th	Day	51% 8% 11% 18% 11% 1	6322
	21st to end	Day	50% 6% 11% 21% 11% 1	6949
June	1st to 10th	Day	53% 7% 9% 18% 13% 1	2800
	11th to 20th	Day	41% 7% 11% 25% 13% 2	2800
	21st to end	Day	48% 8% 12% 20% 11% 1	2800
July	1st to 10th	Day	44% 6% 14% 26% 10%	6290
	11th to 20th	Day	36% 7% 13% 30% 14%	6285
	21st to end	Day	33% 6% 14% 37% 10%	6728
August	1st to 10th	Day	45% 7% 15% 24% 10%	4461
	11th to 20th	Day	42% 6% 14% 28% 9% 1	4162
	21st to end	Day	40% 6% 15% 27% 12%	4312
September	1st to 10th	Day	38% 7% 13% 30% 11%	2400
	11th to 20th	Day	34% 7% 12% 32% 13% 1	2376
	21st to end	Day	43% 7% 14% 27% 10%	2025
October	1st to 10th	Day	47% 5% 12% 30% 7%	3160
	11th to 20th	Day	46% 5% 15% 24% 9%	3140
	21st to end	Day	49% 5% 17% 21% 7%	3181
November	1st to 10th	Day	48% 7% 14% 23% 7% 1	1761
	11th to 20th	Day	56% 6% 12% 15% 9% 1	1602
	21st to end	Day	65% 6% 12% 10% 6% 1	1600
December	1st to 10th	Day	71% 6% 11% 10% 2	2600
	11th to 20th	Day	63% 6% 10% 15% 7%	2986
	21st to end	Day	68% 4 7% 15% 5%	3960
January	1st to 10th	Night	68% 4 11% 13% 4	6349
	11th to 20th	Night	64% 6% 10% 13% 6%	6360
	21st to end	Night	64% 6% 8% 11% 11% 1	6792
February	1st to 10th	Night	61% 5% 10% 16% 7% 1	2600
	11th to 20th	Night	58% 4 10% 19% 8% 1	2534
	21st to end	Night	57% 5% 9% 19% 10% 1	1980
March	1st to 10th	Night	60% 4 7% 15% 12% 3	5340
	11th to 20th	Night	51% 6% 16% 16% 8% 3	4980
	21st to end	Night	51% 4 8% 22% 14% 2	5174
April	1st to 10th	Night	53% 5% 8% 18% 14% 2	2000
	11th to 20th	Night	50% 5% 7% 21% 16% 1	1975
	21st to end	Night	51% 5% 8% 17% 16% 3	1877
May	1st to 10th	Night	56% 8% 8% 18% 8% 3	3600
	11th to 20th	Night	45% 6% 13% 20% 15% 1	3576
	21st to end	Night	49% 7% 12% 17% 14% 1	3927
June	1st to 10th	Night	47% 8% 11% 22% 11% 2	1600
	11th to 20th	Night	40% 6% 11% 24% 17% 2	1600
	21st to end	Night	47% 7% 11% 21% 13% 1	1600
July	1st to 10th	Night	45% 7% 15% 19% 13% 1	3560
	11th to 20th	Night	37% 6% 13% 32% 12%	3552
	21st to end	Night	37% 5% 12% 30% 16%	3809
August	1st to 10th	Night	41% 7% 14% 26% 12% 1	2662
	11th to 20th	Night	40% 7% 16% 29% 9%	2920
	21st to end	Night	39% 7% 14% 27% 12% 2	3586
September	1st to 10th	Night	36% 7% 11% 31% 15% 1	2000
	11th to 20th	Night	33% 7% 12% 36% 10% 1	1981
	21st to end	Night	41% 6% 11% 29% 12%	2150
October	1st to 10th	Night	40% 5% 12% 30% 12%	3800
	11th to 20th	Night	46% 4 15% 26% 9%	3779
	21st to end	Night	49% 5% 16% 21% 8%	4366
November	1st to 10th	Night	51% 5% 13% 21% 9% 1	2537
	11th to 20th	Night	54% 6% 11% 18% 10% 1	2795
	21st to end	Night	60% 6% 13% 15% 5%	2800
December	1st to 10th	Night	68% 5% 13% 11% 3	4580
	11th to 20th	Night	63% 4 7% 14% 10% 1	5208
	21st to end	Night	69% 3 6% 12% 9%	6996

