Olmzog Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Olmzog Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	15% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	131445
	Night	15% 8% 3 7% 4 4 4 4 5% 7% 39%	131063
Transparency	Day	45% 5% 12% 22% 14% 2	134952
	Night	44% 5% 12% 22% 15% 2	135643
Seeing	Day	25% 16% 29% 19% 8% 3	16722
	Night	22% 15% 30% 25% 8% 1	25821

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	8% 7% 3 12% 6% 7% 6% 5% 7% 7% 32%	4661
	11th to 20th	Day	11% 9% 4 9% 5% 5% 6% 4 6% 6% 35%	4650
	21st to end	Day	11% 10% 4 8% 5% 4 4 3 5% 5% 40%	5124
February	1st to 10th	Day	11% 9% 3 10% 6% 5% 5% 4 7% 7% 33%	1919
	11th to 20th	Day	11% 8% 3 9% 6% 5% 5% 4 7% 6% 36%	2048
	21st to end	Day	17% 11% 4 7% 5% 5% 5% 3 5% 5% 35%	1730
March	1st to 10th	Day	17% 11% 3 8% 5% 4 4 4 5% 5% 34%	4576
	11th to 20th	Day	13% 8% 2 7% 5% 5% 5% 5% 6% 7% 38%	4762
	21st to end	Day	19% 11% 2 6% 4 3 3 3 5% 7% 36%	6102
April	1st to 10th	Day	16% 10% 3 8% 6% 4 4 3 5% 6% 35%	2520
	11th to 20th	Day	15% 8% 2 7% 5% 4 5% 4 6% 7% 36%	2494
	21st to end	Day	18% 7% 2 7% 5% 4 5% 4 5% 6% 36%	2679
May	1st to 10th	Day	30% 10% 3 7% 4 4 4 3 5% 6% 24%	5899
	11th to 20th	Day	20% 8% 3 9% 5% 5% 5% 4 5% 6% 30%	6262
	21st to end	Day	19% 7% 2 6% 5% 4 5% 4 7% 8% 33%	6949
June	1st to 10th	Day	18% 5% 2 7% 5% 5% 6% 4 7% 8% 33%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 6% 5% 4 6% 8% 37%	2800
	21st to end	Day	16% 7% 2 7% 5% 6% 5% 4 6% 9% 33%	2800
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 6% 6% 7% 10% 34%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 5% 5% 7% 8% 36%	4448
	11th to 20th	Day	13% 6% 3 7% 4 5% 5% 5% 7% 9% 36%	3978
	21st to end	Day	8% 4 2 6% 5% 5% 6% 4 8% 10% 42%	4312
September	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 3 6% 8% 45%	2400
	11th to 20th	Day	13% 4 2 6% 3 3 4 3 6% 7% 49%	2260
	21st to end	Day	17% 6% 2 5% 4 4 4 4 5% 8% 42%	2200
October	1st to 10th	Day	22% 5% 2 6% 4 4 4 3 5% 5% 41%	2164
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 5% 8% 42%	1950
	21st to end	Day	19% 8% 3 7% 4 5% 4 3 5% 7% 36%	1980
November	1st to 10th	Day	11% 8% 2 6% 5% 3 4 4 4 6% 48%	1851
	11th to 20th	Day	15% 7% 3 5% 3 4 5% 5% 5% 6% 43%	1890
	21st to end	Day	17% 9% 2 7% 4 4 4 4 6% 6% 38%	1797
December	1st to 10th	Day	14% 8% 3 10% 5% 5% 5% 3 6% 6% 35%	2680
	11th to 20th	Day	19% 8% 2 7% 5% 4 4 3 5% 5% 38%	3066
	21st to end	Day	15% 10% 3 8% 5% 5% 4 3 5% 7% 35%	4048
January	1st to 10th	Night	11% 7% 4 9% 5% 5% 5% 4 6% 7% 38%	8234
	11th to 20th	Night	11% 10% 5% 11% 6% 6% 5% 4 5% 6% 30%	8235
	21st to end	Night	11% 8% 3 7% 4 5% 6% 4 5% 7% 40%	9057
February	1st to 10th	Night	11% 9% 3 7% 5% 5% 5% 4 6% 6% 38%	2694
	11th to 20th	Night	9% 8% 3 6% 6% 7% 6% 5% 6% 7% 38%	2458
	21st to end	Night	14% 9% 3 8% 5% 3 5% 3 5% 6% 39%	2076
