Peek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peek Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	9% 8% 3 8% 5% 5% 6% 4 7% 8% 36%	127201
	Night	10% 8% 3 9% 5% 5% 5% 4 6% 8% 37%	124176
Transparency	Day	43% 7% 12% 20% 15% 4	130912
	Night	42% 7% 12% 20% 15% 4	128534
Seeing	Day	19% 13% 23% 22% 15% 8%	16559
	Night	18% 10% 20% 30% 19% 3	26640

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	10% 11% 5% 9% 4 4 4 3 4 7% 39%	4065
	11th to 20th	Day	8% 8% 5% 9% 5% 5% 3 3 3 4 45%	4060
	21st to end	Day	11% 12% 4 7% 4 3 5% 3 4 6% 40%	4466
February	1st to 10th	Day	12% 10% 3 9% 5% 5% 4 4 4 7% 36%	1880
	11th to 20th	Day	14% 13% 4 10% 4 4 4 3 5% 5% 33%	1948
	21st to end	Day	14% 9% 3 8% 4 4 5% 3 5% 7% 37%	1650
March	1st to 10th	Day	14% 10% 5% 8% 6% 4 4 3 5% 5% 36%	4819
	11th to 20th	Day	9% 8% 4 6% 4 4 4 3 4 6% 48%	4980
	21st to end	Day	12% 9% 3 7% 4 4 4 4 5% 7% 42%	5465
April	1st to 10th	Day	11% 8% 3 6% 5% 5% 5% 4 6% 8% 41%	2200
	11th to 20th	Day	12% 8% 2 5% 5% 4 5% 4 6% 7% 41%	2403
	21st to end	Day	12% 7% 3 7% 6% 5% 4 4 6% 7% 40%	2600
May	1st to 10th	Day	9% 9% 3 9% 7% 7% 7% 5% 7% 8% 29%	5862
	11th to 20th	Day	13% 9% 3 9% 7% 7% 7% 5% 8% 9% 25%	5861
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 4 8% 10% 33%	6433
June	1st to 10th	Day	9% 7% 3 7% 7% 7% 7% 6% 8% 8% 33%	2600
	11th to 20th	Day	6% 6% 3 8% 6% 7% 7% 6% 10% 10% 32%	2600
	21st to end	Day	7% 6% 2 7% 6% 7% 7% 6% 9% 10% 33%	2600
July	1st to 10th	Day	5% 5% 2 8% 6% 6% 7% 6% 8% 12% 34%	5900
	11th to 20th	Day	6% 6% 3 9% 7% 7% 8% 6% 9% 10% 27%	5900
	21st to end	Day	8% 8% 4 11% 8% 8% 8% 6% 8% 10% 22%	6424
August	1st to 10th	Day	9% 8% 3 8% 7% 7% 7% 6% 10% 11% 24%	4234
	11th to 20th	Day	7% 6% 2 8% 7% 7% 7% 5% 9% 11% 31%	4174
	21st to end	Day	5% 4 2 6% 5% 5% 5% 5% 7% 9% 44%	4334
September	1st to 10th	Day	8% 7% 3 8% 5% 6% 6% 5% 7% 8% 37%	2200
	11th to 20th	Day	11% 7% 4 8% 6% 5% 6% 5% 6% 8% 34%	2200
	21st to end	Day	11% 6% 3 8% 6% 6% 6% 4 6% 8% 38%	2200
October	1st to 10th	Day	9% 6% 2 7% 4 4 4 3 6% 9% 47%	2165
	11th to 20th	Day	8% 8% 2 6% 4 3 4 2 5% 6% 52%	2000
	21st to end	Day	12% 7% 2 7% 3 3 3 3 5% 6% 49%	2200
November	1st to 10th	Day	7% 7% 3 6% 3 3 3 3 5% 7% 53%	1950
	11th to 20th	Day	9% 8% 2 6% 4 4 4 4 5% 7% 47%	1890
	21st to end	Day	11% 9% 3 7% 4 4 4 3 4 6% 45%	1890
December	1st to 10th	Day	10% 8% 3 7% 5% 4 5% 4 6% 6% 42%	3020
	11th to 20th	Day	12% 9% 4 8% 5% 5% 5% 5% 7% 7% 34%	3463
	21st to end	Day	9% 10% 6% 8% 5% 5% 5% 4 5% 9% 35%	4565
January	1st to 10th	Night	10% 10% 5% 13% 6% 5% 4 4 5% 5% 33%	5903
	11th to 20th	Night	10% 8% 3 12% 8% 5% 4 3 5% 6% 36%	5900
	21st to end	Night	11% 11% 4 8% 4 4 4 4 5% 6% 40%	6490
February	1st to 10th	Night	11% 9% 4 9% 5% 3 4 4 6% 6% 37%	2515
	11th to 20th	Night	17% 11% 4 10% 5% 4 4 3 5% 4 33%	2338
	21st to end	Night	13% 10% 4 9% 6% 3 4 3 6% 7% 35%	1980
