Tamke-Allan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tamke-Allan Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	13% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	126844
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 41%	124200
Transparency	Day	40% 9% 12% 21% 15% 3	130583
	Night	39% 8% 12% 22% 16% 4	128529
Seeing	Day	17% 11% 23% 23% 18% 8%	16226
	Night	16% 9% 23% 30% 19% 3	26261

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% 11% 4 6% 4 3 4 4 5% 6% 37%	4056
	11th to 20th	Day	15% 8% 6% 8% 3 4 4 3 4 5% 39%	4060
	21st to end	Day	20% 9% 2 6% 4 3 3 4 5% 7% 37%	4540
February	1st to 10th	Day	18% 11% 3 6% 5% 4 4 3 5% 4 36%	2000
	11th to 20th	Day	23% 8% 4 6% 4 4 3 3 4 6% 35%	1948
	21st to end	Day	20% 8% 3 7% 4 3 3 3 5% 5% 38%	1650
March	1st to 10th	Day	19% 10% 3 7% 4 5% 4 4 5% 6% 34%	4544
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 5% 6% 48%	4980
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 6% 44%	5465
April	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 4 6% 6% 44%	2325
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 5% 7% 45%	2374
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 6% 8% 42%	2578
May	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 5% 6% 8% 29%	5862
	11th to 20th	Day	15% 9% 4 9% 6% 5% 6% 4 6% 7% 29%	5861
	21st to end	Day	15% 7% 3 7% 4 4 4 3 6% 7% 38%	6433
June	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 10% 39%	2600
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 7% 9% 11% 41%	2600
	21st to end	Day	7% 5% 2 5% 5% 5% 7% 5% 8% 12% 40%	2600
July	1st to 10th	Day	5% 4 2 7% 5% 6% 6% 6% 8% 11% 40%	5900
	11th to 20th	Day	8% 6% 2 7% 6% 7% 7% 6% 10% 12% 31%	5900
	21st to end	Day	12% 8% 3 9% 6% 6% 7% 5% 8% 9% 24%	6424
August	1st to 10th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 11% 39%	4234
	11th to 20th	Day	6% 5% 2 6% 5% 5% 7% 5% 8% 11% 40%	3979
	21st to end	Day	6% 4 1 5% 5% 4 5% 5% 7% 8% 51%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 8% 42%	2250
	11th to 20th	Day	14% 5% 2 6% 4 5% 4 5% 6% 8% 42%	2200
	21st to end	Day	14% 5% 2 6% 4 4 4 4 4 6% 48%	2200
October	1st to 10th	Day	10% 5% 2 5% 4 3 3 3 5% 5% 55%	2200
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 4 6% 55%	2085
	21st to end	Day	15% 5% 2 4 4 3 3 4 6% 7% 48%	2130
November	1st to 10th	Day	11% 7% 2 4 3 3 3 3 4 6% 53%	1852
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 6% 49%	1890
	21st to end	Day	16% 7% 2 5% 4 4 3 4 5% 5% 45%	1890
December	1st to 10th	Day	15% 7% 3 6% 4 3 4 4 4 5% 44%	3020
	11th to 20th	Day	20% 12% 3 8% 5% 4 4 3 5% 5% 31%	3436
	21st to end	Day	19% 10% 3 7% 4 4 4 3 6% 9% 31%	4466
January	1st to 10th	Night	16% 8% 3 8% 4 4 4 3 5% 5% 40%	5889
	11th to 20th	Night	18% 7% 2 6% 4 3 4 3 5% 5% 42%	5900
	21st to end	Night	16% 10% 3 6% 4 4 4 4 6% 6% 39%	6398
February	1st to 10th	Night	17% 8% 3 6% 4 5% 4 3 6% 6% 38%	2400
