Amazing Grace Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amazing Grace Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	18% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	126371
	Night	18% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	123010
Transparency	Day	48% 6% 12% 20% 12% 2	129865
	Night	48% 6% 12% 20% 12% 2	127603
Seeing	Day	26% 18% 28% 18% 8% 2	16027
	Night	24% 15% 24% 26% 9% 1	24852

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% 10% 6% 10% 8% 6% 7% 4 5% 7% 26%	3597
	11th to 20th	Day	12% 11% 5% 12% 8% 5% 5% 4 5% 6% 27%	3570
	21st to end	Day	12% 10% 6% 12% 5% 5% 4 4 5% 5% 32%	3960
February	1st to 10th	Day	15% 13% 3 10% 6% 6% 5% 4 5% 7% 26%	1764
	11th to 20th	Day	12% 10% 4 9% 8% 5% 6% 4 6% 7% 30%	1948
	21st to end	Day	16% 11% 4 10% 5% 4 5% 4 5% 6% 29%	1650
March	1st to 10th	Day	19% 14% 3 11% 6% 5% 4 3 4 4 26%	4476
	11th to 20th	Day	16% 10% 5% 9% 5% 5% 4 4 4 5% 31%	4722
	21st to end	Day	24% 11% 3 7% 4 4 4 3 5% 7% 29%	5849
April	1st to 10th	Day	21% 11% 3 8% 5% 5% 4 3 4 5% 30%	2400
	11th to 20th	Day	18% 9% 3 6% 5% 4 6% 3 5% 6% 34%	2399
	21st to end	Day	21% 9% 3 8% 5% 4 5% 3 5% 7% 29%	2600
May	1st to 10th	Day	35% 10% 3 8% 5% 4 4 3 4 5% 20%	5886
	11th to 20th	Day	21% 9% 3 9% 6% 6% 5% 4 6% 7% 25%	6216
	21st to end	Day	19% 7% 3 7% 5% 4 5% 4 7% 8% 31%	6949
June	1st to 10th	Day	20% 8% 2 8% 6% 5% 6% 5% 6% 8% 26%	2800
	11th to 20th	Day	18% 7% 3 6% 5% 5% 6% 5% 5% 7% 34%	2800
	21st to end	Day	19% 8% 3 8% 6% 5% 5% 4 5% 7% 30%	2800
July	1st to 10th	Day	16% 6% 2 7% 5% 5% 6% 5% 8% 10% 30%	6360
	11th to 20th	Day	15% 6% 2 7% 5% 5% 5% 5% 7% 9% 35%	6360
	21st to end	Day	17% 4 2 5% 4 4 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	15% 6% 3 7% 6% 6% 5% 5% 7% 8% 33%	4412
	11th to 20th	Day	18% 7% 2 7% 5% 5% 6% 5% 8% 9% 28%	4150
	21st to end	Day	11% 4 1 6% 4 5% 6% 5% 7% 9% 41%	4312
September	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 44%	2400
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 4 6% 49%	2222
	21st to end	Day	17% 6% 1 6% 5% 5% 4 3 5% 7% 41%	2200
October	1st to 10th	Day	23% 8% 3 7% 4 4 4 4 4 6% 33%	2123
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 3 4 6% 37%	1807
	21st to end	Day	24% 11% 4 7% 4 5% 4 4 4 4 29%	1881
November	1st to 10th	Day	14% 7% 4 7% 6% 4 4 3 6% 7% 37%	1761
	11th to 20th	Day	18% 10% 3 6% 5% 6% 5% 4 5% 7% 32%	1800
	21st to end	Day	21% 9% 2 6% 5% 6% 5% 5% 6% 6% 29%	1725
December	1st to 10th	Day	17% 13% 5% 14% 7% 7% 6% 4 5% 5% 18%	2600
	11th to 20th	Day	21% 13% 3 7% 4 4 5% 3 5% 6% 29%	2986
	21st to end	Day	21% 12% 4 8% 5% 4 5% 3 4 5% 30%	3960
January	1st to 10th	Night	12% 10% 6% 13% 8% 5% 5% 4 5% 6% 27%	6349
	11th to 20th	Night	11% 14% 6% 14% 9% 5% 6% 4 6% 6% 20%	6315
	21st to end	Night	12% 11% 4 10% 6% 5% 6% 4 6% 7% 28%	6996
