Cahall Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cahall Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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	17% 8% 2 6% 4 4 4 4 5% 7% 39%	126697
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	123995
Transparency	Day	44% 4 11% 23% 15% 2	130231
	Night	45% 4 11% 22% 16% 2	128549
Seeing	Day	21% 16% 33% 21% 7% 2	16475
	Night	21% 14% 33% 24% 7% 1	25345

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	11% 8% 3 7% 4 3 4 3 5% 6% 46%	3597
	11th to 20th	Day	15% 8% 3 6% 2 3 3 3 4 5% 49%	3570
	21st to end	Day	17% 9% 2 6% 4 3 4 3 5% 4 42%	3960
February	1st to 10th	Day	19% 8% 3 5% 5% 4 4 5% 5% 6% 36%	1885
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 6% 42%	1948
	21st to end	Day	18% 10% 2 7% 4 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	20% 13% 2 5% 3 4 4 3 4 6% 36%	4476
	11th to 20th	Day	14% 10% 3 6% 4 3 5% 4 5% 7% 38%	4811
	21st to end	Day	25% 9% 2 7% 5% 4 4 3 5% 6% 30%	5712
April	1st to 10th	Day	20% 10% 3 6% 4 4 4 3 5% 5% 37%	2400
	11th to 20th	Day	18% 7% 2 5% 4 4 4 3 6% 6% 39%	2402
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 7% 39%	2600
May	1st to 10th	Day	27% 12% 3 7% 4 3 4 3 4 6% 26%	5886
	11th to 20th	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 31%	5876
	21st to end	Day	20% 7% 3 6% 4 4 4 3 5% 7% 36%	6949
June	1st to 10th	Day	19% 6% 2 6% 4 4 5% 4 7% 9% 33%	2800
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 5% 6% 8% 37%	2800
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2800
July	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 7% 10% 32%	6360
	11th to 20th	Day	8% 6% 3 7% 5% 5% 6% 5% 8% 11% 36%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 8% 43%	6926
August	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 6% 6% 9% 31%	4235
	11th to 20th	Day	13% 7% 3 7% 4 5% 5% 4 6% 8% 38%	4151
	21st to end	Day	8% 4 2 5% 4 5% 6% 5% 7% 9% 46%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 3 6% 6% 46%	2379
	11th to 20th	Day	14% 6% 2 4 3 3 3 2 5% 6% 52%	2200
	21st to end	Day	18% 7% 2 6% 4 3 5% 4 4 6% 42%	2200
October	1st to 10th	Day	22% 7% 2 5% 3 4 3 3 4 5% 43%	2200
	11th to 20th	Day	16% 7% 2 5% 3 3 3 3 4 7% 46%	1965
	21st to end	Day	19% 7% 2 5% 3 4 4 3 5% 6% 42%	1980
November	1st to 10th	Day	17% 6% 2 5% 4 3 3 3 4 5% 49%	1851
	11th to 20th	Day	16% 8% 2 5% 3 3 3 3 4 6% 47%	1890
	21st to end	Day	18% 7% 2 5% 4 3 4 4 4 6% 42%	1890
December	1st to 10th	Day	14% 9% 2 5% 3 3 3 3 4 5% 49%	2730
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 4 5% 48%	2986
	21st to end	Day	20% 11% 3 7% 4 4 3 2 4 5% 38%	3960
January	1st to 10th	Night	14% 7% 2 4 3 3 3 2 4 6% 51%	6349
	11th to 20th	Night	14% 9% 4 8% 5% 4 4 3 4 5% 40%	6315
	21st to end	Night	15% 11% 3 8% 5% 4 4 3 4 5% 37%	6996
February	1st to 10th	Night	16% 10% 2 7% 4 4 4 3 5% 6% 39%	2511