March	1st to 10th	Night	18% 10% 2 5% 4 4 4 4 5% 7% 36%	5484
	11th to 20th	Night	12% 9% 3 7% 4 5% 5% 4 5% 7% 39%	5412
	21st to end	Night	20% 9% 3 6% 4 3 3 3 6% 6% 35%	5072
April	1st to 10th	Night	17% 8% 2 6% 5% 5% 5% 4 6% 6% 37%	2100
	11th to 20th	Night	16% 8% 3 6% 4 5% 4 3 5% 6% 40%	2075
	21st to end	Night	19% 8% 3 6% 3 4 3 3 5% 6% 41%	1939
May	1st to 10th	Night	31% 10% 3 5% 3 3 4 4 5% 6% 26%	4064
	11th to 20th	Night	15% 10% 4 9% 6% 5% 6% 4 6% 6% 30%	3674
	21st to end	Night	20% 7% 2 7% 5% 4 4 4 5% 8% 33%	3926
June	1st to 10th	Night	17% 7% 2 5% 5% 4 5% 2 5% 7% 41%	1600
	11th to 20th	Night	17% 6% 2 5% 4 4 5% 4 5% 9% 40%	1600
	21st to end	Night	15% 6% 2 5% 3 3 4 5% 6% 8% 43%	1600
July	1st to 10th	Night	10% 6% 2 6% 5% 5% 4 5% 6% 9% 43%	3600
	11th to 20th	Night	10% 6% 2 7% 4 6% 5% 5% 7% 8% 39%	3600
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 6% 52%	3926
August	1st to 10th	Night	21% 5% 2 6% 3 5% 4 3 5% 7% 38%	2670
	11th to 20th	Night	14% 7% 2 7% 4 4 4 3 6% 7% 42%	3065
	21st to end	Night	9% 3 1 3 3 3 3 3 6% 8% 58%	3586
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 8% 50%	2000
	11th to 20th	Night	13% 4 2 4 3 3 4 3 4 6% 54%	2140
	21st to end	Night	16% 5% 2 6% 4 4 3 3 4 6% 47%	2200
October	1st to 10th	Night	21% 5% 1 5% 3 4 3 3 5% 6% 43%	2233
	11th to 20th	Night	15% 5% 2 5% 4 4 4 3 6% 6% 46%	2449
	21st to end	Night	21% 7% 2 5% 4 4 5% 4 4 6% 38%	2860
November	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 6% 47%	2667
	11th to 20th	Night	16% 6% 3 6% 4 4 4 4 5% 6% 42%	2730
	21st to end	Night	13% 8% 3 6% 4 4 4 4 5% 6% 44%	2819
December	1st to 10th	Night	13% 10% 3 8% 5% 5% 5% 4 5% 7% 35%	4720
	11th to 20th	Night	20% 9% 3 6% 4 3 3 3 4 5% 38%	5348
	21st to end	Night	17% 10% 4 9% 5% 3 4 3 5% 5% 35%	7150

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	46% 5% 9% 27% 11% 1	4662
	11th to 20th	Day	47% 5% 9% 18% 18% 3	4680
	21st to end	Day	43% 4 8% 24% 18% 2	5124
February	1st to 10th	Day	48% 6% 13% 19% 13% 2	1919
	11th to 20th	Day	46% 5% 10% 22% 14% 3	2057
	21st to end	Day	51% 4 10% 20% 14% 2	1730
March	1st to 10th	Day	52% 3 11% 14% 12% 8%	4579
	11th to 20th	Day	43% 5% 16% 23% 10% 3	4762
	21st to end	Day	49% 3 6% 16% 21% 5%	6102
April	1st to 10th	Day	50% 4 9% 20% 14% 3	2520
	11th to 20th	Day	45% 6% 12% 19% 16% 2	2494
	21st to end	Day	48% 5% 9% 20% 15% 3	2679
May	1st to 10th	Day	62% 5% 10% 14% 8% 1	5910
	11th to 20th	Day	53% 5% 14% 16% 10% 1	6266
	21st to end	Day	46% 6% 13% 23% 11% 1	6949
June	1st to 10th	Day	46% 7% 12% 21% 12% 1	2800
	11th to 20th	Day	44% 6% 15% 20% 14% 2	2800
	21st to end	Day	46% 8% 13% 21% 12% 1	2800
July	1st to 10th	Day	43% 7% 14% 22% 14% 1	6290
	11th to 20th	Day	38% 7% 17% 25% 12% 1	6285
	21st to end	Day	39% 5% 13% 27% 15% 1	6728
August	1st to 10th	Day	44% 6% 16% 22% 11% 1	4434
	11th to 20th	Day	42% 5% 16% 29% 8% 1	4017