February	1st to 10th	Night	14% 11% 4 9% 7% 5% 5% 4 6% 7% 27%	2635
	11th to 20th	Night	11% 10% 4 8% 5% 6% 6% 6% 6% 8% 30%	2338
	21st to end	Night	12% 11% 3 8% 6% 6% 5% 5% 7% 7% 31%	1980
March	1st to 10th	Night	20% 13% 4 8% 6% 5% 5% 4 5% 6% 25%	5364
	11th to 20th	Night	16% 11% 5% 9% 5% 5% 5% 3 4 6% 30%	5223
	21st to end	Night	23% 13% 3 7% 4 4 4 3 5% 6% 28%	5068
April	1st to 10th	Night	20% 10% 3 7% 5% 5% 5% 3 4 6% 33%	2000
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 4 5% 6% 33%	1950
	21st to end	Night	21% 9% 2 6% 4 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	36% 10% 3 6% 4 4 4 3 5% 5% 20%	4054
	11th to 20th	Night	19% 12% 4 8% 5% 5% 5% 4 5% 6% 27%	3720
	21st to end	Night	20% 9% 3 7% 5% 5% 5% 4 6% 7% 31%	3926
June	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 4 4 8% 31%	1600
	11th to 20th	Night	18% 8% 3 5% 4 4 4 4 5% 6% 37%	1600
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 5% 7% 36%	1600
July	1st to 10th	Night	15% 5% 2 6% 5% 5% 5% 5% 8% 10% 35%	3600
	11th to 20th	Night	20% 6% 3 7% 4 3 4 3 5% 7% 37%	3600
	21st to end	Night	13% 4 2 5% 4 4 4 4 6% 7% 45%	3926
August	1st to 10th	Night	21% 9% 3 7% 4 4 4 3 6% 5% 34%	2703
	11th to 20th	Night	18% 8% 3 7% 4 4 5% 4 5% 8% 35%	2894
	21st to end	Night	10% 5% 2 5% 4 6% 5% 4 7% 9% 43%	3586
September	1st to 10th	Night	14% 5% 2 5% 4 3 4 4 5% 7% 48%	2000
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 5% 6% 49%	2177
	21st to end	Night	18% 6% 2 5% 4 4 4 4 6% 6% 42%	2200
October	1st to 10th	Night	20% 7% 2 6% 5% 5% 5% 3 6% 6% 34%	2124
	11th to 20th	Night	17% 7% 2 8% 5% 5% 6% 3 5% 7% 34%	2370
	21st to end	Night	23% 9% 3 6% 5% 4 5% 3 6% 7% 29%	2717
November	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 4 6% 6% 37%	2537
	11th to 20th	Night	16% 7% 4 7% 5% 5% 4 4 5% 9% 34%	2600
	21st to end	Night	16% 10% 4 7% 5% 5% 6% 4 7% 7% 30%	2674
December	1st to 10th	Night	18% 11% 4 12% 7% 6% 6% 4 5% 6% 21%	4580
	11th to 20th	Night	24% 10% 3 7% 5% 4 4 4 5% 6% 27%	5208
	21st to end	Night	20% 13% 4 9% 4 4 4 3 4 5% 29%	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	51% 5% 12% 22% 9%	3598
	11th to 20th	Day	51% 7% 10% 20% 11% 3	3600
	21st to end	Day	49% 5% 9% 18% 16% 3	3960
February	1st to 10th	Day	52% 5% 13% 18% 9% 3	1764
	11th to 20th	Day	50% 5% 12% 18% 11% 3	1957
	21st to end	Day	53% 4 11% 16% 13% 2	1650
March	1st to 10th	Day	58% 4 9% 12% 10% 6%	4479
	11th to 20th	Day	51% 5% 13% 17% 11% 3	4722
	21st to end	Day	52% 4 5% 17% 18% 3	5849
April	1st to 10th	Day	53% 3 9% 19% 13% 3	2400
	11th to 20th	Day	47% 5% 12% 20% 15% 1	2399
	21st to end	Day	54% 5% 8% 18% 13% 3	2600
May	1st to 10th	Day	66% 5% 8% 14% 6% 1	5897
	11th to 20th	Day	54% 6% 16% 15% 8% 1	6220
	21st to end	Day	47% 7% 14% 21% 11% 1	6949
June	1st to 10th	Day	52% 6% 13% 20% 9%	2800