	11th to 20th	Night	23% 8% 3 6% 4 4 4 3 5% 5% 35%	2338
	21st to end	Night	22% 8% 3 5% 5% 3 4 4 4 6% 36%	1980
March	1st to 10th	Night	18% 10% 3 7% 5% 4 4 3 5% 7% 34%	5309
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 5% 8% 51%	4980
	21st to end	Night	14% 7% 3 7% 5% 4 4 4 5% 6% 42%	5467
April	1st to 10th	Night	12% 5% 1 5% 4 4 4 3 5% 7% 48%	2090
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 6% 49%	1975
	21st to end	Night	11% 6% 2 7% 5% 4 4 4 6% 8% 44%	1820
May	1st to 10th	Night	14% 8% 3 9% 6% 5% 6% 4 7% 9% 30%	4033
	11th to 20th	Night	18% 9% 3 8% 5% 5% 4 4 6% 6% 30%	4032
	21st to end	Night	14% 7% 3 7% 4 4 5% 4 6% 8% 39%	4421
June	1st to 10th	Night	7% 6% 2 7% 5% 5% 5% 6% 7% 8% 42%	1800
	11th to 20th	Night	5% 3 1 5% 4 4 4 5% 9% 11% 49%	1800
	21st to end	Night	6% 5% 2 6% 6% 5% 5% 5% 6% 11% 43%	1800
July	1st to 10th	Night	7% 6% 2 6% 5% 5% 6% 5% 9% 12% 37%	4058
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 12% 38%	4057
	21st to end	Night	10% 9% 3 9% 6% 6% 6% 6% 8% 12% 24%	4427
August	1st to 10th	Night	6% 4 2 6% 4 4 5% 5% 7% 11% 45%	2915
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 9% 49%	3065
	21st to end	Night	6% 4 2 4 3 4 4 4 6% 9% 56%	3586
September	1st to 10th	Night	9% 7% 3 6% 5% 5% 5% 4 6% 7% 44%	2130
	11th to 20th	Night	11% 6% 2 8% 4 3 4 3 6% 9% 43%	2200
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 6% 49%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 4 6% 56%	2200
	11th to 20th	Night	13% 5% 1 4 3 3 3 3 4 6% 55%	2314
	21st to end	Night	15% 6% 2 4 3 4 3 3 3 5% 52%	2706
November	1st to 10th	Night	12% 6% 1 5% 4 3 4 3 5% 6% 51%	2666
	11th to 20th	Night	12% 7% 3 5% 3 3 3 3 4 6% 50%	2730
	21st to end	Night	16% 8% 3 5% 3 3 4 3 5% 7% 42%	2730
December	1st to 10th	Night	19% 7% 3 7% 4 4 4 3 5% 6% 39%	4380
	11th to 20th	Night	21% 10% 3 6% 4 4 4 3 5% 5% 36%	4914
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	6490

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	45% 6% 9% 23% 16% 1	4057
	11th to 20th	Day	46% 5% 7% 19% 22% 2	4060
	21st to end	Day	45% 6% 9% 23% 15% 2	4540
February	1st to 10th	Day	50% 5% 9% 18% 16% 2	2000
	11th to 20th	Day	52% 6% 8% 19% 13% 3	1958
	21st to end	Day	48% 7% 8% 16% 16% 6%	1650
March	1st to 10th	Day	48% 8% 7% 14% 16% 8%	4544
	11th to 20th	Day	36% 6% 7% 21% 21% 9%	4980
	21st to end	Day	40% 7% 8% 29% 14% 1	5465
April	1st to 10th	Day	38% 8% 12% 22% 16% 3	2325
	11th to 20th	Day	38% 6% 11% 17% 23% 5%	2374
	21st to end	Day	39% 8% 13% 25% 13% 2	2578
May	1st to 10th	Day	48% 10% 12% 19% 9% 1	5897
	11th to 20th	Day	50% 11% 14% 17% 7%	5865
	21st to end	Day	43% 9% 12% 20% 14% 3	6447
June	1st to 10th	Day	37% 12% 17% 19% 13% 2	2600
	11th to 20th	Day	31% 14% 18% 21% 14% 2	2600
	21st to end	Day	33% 14% 18% 19% 13% 3	2600
July	1st to 10th	Day	33% 13% 18% 21% 14% 1	5835
	11th to 20th	Day	39% 13% 23% 19% 5% 1	5831
	21st to end	Day	50% 12% 16% 16% 5%	6240
August	1st to 10th	Day	33% 11% 17% 29% 10%	4221