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 4 5% 5% 39%	2338
	21st to end	Night	18% 8% 2 6% 4 4 4 3 5% 5% 41%	1980
March	1st to 10th	Night	24% 10% 2 4 4 4 3 3 4 5% 37%	5364
	11th to 20th	Night	16% 11% 3 7% 4 4 4 3 4 6% 37%	5147
	21st to end	Night	24% 11% 2 6% 4 4 4 3 4 4 32%	5202
April	1st to 10th	Night	19% 8% 3 7% 4 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	18% 9% 2 6% 5% 4 4 3 5% 6% 38%	1945
	21st to end	Night	20% 7% 3 7% 4 4 4 3 4 6% 39%	1800
May	1st to 10th	Night	28% 12% 3 7% 4 3 3 3 5% 6% 26%	4054
	11th to 20th	Night	18% 9% 2 8% 5% 5% 6% 4 7% 6% 28%	4033
	21st to end	Night	20% 7% 4 8% 5% 5% 5% 4 6% 6% 31%	3939
June	1st to 10th	Night	17% 8% 3 6% 4 5% 4 4 6% 7% 37%	1600
	11th to 20th	Night	16% 8% 2 7% 4 4 4 4 7% 8% 35%	1600
	21st to end	Night	16% 6% 2 6% 3 4 4 4 7% 9% 39%	1600
July	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 7% 10% 36%	3600
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 7% 10% 37%	3600
	21st to end	Night	12% 7% 3 7% 5% 4 5% 4 5% 6% 42%	3926
August	1st to 10th	Night	18% 8% 2 7% 5% 5% 5% 3 7% 7% 31%	2882
	11th to 20th	Night	8% 8% 3 8% 5% 5% 6% 6% 6% 8% 37%	2894
	21st to end	Night	9% 4 1 4 4 3 4 3 6% 8% 54%	3586
September	1st to 10th	Night	12% 5% 2 6% 5% 4 4 4 6% 7% 45%	2020
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 53%	2200
	21st to end	Night	18% 6% 3 7% 3 3 3 4 5% 6% 42%	2200
October	1st to 10th	Night	20% 6% 2 5% 4 4 3 3 3 6% 44%	2200
	11th to 20th	Night	15% 5% 2 6% 4 3 4 3 4 6% 47%	2430
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 6% 40%	2860
November	1st to 10th	Night	15% 5% 2 6% 4 3 3 4 5% 6% 48%	2667
	11th to 20th	Night	16% 8% 2 5% 3 3 4 3 5% 5% 46%	2730
	21st to end	Night	16% 7% 2 5% 4 4 3 4 5% 6% 44%	2730
December	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 4 5% 45%	4493
	11th to 20th	Night	21% 9% 3 6% 3 3 3 3 5% 5% 41%	5208
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 6% 40%	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	40% 3 7% 28% 20% 2	3598
	11th to 20th	Day	40% 3 10% 23% 19% 5%	3600
	21st to end	Day	46% 2 10% 21% 18% 2	3960
February	1st to 10th	Day	49% 3 12% 19% 14% 2	1885
	11th to 20th	Day	43% 4 12% 23% 16% 2	1957
	21st to end	Day	47% 4 10% 22% 15% 3	1650
March	1st to 10th	Day	51% 3 10% 15% 12% 9%	4479
	11th to 20th	Day	45% 4 11% 27% 11% 2	4811
	21st to end	Day	56% 3 6% 15% 13% 7%	5712
April	1st to 10th	Day	49% 3 8% 19% 18% 3	2400
	11th to 20th	Day	45% 3 9% 22% 17% 3	2402
	21st to end	Day	45% 3 9% 22% 17% 4	2600
May	1st to 10th	Day	60% 4 13% 15% 7% 1	5897
	11th to 20th	Day	53% 5% 13% 18% 10% 2	5880
	21st to end	Day	46% 5% 11% 24% 13% 1	6949
June	1st to 10th	Day	45% 6% 12% 24% 12% 1	2800
	11th to 20th	Day	42% 5% 14% 22% 15% 2	2800
	21st to end	Day	43% 6% 13% 23% 14% 1	2800
July	1st to 10th	Day	46% 7% 12% 20% 13% 1	6290
	11th to 20th	Day	40% 6% 16% 25% 11% 1	6285
	21st to end	Day	40% 5% 12% 27% 15% 1	6728