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	22% 20% 38% 16% 4 1	200
	11th to 20th	Day	25% 20% 34% 17% 4 1	200
	21st to end	Day	19% 16% 41% 20% 4	263
February	1st to 10th	Day	28% 24% 33% 12% 2	300
	11th to 20th	Day	24% 31% 36% 9% 1	300
	21st to end	Day	30% 22% 30% 16% 3	414
March	1st to 10th	Day	26% 20% 32% 18% 4	500
	11th to 20th	Day	25% 21% 33% 17% 3 1	500
	21st to end	Day	31% 20% 27% 14% 7% 1	550
April	1st to 10th	Day	31% 20% 28% 14% 6% 1	500
	11th to 20th	Day	33% 17% 23% 15% 9% 2	492
	21st to end	Day	29% 19% 30% 14% 6% 2	500
May	1st to 10th	Day	24% 16% 33% 19% 7% 2	503
	11th to 20th	Day	18% 17% 33% 21% 9% 3	527
	21st to end	Day	21% 14% 30% 23% 11% 1	573
June	1st to 10th	Day	23% 12% 32% 22% 9% 2	530
	11th to 20th	Day	14% 15% 33% 25% 12% 3	530
	21st to end	Day	19% 14% 32% 23% 9% 3	530
July	1st to 10th	Day	11% 8% 29% 34% 15% 3	525
	11th to 20th	Day	12% 8% 31% 31% 15% 4	524
	21st to end	Day	13% 8% 32% 28% 15% 4	576
August	1st to 10th	Day	13% 7% 26% 31% 19% 4	526
	11th to 20th	Day	13% 7% 25% 29% 18% 8%	519
	21st to end	Day	15% 11% 28% 24% 14% 8%	583
September	1st to 10th	Day	15% 12% 29% 19% 16% 9%	530
	11th to 20th	Day	11% 7% 25% 23% 21% 13%	522
	21st to end	Day	18% 11% 28% 22% 16% 6%	490
October	1st to 10th	Day	24% 13% 26% 20% 12% 5%	490
	11th to 20th	Day	25% 20% 27% 19% 8% 1	490
	21st to end	Day	28% 19% 27% 20% 6%	462
November	1st to 10th	Day	24% 19% 31% 18% 8% 1	395
	11th to 20th	Day	26% 19% 34% 14% 6% 1	202
	21st to end	Day	30% 19% 35% 11% 4 1	200
December	1st to 10th	Day	32% 14% 36% 15% 3 1	200
	11th to 20th	Day	29% 27% 31% 10% 4	200
	21st to end	Day	30% 19% 32% 14% 4 1	220
January	1st to 10th	Night	27% 15% 35% 20% 4	800
	11th to 20th	Night	27% 17% 28% 22% 6% 1	800
	21st to end	Night	19% 12% 38% 26% 5%	880
February	1st to 10th	Night	22% 16% 36% 22% 4	800
	11th to 20th	Night	22% 17% 39% 19% 4	783
	21st to end	Night	24% 18% 30% 24% 4	660
March	1st to 10th	Night	25% 14% 33% 24% 3	800
	11th to 20th	Night	22% 19% 32% 22% 5%	800
	21st to end	Night	24% 16% 27% 25% 7%	880
April	1st to 10th	Night	27% 14% 31% 24% 5%	800
	11th to 20th	Night	24% 19% 27% 22% 8%	791
	21st to end	Night	26% 12% 29% 25% 7% 1	677
May	1st to 10th	Night	24% 8% 28% 31% 9% 1	400
	11th to 20th	Night	20% 12% 28% 31% 9%	394
	21st to end	Night	15% 7% 37% 29% 13%	433
June	1st to 10th	Night	17% 6% 32% 35% 11%	400
	11th to 20th	Night	16% 8% 28% 34% 13% 1	400
	21st to end	Night	21% 6% 30% 31% 13% 1	400
July	1st to 10th	Night	9% 6% 26% 44% 14% 1	390
	11th to 20th	Night	13% 5% 23% 44% 14% 1	388
	21st to end	Night	10% 5% 24% 46% 14% 2	438
August	1st to 10th	Night	10% 4 20% 43% 22%	445
	11th to 20th	Night	11% 3 19% 39% 27% 2	599
	21st to end	Night	12% 7% 24% 36% 19% 2	880
September	1st to 10th	Night	13% 5% 30% 31% 19% 2	800
	11th to 20th	Night	15% 7% 19% 32% 22% 5%	793
	21st to end	Night	18% 9% 24% 26% 20% 3	760
October	1st to 10th	Night	19% 12% 25% 27% 15% 1	760
	11th to 20th	Night	25% 12% 29% 25% 8% 1	760
	21st to end	Night	26% 14% 27% 25% 6%	836
November	1st to 10th	Night	27% 13% 28% 24% 8%	780
	11th to 20th	Night	23% 16% 33% 18% 9% 1	800
	21st to end	Night	30% 16% 26% 22% 6%	800
December	1st to 10th	Night	30% 15% 30% 21% 4	800
	11th to 20th	Night	28% 16% 29% 22% 5% 1	800
	21st to end	Night	33% 15% 29% 19% 4	880

Page generated at 2025-09-16 01:49:17. Version ac07

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