	11th to 20th	Day	32% 12% 18% 24% 13% 1	4018
	21st to end	Day	28% 9% 14% 31% 17% 2	4311
September	1st to 10th	Day	37% 10% 13% 19% 16% 5%	2250
	11th to 20th	Day	38% 9% 13% 21% 16% 3	2200
	21st to end	Day	36% 7% 13% 21% 17% 6%	2200
October	1st to 10th	Day	22% 6% 7% 22% 31% 12%	3590
	11th to 20th	Day	22% 4 7% 25% 32% 11%	3377
	21st to end	Day	28% 6% 10% 25% 22% 8%	3419
November	1st to 10th	Day	32% 5% 9% 23% 25% 5%	1852
	11th to 20th	Day	35% 4 8% 25% 23% 5%	1890
	21st to end	Day	41% 5% 10% 23% 19% 3	1890
December	1st to 10th	Day	41% 5% 6% 23% 22% 2	3020
	11th to 20th	Day	55% 6% 9% 18% 11% 2	3433
	21st to end	Day	48% 6% 16% 21% 8% 1	4466
January	1st to 10th	Night	42% 7% 10% 22% 17% 1	5889
	11th to 20th	Night	42% 7% 7% 19% 20% 5%	5900
	21st to end	Night	44% 6% 8% 20% 18% 5%	6398
February	1st to 10th	Night	46% 6% 10% 20% 15% 2	2400
	11th to 20th	Night	51% 5% 8% 18% 14% 4	2339
	21st to end	Night	50% 6% 7% 17% 15% 4	1980
March	1st to 10th	Night	47% 7% 9% 16% 17% 4	5309
	11th to 20th	Night	32% 6% 7% 21% 22% 11%	4980
	21st to end	Night	43% 6% 9% 27% 14% 2	5467
April	1st to 10th	Night	34% 7% 12% 23% 21% 3	2090
	11th to 20th	Night	35% 5% 9% 21% 23% 6%	1975
	21st to end	Night	37% 9% 12% 26% 15% 1	1820
May	1st to 10th	Night	47% 11% 14% 20% 8% 1	4060
	11th to 20th	Night	51% 9% 13% 16% 9%	4033
	21st to end	Night	40% 11% 13% 22% 13% 1	4429
June	1st to 10th	Night	36% 11% 14% 23% 14% 2	1800
	11th to 20th	Night	25% 11% 22% 27% 13% 2	1800
	21st to end	Night	33% 11% 15% 24% 13% 4	1800
July	1st to 10th	Night	36% 11% 20% 21% 11% 1	4015
	11th to 20th	Night	35% 12% 26% 20% 5% 2	4006
	21st to end	Night	48% 14% 19% 14% 5% 1	4297
August	1st to 10th	Night	30% 10% 18% 30% 12% 1	2904
	11th to 20th	Night	30% 10% 20% 26% 13% 1	3092
	21st to end	Night	24% 8% 14% 38% 15% 1	3586
September	1st to 10th	Night	35% 11% 16% 17% 16% 6%	2130
	11th to 20th	Night	38% 9% 14% 19% 20% 1	2200
	21st to end	Night	35% 8% 11% 21% 20% 5%	2200
October	1st to 10th	Night	24% 6% 8% 21% 27% 14%	3590
	11th to 20th	Night	20% 4 6% 24% 32% 14%	3761
	21st to end	Night	26% 4 7% 26% 29% 9%	4367
November	1st to 10th	Night	34% 5% 8% 22% 23% 8%	2666
	11th to 20th	Night	35% 5% 8% 21% 24% 7%	2730
	21st to end	Night	42% 5% 7% 24% 19% 3	2730
December	1st to 10th	Night	45% 6% 8% 19% 21% 2	4380
	11th to 20th	Night	48% 6% 9% 21% 15% 1	4916
	21st to end	Night	48% 8% 15% 19% 9% 1	6490

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	26% 25% 30% 16% 3	300
	11th to 20th	Day	29% 25% 28% 14% 3 1	300
	21st to end	Day	22% 23% 41% 10% 4	330
February	1st to 10th	Day	26% 26% 26% 15% 5%	300
	11th to 20th	Day	37% 23% 30% 8% 1 1	300
	21st to end	Day	35% 22% 33% 8% 2	249
March	1st to 10th	Day	26% 22% 32% 15% 4	325
	11th to 20th	Day	23% 19% 32% 19% 5% 1	500
	21st to end	Day	24% 16% 34% 18% 7%	550
April	1st to 10th	Day	21% 19% 26% 22% 10% 1	500
	11th to 20th	Day	19% 15% 30% 23% 11% 1	492