March	1st to 10th	Night	14% 12% 4 10% 5% 4 4 3 5% 5% 34%	5034
	11th to 20th	Night	8% 7% 3 6% 4 3 3 4 6% 8% 47%	4980
	21st to end	Night	10% 8% 3 7% 5% 5% 5% 3 5% 7% 42%	5467
April	1st to 10th	Night	10% 8% 4 7% 4 3 3 5% 6% 8% 42%	2200
	11th to 20th	Night	9% 9% 2 7% 5% 4 5% 3 5% 6% 45%	1960
	21st to end	Night	10% 8% 3 6% 5% 5% 4 3 5% 8% 43%	1800
May	1st to 10th	Night	11% 8% 3 8% 5% 6% 6% 5% 8% 11% 29%	4033
	11th to 20th	Night	13% 10% 5% 11% 6% 5% 6% 4 7% 8% 26%	4032
	21st to end	Night	12% 8% 3 9% 5% 5% 5% 4 7% 9% 32%	4421
June	1st to 10th	Night	6% 6% 3 7% 6% 6% 5% 6% 7% 12% 36%	1800
	11th to 20th	Night	5% 5% 1 6% 5% 5% 6% 6% 10% 12% 38%	1800
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 4 8% 11% 40%	1800
July	1st to 10th	Night	6% 5% 3 8% 6% 6% 7% 6% 9% 11% 33%	4058
	11th to 20th	Night	5% 6% 3 8% 6% 7% 7% 7% 11% 11% 28%	4057
	21st to end	Night	8% 9% 4 11% 8% 7% 7% 5% 8% 9% 24%	4427
August	1st to 10th	Night	6% 6% 3 8% 6% 6% 7% 6% 10% 12% 29%	2915
	11th to 20th	Night	7% 7% 3 10% 7% 6% 7% 5% 7% 10% 32%	2875
	21st to end	Night	5% 5% 2 6% 4 5% 6% 5% 8% 10% 46%	3525
September	1st to 10th	Night	8% 6% 3 8% 6% 6% 5% 5% 6% 9% 39%	2200
	11th to 20th	Night	10% 7% 3 9% 5% 4 5% 4 7% 9% 36%	2200
	21st to end	Night	10% 7% 3 7% 5% 5% 4 3 5% 7% 45%	2200
October	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	2235
	11th to 20th	Night	9% 7% 3 6% 4 4 3 4 5% 7% 49%	2400
	21st to end	Night	9% 8% 3 5% 3 4 4 3 4 7% 50%	2640
November	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	2566
	11th to 20th	Night	10% 8% 3 6% 4 4 3 4 6% 6% 46%	2730
	21st to end	Night	11% 10% 3 6% 4 3 4 4 5% 6% 43%	2730
December	1st to 10th	Night	11% 8% 3 10% 6% 4 5% 3 6% 6% 39%	4380
	11th to 20th	Night	13% 10% 6% 11% 6% 5% 5% 3 5% 5% 31%	4952
	21st to end	Night	11% 9% 5% 11% 5% 4 5% 4 5% 6% 34%	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	43% 4 7% 25% 20% 2	4066
	11th to 20th	Day	42% 4 5% 16% 31% 3	4060
	21st to end	Day	46% 4 6% 19% 21% 4	4466
February	1st to 10th	Day	49% 5% 10% 19% 16% 2	1880
	11th to 20th	Day	53% 4 9% 16% 15% 3	1958
	21st to end	Day	47% 6% 8% 16% 18% 6%	1650
March	1st to 10th	Day	48% 6% 6% 14% 18% 9%	4819
	11th to 20th	Day	37% 4 6% 17% 22% 14%	4980
	21st to end	Day	42% 4 8% 27% 16% 2	5465
April	1st to 10th	Day	40% 7% 12% 22% 17% 3	2200
	11th to 20th	Day	40% 6% 9% 20% 20% 5%	2403
	21st to end	Day	42% 6% 12% 24% 14% 2	2600
May	1st to 10th	Day	49% 9% 13% 19% 9% 1	5897
	11th to 20th	Day	52% 9% 16% 17% 6%	5865
	21st to end	Day	44% 9% 13% 18% 14% 2	6447
June	1st to 10th	Day	44% 13% 16% 17% 9% 1	2600
	11th to 20th	Day	42% 13% 16% 16% 12% 1	2600
	21st to end	Day	42% 11% 16% 18% 11% 2	2600
July	1st to 10th	Day	38% 11% 19% 22% 10%	5835
	11th to 20th	Day	46% 11% 24% 15% 4 1	5831
	21st to end	Day	53% 11% 16% 13% 6%	6240
August	1st to 10th	Day	48% 9% 18% 22% 4	4221
	11th to 20th	Day	42% 9% 20% 18% 9% 2	4213