	21st to end	Day	34% 6% 15% 29% 14% 1	4312
September	1st to 10th	Day	36% 5% 13% 27% 17% 1	2400
	11th to 20th	Day	35% 4 11% 25% 22% 2	2260
	21st to end	Day	40% 5% 10% 25% 16% 3	2200
October	1st to 10th	Day	42% 5% 9% 23% 20% 2	3528
	11th to 20th	Day	35% 4 10% 29% 20% 3	3153
	21st to end	Day	35% 5% 11% 31% 15% 4	3180
November	1st to 10th	Day	38% 4 14% 26% 15% 2	1851
	11th to 20th	Day	41% 6% 9% 22% 19% 3	1890
	21st to end	Day	45% 4 16% 23% 10% 2	1797
December	1st to 10th	Day	49% 4 12% 25% 8% 1	2680
	11th to 20th	Day	48% 4 10% 21% 15% 2	3066
	21st to end	Day	49% 4 10% 18% 15% 4	4048
January	1st to 10th	Night	44% 4 13% 25% 12% 2	8234
	11th to 20th	Night	52% 5% 11% 14% 14% 4	8280
	21st to end	Night	43% 4 9% 20% 20% 4	9057
February	1st to 10th	Night	46% 5% 14% 22% 11% 2	2694
	11th to 20th	Night	43% 6% 12% 22% 14% 2	2460
	21st to end	Night	46% 3 12% 24% 12% 4	2076
March	1st to 10th	Night	47% 4 11% 19% 16% 4	5494
	11th to 20th	Night	45% 4 18% 20% 10% 2	5413
	21st to end	Night	50% 3 6% 15% 23% 4	5072
April	1st to 10th	Night	47% 4 11% 20% 16% 2	2100
	11th to 20th	Night	45% 5% 10% 23% 16% 2	2075
	21st to end	Night	44% 5% 10% 21% 17% 4	1939
May	1st to 10th	Night	58% 5% 13% 15% 8% 1	4070
	11th to 20th	Night	53% 7% 13% 17% 9% 2	3675
	21st to end	Night	50% 6% 13% 19% 11% 1	3927
June	1st to 10th	Night	44% 6% 13% 26% 12% 1	1600
	11th to 20th	Night	42% 6% 17% 23% 12% 1	1600
	21st to end	Night	38% 8% 16% 24% 14% 1	1600
July	1st to 10th	Night	37% 7% 16% 24% 15% 1	3560
	11th to 20th	Night	40% 8% 15% 20% 15% 1	3552
	21st to end	Night	33% 4 13% 33% 16% 1	3809
August	1st to 10th	Night	46% 6% 15% 22% 10%	2660
	11th to 20th	Night	42% 5% 18% 25% 9%	3092
	21st to end	Night	26% 4 16% 38% 16% 1	3586
September	1st to 10th	Night	33% 6% 14% 28% 19% 1	2000
	11th to 20th	Night	32% 4 10% 29% 23% 2	2140
	21st to end	Night	40% 4 11% 27% 16% 2	2200
October	1st to 10th	Night	40% 4 14% 24% 17% 2	3649
	11th to 20th	Night	29% 4 12% 27% 24% 4	3985
	21st to end	Night	36% 3 12% 34% 13% 1	4610
November	1st to 10th	Night	38% 5% 12% 25% 17% 3	2667
	11th to 20th	Night	43% 4 10% 23% 17% 3	2730
	21st to end	Night	40% 4 18% 25% 12% 1	2819
December	1st to 10th	Night	47% 5% 11% 27% 10%	4720
	11th to 20th	Night	49% 3 13% 15% 17% 2	5348
	21st to end	Night	52% 4 9% 18% 13% 4	7150

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	31% 22% 27% 15% 4	220
	11th to 20th	Day	34% 19% 35% 7% 3 1	220
	21st to end	Day	26% 26% 31% 12% 4 1	242
February	1st to 10th	Day	32% 28% 27% 12% 1	364
	11th to 20th	Day	32% 23% 32% 13% 1	536
	21st to end	Day	30% 24% 29% 14% 3	446
March	1st to 10th	Day	31% 20% 31% 16% 1	540
	11th to 20th	Day	32% 24% 28% 12% 3 1	540
	21st to end	Day	32% 23% 29% 9% 4 2	594
April	1st to 10th	Day	36% 19% 29% 10% 5% 1	540
	11th to 20th	Day	34% 22% 27% 10% 6% 1	532
	21st to end	Day	31% 18% 30% 15% 4 2	540