August	1st to 10th	Day	47% 6% 15% 22% 9% 1	4222
	11th to 20th	Day	42% 5% 14% 27% 11% 1	4190
	21st to end	Day	31% 6% 13% 29% 19% 1	4312
September	1st to 10th	Day	37% 5% 10% 25% 22% 1	2379
	11th to 20th	Day	34% 4 8% 29% 23% 3	2200
	21st to end	Day	43% 4 8% 22% 21% 2	2200
October	1st to 10th	Day	39% 5% 9% 26% 18% 3	3590
	11th to 20th	Day	33% 2 8% 29% 24% 4	3168
	21st to end	Day	32% 3 12% 30% 20% 3	3180
November	1st to 10th	Day	40% 3 12% 26% 17% 2	1851
	11th to 20th	Day	40% 3 8% 25% 22% 3	1890
	21st to end	Day	43% 4 12% 26% 13% 2	1890
December	1st to 10th	Day	40% 2 14% 28% 15% 1	2730
	11th to 20th	Day	41% 3 15% 17% 19% 5%	2986
	21st to end	Day	51% 2 9% 21% 13% 4	3960
January	1st to 10th	Night	35% 2 10% 35% 17% 2	6349
	11th to 20th	Night	47% 3 11% 22% 13% 4	6360
	21st to end	Night	51% 2 8% 17% 18% 4	6996
February	1st to 10th	Night	47% 3 13% 21% 14% 2	2511
	11th to 20th	Night	47% 4 8% 21% 17% 3	2340
	21st to end	Night	45% 3 8% 24% 17% 4	1980
March	1st to 10th	Night	51% 2 8% 16% 16% 7%	5374
	11th to 20th	Night	49% 3 14% 19% 13% 2	5148
	21st to end	Night	56% 2 4 13% 19% 6%	5202
April	1st to 10th	Night	50% 4 8% 20% 16% 4	2000
	11th to 20th	Night	48% 3 7% 21% 17% 3	1945
	21st to end	Night	47% 4 9% 20% 17% 4	1800
May	1st to 10th	Night	60% 3 14% 14% 7% 2	4060
	11th to 20th	Night	54% 6% 14% 16% 8% 2	4034
	21st to end	Night	53% 4 12% 19% 10% 1	3940
June	1st to 10th	Night	47% 5% 14% 22% 12% 1	1600
	11th to 20th	Night	47% 5% 12% 22% 14% 1	1600
	21st to end	Night	41% 6% 14% 25% 14% 1	1600
July	1st to 10th	Night	43% 6% 12% 22% 17%	3560
	11th to 20th	Night	40% 7% 18% 23% 11% 1	3552
	21st to end	Night	40% 5% 11% 28% 14% 1	3809
August	1st to 10th	Night	50% 7% 14% 19% 10% 1	2872
	11th to 20th	Night	41% 8% 14% 26% 10%	2921
	21st to end	Night	27% 5% 15% 29% 23% 1	3586
September	1st to 10th	Night	38% 4 11% 26% 19% 1	2020
	11th to 20th	Night	34% 3 10% 26% 25% 3	2200
	21st to end	Night	43% 4 8% 24% 20% 2	2200
October	1st to 10th	Night	39% 4 12% 27% 15% 3	3590
	11th to 20th	Night	28% 2 10% 28% 29% 3	3966
	21st to end	Night	34% 3 11% 30% 20% 2	4610
November	1st to 10th	Night	38% 4 12% 25% 18% 3	2667
	11th to 20th	Night	41% 3 10% 22% 21% 4	2730
	21st to end	Night	41% 3 13% 27% 13% 2	2730
December	1st to 10th	Night	43% 2 12% 28% 14% 1	4493
	11th to 20th	Night	47% 3 14% 15% 19% 1	5208
	21st to end	Night	46% 4 8% 25% 17% 1	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	30% 24% 28% 15% 4	200
	11th to 20th	Day	24% 27% 38% 10% 2 1	200
	21st to end	Day	25% 23% 39% 10% 3	220
February	1st to 10th	Day	28% 20% 37% 13% 1	430
	11th to 20th	Day	28% 28% 34% 9% 1	496
	21st to end	Day	23% 24% 37% 12% 3	414
March	1st to 10th	Day	26% 25% 33% 13% 3	500
	11th to 20th	Day	25% 25% 34% 12% 4 1	500
	21st to end	Day	28% 23% 33% 9% 6% 1	550
April	1st to 10th	Day	29% 21% 30% 13% 6% 1	500
	11th to 20th	Day	27% 22% 33% 16% 3	492