	21st to end	Day	34% 8% 13% 33% 10% 1	4334
September	1st to 10th	Day	41% 10% 13% 17% 16% 3	2200
	11th to 20th	Day	46% 8% 11% 19% 14% 2	2200
	21st to end	Day	44% 7% 13% 17% 15% 4	2200
October	1st to 10th	Day	29% 5% 6% 20% 27% 12%	3529
	11th to 20th	Day	26% 3 5% 24% 23% 18%	3240
	21st to end	Day	28% 3 8% 27% 23% 11%	3538
November	1st to 10th	Day	31% 5% 11% 19% 27% 7%	1950
	11th to 20th	Day	36% 5% 7% 23% 25% 5%	1890
	21st to end	Day	40% 5% 7% 24% 19% 4	1890
December	1st to 10th	Day	41% 6% 8% 17% 25% 4	3020
	11th to 20th	Day	46% 4 12% 17% 18% 3	3460
	21st to end	Day	46% 4 12% 23% 13% 2	4565
January	1st to 10th	Night	51% 4 7% 20% 17% 1	5903
	11th to 20th	Night	46% 6% 5% 15% 22% 6%	5900
	21st to end	Night	45% 4 7% 20% 21% 4	6490
February	1st to 10th	Night	46% 6% 10% 20% 16% 2	2515
	11th to 20th	Night	53% 4 8% 17% 13% 4	2339
	21st to end	Night	48% 6% 9% 17% 15% 4	1980
March	1st to 10th	Night	50% 7% 5% 14% 18% 5%	5034
	11th to 20th	Night	34% 4 8% 21% 22% 11%	4980
	21st to end	Night	41% 5% 11% 27% 13% 3	5467
April	1st to 10th	Night	40% 7% 13% 22% 16% 3	2200
	11th to 20th	Night	38% 6% 9% 23% 19% 4	1960
	21st to end	Night	39% 6% 14% 26% 13% 1	1800
May	1st to 10th	Night	46% 9% 16% 22% 6% 1	4060
	11th to 20th	Night	53% 10% 16% 17% 4	4033
	21st to end	Night	44% 11% 14% 21% 9%	4429
June	1st to 10th	Night	37% 14% 19% 21% 9% 1	1800
	11th to 20th	Night	32% 14% 23% 21% 9% 1	1800
	21st to end	Night	34% 12% 19% 21% 12% 2	1800
July	1st to 10th	Night	39% 11% 19% 24% 7%	4015
	11th to 20th	Night	41% 14% 26% 15% 3 1	4006
	21st to end	Night	53% 11% 17% 13% 6%	4297
August	1st to 10th	Night	40% 11% 27% 18% 4	2904
	11th to 20th	Night	43% 11% 22% 16% 7%	2902
	21st to end	Night	31% 8% 18% 33% 9% 1	3525
September	1st to 10th	Night	40% 10% 14% 20% 14% 3	2200
	11th to 20th	Night	42% 10% 13% 19% 14% 1	2200
	21st to end	Night	38% 8% 12% 24% 15% 3	2200
October	1st to 10th	Night	31% 6% 8% 21% 23% 11%	3651
	11th to 20th	Night	25% 5% 7% 21% 26% 15%	3899
	21st to end	Night	27% 3 8% 28% 22% 12%	4252
November	1st to 10th	Night	36% 6% 10% 22% 21% 6%	2566
	11th to 20th	Night	37% 5% 7% 22% 23% 6%	2730
	21st to end	Night	41% 4 7% 26% 19% 4	2730
December	1st to 10th	Night	44% 7% 8% 18% 21% 2	4380
	11th to 20th	Night	54% 5% 10% 17% 13% 2	4954
	21st to end	Night	48% 6% 14% 19% 12% 1	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	32% 27% 29% 10% 2 1	303
	11th to 20th	Day	33% 22% 30% 13% 2	300
	21st to end	Day	30% 30% 28% 9% 3	330
February	1st to 10th	Day	29% 26% 29% 14% 3	300
	11th to 20th	Day	45% 25% 22% 6% 1	300
	21st to end	Day	38% 22% 30% 8% 1	249
March	1st to 10th	Day	30% 21% 25% 18% 5% 1	445
	11th to 20th	Day	30% 22% 24% 19% 4 1	500
	21st to end	Day	26% 23% 28% 18% 5%	550
April	1st to 10th	Day	25% 18% 26% 20% 8% 2	500
	11th to 20th	Day	20% 22% 23% 19% 12% 3	492
	21st to end	Day	22% 15% 25% 20% 13% 5%	500