	21st to end	Day	19% 12% 29% 26% 11% 3	500
May	1st to 10th	Day	17% 15% 32% 22% 11% 4	503
	11th to 20th	Day	12% 9% 26% 28% 16% 9%	527
	21st to end	Day	17% 9% 18% 23% 21% 12%	573
June	1st to 10th	Day	12% 7% 21% 25% 23% 12%	530
	11th to 20th	Day	6% 5% 15% 29% 30% 15%	530
	21st to end	Day	8% 2 18% 27% 28% 16%	530
July	1st to 10th	Day	8% 4 11% 27% 30% 20%	525
	11th to 20th	Day	10% 4 10% 26% 30% 20%	524
	21st to end	Day	11% 5% 9% 25% 30% 19%	576
August	1st to 10th	Day	12% 5% 12% 25% 30% 16%	526
	11th to 20th	Day	11% 3 17% 29% 29% 11%	519
	21st to end	Day	10% 3 13% 22% 32% 20%	583
September	1st to 10th	Day	10% 3 16% 24% 32% 15%	530
	11th to 20th	Day	14% 3 10% 25% 32% 15%	530
	21st to end	Day	18% 5% 16% 25% 28% 8%	530
October	1st to 10th	Day	12% 5% 22% 27% 24% 10%	530
	11th to 20th	Day	10% 13% 23% 31% 19% 3	530
	21st to end	Day	19% 10% 25% 28% 15% 3	513
November	1st to 10th	Day	18% 12% 28% 26% 13% 3	330
	11th to 20th	Day	15% 14% 36% 29% 6%	330
	21st to end	Day	19% 13% 36% 21% 9% 1	330
December	1st to 10th	Day	27% 18% 32% 18% 5%	330
	11th to 20th	Day	25% 16% 30% 22% 7%	321
	21st to end	Day	29% 18% 29% 18% 6% 1	330
January	1st to 10th	Night	22% 17% 33% 24% 4	800
	11th to 20th	Night	23% 16% 33% 24% 4	800
	21st to end	Night	19% 15% 37% 27% 2	880
February	1st to 10th	Night	21% 14% 35% 27% 4	800
	11th to 20th	Night	29% 15% 33% 21% 2	783
	21st to end	Night	24% 17% 34% 23% 1	660
March	1st to 10th	Night	20% 13% 32% 32% 4	800
	11th to 20th	Night	19% 14% 33% 26% 7% 1	800
	21st to end	Night	20% 12% 30% 29% 8%	880
April	1st to 10th	Night	14% 14% 32% 30% 9%	690
	11th to 20th	Night	16% 10% 30% 32% 12%	593
	21st to end	Night	12% 8% 32% 37% 10% 1	600
May	1st to 10th	Night	12% 7% 27% 36% 16% 2	600
	11th to 20th	Night	13% 6% 23% 33% 21% 4	591
	21st to end	Night	8% 5% 16% 33% 33% 4	650
June	1st to 10th	Night	9% 3 16% 36% 32% 5%	600
	11th to 20th	Night	5% 3 14% 34% 37% 8%	600
	21st to end	Night	7% 1 10% 34% 39% 10%	600
July	1st to 10th	Night	10% 3 8% 25% 42% 11%	585
	11th to 20th	Night	8% 4 8% 27% 43% 10%	582
	21st to end	Night	8% 3 10% 28% 41% 10%	657
August	1st to 10th	Night	10% 2 8% 28% 44% 9%	597
	11th to 20th	Night	10% 2 9% 31% 42% 5%	594
	21st to end	Night	10% 1 8% 31% 41% 9%	660
September	1st to 10th	Night	10% 3 12% 30% 38% 7%	730
	11th to 20th	Night	12% 3 13% 31% 35% 6%	800
	21st to end	Night	15% 3 17% 32% 31% 2	800
October	1st to 10th	Night	11% 7% 18% 27% 31% 6%	800
	11th to 20th	Night	15% 6% 24% 35% 19% 2	800
	21st to end	Night	16% 7% 23% 36% 16% 1	880
November	1st to 10th	Night	14% 8% 22% 36% 16% 3	820
	11th to 20th	Night	14% 12% 31% 33% 10% 1	840
	21st to end	Night	20% 11% 28% 31% 10%	840
December	1st to 10th	Night	24% 14% 27% 27% 7%	840
	11th to 20th	Night	22% 11% 30% 29% 7%	829
	21st to end	Night	26% 14% 24% 31% 5%	880

Page generated at 2025-12-15 01:13:49. Version ac07

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