	21st to end	Day	25% 15% 39% 17% 3 1	500
May	1st to 10th	Day	30% 21% 32% 13% 3 1	503
	11th to 20th	Day	20% 14% 41% 18% 5% 2	527
	21st to end	Day	18% 12% 35% 28% 6% 2	573
June	1st to 10th	Day	21% 11% 34% 25% 7% 2	530
	11th to 20th	Day	16% 12% 41% 22% 7% 2	530
	21st to end	Day	14% 10% 38% 27% 10% 1	530
July	1st to 10th	Day	18% 7% 30% 30% 13% 2	525
	11th to 20th	Day	13% 6% 29% 37% 13% 2	524
	21st to end	Day	11% 7% 34% 36% 11% 1	576
August	1st to 10th	Day	12% 7% 32% 36% 12% 2	526
	11th to 20th	Day	15% 7% 28% 31% 16% 3	519
	21st to end	Day	13% 8% 33% 29% 14% 4	583
September	1st to 10th	Day	17% 10% 28% 28% 13% 4	530
	11th to 20th	Day	18% 10% 25% 25% 15% 7%	530
	21st to end	Day	22% 11% 30% 23% 10% 5%	530
October	1st to 10th	Day	24% 10% 27% 24% 11% 4	530
	11th to 20th	Day	19% 15% 31% 22% 10% 2	461
	21st to end	Day	25% 15% 34% 18% 7% 2	462
November	1st to 10th	Day	21% 18% 32% 16% 12% 1	425
	11th to 20th	Day	21% 24% 35% 15% 4	430
	21st to end	Day	24% 23% 32% 15% 5% 1	430
December	1st to 10th	Day	26% 21% 34% 14% 4	279
	11th to 20th	Day	31% 25% 35% 7% 3	200
	21st to end	Day	24% 19% 37% 16% 3 2	220
January	1st to 10th	Night	23% 20% 39% 15% 3	800
	11th to 20th	Night	26% 19% 36% 17% 2	800
	21st to end	Night	19% 21% 36% 19% 4	880
February	1st to 10th	Night	24% 20% 36% 19% 1	800
	11th to 20th	Night	27% 19% 39% 13% 1	783
	21st to end	Night	23% 22% 35% 17% 3	660
March	1st to 10th	Night	25% 19% 35% 18% 3	800
	11th to 20th	Night	23% 18% 38% 17% 4 1	800
	21st to end	Night	26% 16% 34% 19% 5%	880
April	1st to 10th	Night	25% 17% 35% 20% 4	800
	11th to 20th	Night	25% 15% 38% 18% 4	761
	21st to end	Night	24% 15% 36% 23% 3	600
May	1st to 10th	Night	29% 11% 34% 20% 6% 1	600
	11th to 20th	Night	20% 12% 39% 22% 6% 1	592
	21st to end	Night	20% 8% 37% 28% 7%	446
June	1st to 10th	Night	20% 7% 33% 33% 7% 1	400
	11th to 20th	Night	18% 8% 34% 34% 7% 1	400
	21st to end	Night	17% 5% 33% 37% 8%	400
July	1st to 10th	Night	12% 3 32% 42% 9% 1	390
	11th to 20th	Night	14% 5% 27% 44% 9% 1	388
	21st to end	Night	13% 3 32% 43% 10%	438
August	1st to 10th	Night	12% 4 31% 42% 11%	577
	11th to 20th	Night	12% 4 27% 44% 12% 1	601
	21st to end	Night	14% 5% 23% 39% 18% 2	880
September	1st to 10th	Night	14% 5% 28% 38% 14% 1	800
	11th to 20th	Night	16% 6% 27% 31% 18% 3	800
	21st to end	Night	22% 6% 26% 31% 13% 3	800
October	1st to 10th	Night	23% 9% 27% 27% 13% 2	800
	11th to 20th	Night	19% 12% 33% 24% 10% 2	800
	21st to end	Night	24% 14% 34% 21% 7%	880
November	1st to 10th	Night	19% 15% 34% 24% 8% 1	820
	11th to 20th	Night	21% 22% 31% 19% 6% 1	840
	21st to end	Night	22% 22% 33% 19% 5%	840
December	1st to 10th	Night	26% 17% 35% 18% 4	809
	11th to 20th	Night	24% 20% 33% 19% 3	800
	21st to end	Night	24% 16% 39% 18% 3	880

Page generated at 2025-11-30 01:00:51. Version ac07

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