May	1st to 10th	Day	21% 18% 24% 18% 13% 6%	503
	11th to 20th	Day	17% 13% 24% 22% 15% 8%	527
	21st to end	Day	15% 9% 22% 24% 18% 12%	573
June	1st to 10th	Day	12% 9% 26% 25% 17% 11%	530
	11th to 20th	Day	8% 7% 23% 25% 22% 16%	530
	21st to end	Day	10% 7% 23% 26% 18% 16%	530
July	1st to 10th	Day	10% 3 15% 25% 27% 20%	525
	11th to 20th	Day	9% 3 13% 26% 26% 23%	524
	21st to end	Day	10% 3 15% 25% 28% 20%	576
August	1st to 10th	Day	18% 4 17% 22% 24% 16%	526
	11th to 20th	Day	10% 4 16% 27% 26% 17%	519
	21st to end	Day	12% 4 13% 25% 25% 21%	583
September	1st to 10th	Day	9% 4 16% 27% 27% 16%	530
	11th to 20th	Day	13% 5% 17% 22% 29% 15%	530
	21st to end	Day	15% 7% 19% 31% 25% 4	530
October	1st to 10th	Day	14% 8% 18% 29% 22% 9%	530
	11th to 20th	Day	14% 13% 23% 26% 21% 3	530
	21st to end	Day	20% 15% 20% 29% 13% 3	583
November	1st to 10th	Day	16% 15% 25% 29% 11% 4	428
	11th to 20th	Day	22% 18% 35% 17% 8% 1	330
	21st to end	Day	25% 17% 33% 14% 9% 2	330
December	1st to 10th	Day	28% 24% 31% 13% 3	330
	11th to 20th	Day	24% 21% 27% 22% 6% 1	330
	21st to end	Day	25% 22% 32% 14% 6% 1	363
January	1st to 10th	Night	26% 17% 28% 24% 5%	805
	11th to 20th	Night	25% 17% 29% 25% 5%	800
	21st to end	Night	23% 17% 31% 26% 3	880
February	1st to 10th	Night	23% 13% 31% 30% 4	800
	11th to 20th	Night	28% 20% 30% 20% 1	783
	21st to end	Night	24% 22% 33% 20% 2	660
March	1st to 10th	Night	22% 17% 29% 28% 5%	800
	11th to 20th	Night	25% 15% 27% 24% 8% 1	800
	21st to end	Night	21% 15% 31% 25% 8%	880
April	1st to 10th	Night	22% 16% 25% 28% 9% 1	800
	11th to 20th	Night	19% 12% 27% 28% 13% 1	683
	21st to end	Night	17% 9% 21% 39% 13% 1	600
May	1st to 10th	Night	16% 10% 22% 33% 18% 1	600
	11th to 20th	Night	14% 7% 19% 35% 23% 3	591
	21st to end	Night	10% 5% 13% 32% 35% 4	650
June	1st to 10th	Night	10% 3 12% 38% 32% 6%	600
	11th to 20th	Night	7% 3 10% 35% 37% 9%	600
	21st to end	Night	8% 3 10% 35% 39% 7%	600
July	1st to 10th	Night	10% 1 11% 25% 45% 8%	585
	11th to 20th	Night	8% 3 6% 24% 46% 14%	582
	21st to end	Night	7% 2 8% 26% 44% 13%	657
August	1st to 10th	Night	11% 2 5% 31% 44% 8%	597
	11th to 20th	Night	10% 3 7% 30% 42% 8%	594
	21st to end	Night	11% 1 8% 27% 44% 9%	709
September	1st to 10th	Night	11% 3 11% 29% 39% 7%	800
	11th to 20th	Night	15% 4 11% 28% 36% 7%	800
	21st to end	Night	17% 4 15% 32% 30% 3	800
October	1st to 10th	Night	15% 7% 16% 28% 27% 7%	800
	11th to 20th	Night	15% 8% 21% 36% 17% 2	800
	21st to end	Night	17% 11% 18% 38% 14% 3	880
November	1st to 10th	Night	17% 8% 17% 38% 18% 3	820
	11th to 20th	Night	20% 11% 23% 34% 11% 2	840
	21st to end	Night	22% 11% 23% 33% 11% 1	840
December	1st to 10th	Night	24% 14% 26% 29% 6%	840
	11th to 20th	Night	20% 13% 28% 32% 6% 1	840
	21st to end	Night	25% 13% 26% 30% 5%	924

Page generated at 2025-12-30 01:14:11. Version ac07

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