May	1st to 10th	Day	34% 18% 27% 13% 6% 2	507
	11th to 20th	Day	24% 16% 35% 18% 6% 2	527
	21st to end	Day	21% 14% 31% 22% 9% 2	573
June	1st to 10th	Day	20% 15% 32% 23% 8% 2	530
	11th to 20th	Day	19% 13% 33% 23% 11% 1	530
	21st to end	Day	19% 15% 35% 19% 9% 3	530
July	1st to 10th	Day	14% 12% 31% 26% 14% 3	525
	11th to 20th	Day	13% 8% 34% 28% 14% 3	524
	21st to end	Day	15% 10% 35% 26% 11% 3	576
August	1st to 10th	Day	12% 8% 26% 32% 16% 6%	526
	11th to 20th	Day	16% 8% 28% 26% 15% 7%	519
	21st to end	Day	13% 11% 29% 27% 14% 7%	583
September	1st to 10th	Day	18% 10% 24% 24% 17% 7%	530
	11th to 20th	Day	15% 10% 24% 23% 17% 10%	530
	21st to end	Day	20% 11% 28% 24% 11% 6%	530
October	1st to 10th	Day	25% 13% 23% 20% 12% 7%	530
	11th to 20th	Day	21% 19% 25% 23% 10% 2	502
	21st to end	Day	32% 13% 26% 21% 7% 1	462
November	1st to 10th	Day	26% 17% 26% 22% 8% 1	425
	11th to 20th	Day	29% 20% 27% 20% 3 1	430
	21st to end	Day	32% 19% 26% 18% 4 1	337
December	1st to 10th	Day	29% 21% 28% 17% 4	220
	11th to 20th	Day	40% 19% 30% 8% 3	220
	21st to end	Day	31% 18% 31% 16% 3	242
January	1st to 10th	Night	22% 22% 33% 19% 3	840
	11th to 20th	Night	26% 18% 32% 21% 2	840
	21st to end	Night	21% 18% 36% 21% 4	924
February	1st to 10th	Night	21% 19% 38% 20% 3	840
	11th to 20th	Night	24% 21% 37% 17% 1	823
	21st to end	Night	26% 20% 33% 18% 2	692
March	1st to 10th	Night	25% 21% 34% 18% 2	840
	11th to 20th	Night	25% 20% 31% 20% 3	840
	21st to end	Night	25% 18% 33% 19% 5%	924
April	1st to 10th	Night	27% 18% 32% 19% 4	840
	11th to 20th	Night	23% 20% 31% 20% 6%	831
	21st to end	Night	26% 19% 29% 22% 5%	679
May	1st to 10th	Night	30% 12% 29% 22% 7%	604
	11th to 20th	Night	20% 12% 35% 27% 5%	441
	21st to end	Night	20% 6% 29% 32% 11% 1	433
June	1st to 10th	Night	20% 9% 31% 29% 12%	400
	11th to 20th	Night	17% 8% 33% 32% 10%	400
	21st to end	Night	16% 7% 28% 37% 11% 1	400
July	1st to 10th	Night	13% 6% 26% 43% 12%	390
	11th to 20th	Night	14% 4 24% 41% 15% 2	388
	21st to end	Night	11% 5% 27% 42% 15%	438
August	1st to 10th	Night	14% 4 22% 36% 22% 1	443
	11th to 20th	Night	14% 6% 23% 35% 19% 3	707
	21st to end	Night	15% 6% 21% 36% 20% 2	880
September	1st to 10th	Night	15% 6% 24% 35% 18% 2	800
	11th to 20th	Night	14% 7% 22% 33% 19% 5%	800
	21st to end	Night	20% 7% 24% 31% 17% 2	800
October	1st to 10th	Night	24% 11% 21% 27% 14% 3	800
	11th to 20th	Night	20% 11% 32% 25% 11% 1	800
	21st to end	Night	26% 15% 29% 24% 6%	880
November	1st to 10th	Night	21% 17% 29% 25% 7% 1	820
	11th to 20th	Night	21% 18% 32% 21% 6% 1	840
	21st to end	Night	22% 18% 30% 25% 5% 1	840
December	1st to 10th	Night	24% 18% 33% 19% 5%	840
	11th to 20th	Night	27% 17% 32% 20% 4	840
	21st to end	Night	26% 21% 30% 19% 3	924

Page generated at 2026-04-29 02:21:43. Version ac07

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