	11th to 20th	Day	46% 6% 12% 20% 14% 2	2800
	21st to end	Day	51% 6% 11% 20% 10% 1	2800
July	1st to 10th	Day	44% 7% 16% 22% 11%	6290
	11th to 20th	Day	42% 8% 15% 23% 13% 1	6285
	21st to end	Day	39% 6% 13% 30% 13%	6728
August	1st to 10th	Day	45% 7% 16% 22% 9%	4398
	11th to 20th	Day	47% 7% 16% 23% 7%	4189
	21st to end	Day	33% 7% 17% 27% 15% 1	4312
September	1st to 10th	Day	35% 6% 13% 30% 15% 1	2400
	11th to 20th	Day	35% 4 9% 31% 19% 2	2222
	21st to end	Day	41% 4 13% 25% 16% 2	2200
October	1st to 10th	Day	45% 6% 10% 23% 15% 1	3513
	11th to 20th	Day	42% 3 14% 24% 14% 2	2971
	21st to end	Day	40% 3 12% 27% 15% 3	3081
November	1st to 10th	Day	43% 4 15% 22% 14% 1	1761
	11th to 20th	Day	46% 4 14% 17% 17% 2	1800
	21st to end	Day	48% 6% 18% 18% 8% 1	1725
December	1st to 10th	Day	57% 8% 14% 17% 4 1	2600
	11th to 20th	Day	49% 6% 10% 17% 18% 1	2986
	21st to end	Day	55% 4 8% 16% 13% 5%	3960
January	1st to 10th	Night	52% 6% 12% 20% 9% 1	6349
	11th to 20th	Night	57% 7% 12% 15% 8% 1	6360
	21st to end	Night	49% 6% 10% 17% 16% 3	6996
February	1st to 10th	Night	50% 5% 13% 19% 11% 2	2635
	11th to 20th	Night	45% 5% 13% 22% 13% 1	2340
	21st to end	Night	46% 4 13% 22% 11% 3	1980
March	1st to 10th	Night	54% 5% 11% 14% 13% 3	5374
	11th to 20th	Night	52% 5% 15% 16% 11% 2	5224
	21st to end	Night	52% 5% 6% 16% 18% 3	5068
April	1st to 10th	Night	51% 4 10% 19% 14% 2	2000
	11th to 20th	Night	50% 4 8% 24% 14% 1	1950
	21st to end	Night	47% 4 12% 19% 15% 3	1800
May	1st to 10th	Night	65% 5% 10% 13% 7% 1	4060
	11th to 20th	Night	55% 8% 10% 18% 8% 1	3721
	21st to end	Night	49% 6% 15% 19% 9% 1	3927
June	1st to 10th	Night	50% 6% 13% 19% 12%	1600
	11th to 20th	Night	47% 5% 15% 20% 12% 2	1600
	21st to end	Night	45% 7% 14% 22% 11% 1	1600
July	1st to 10th	Night	40% 7% 19% 23% 11%	3560
	11th to 20th	Night	44% 7% 13% 22% 13%	3552
	21st to end	Night	34% 5% 15% 34% 11% 1	3809
August	1st to 10th	Night	50% 6% 13% 22% 9%	2693
	11th to 20th	Night	44% 7% 17% 20% 11%	2921
	21st to end	Night	29% 8% 19% 28% 15%	3586
September	1st to 10th	Night	32% 6% 14% 31% 16%	2000
	11th to 20th	Night	34% 5% 11% 30% 19% 2	2177
	21st to end	Night	41% 4 12% 27% 14% 1	2200
October	1st to 10th	Night	44% 6% 14% 25% 11% 1	3514
	11th to 20th	Night	38% 6% 12% 25% 17% 3	3945
	21st to end	Night	41% 5% 13% 28% 12% 2	4467
November	1st to 10th	Night	42% 6% 12% 25% 13% 2	2537
	11th to 20th	Night	44% 5% 12% 23% 15% 2	2600
	21st to end	Night	46% 6% 16% 22% 9% 1	2674
December	1st to 10th	Night	54% 7% 14% 21% 4	4580
	11th to 20th	Night	52% 6% 10% 15% 17% 1	5208
	21st to end	Night	55% 4 8% 18% 12% 3	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	31% 26% 27% 14% 3 1	200
	11th to 20th	Day	33% 28% 27% 11% 2 1	200
	21st to end	Day	23% 27% 32% 13% 5%	220
February	1st to 10th	Day	36% 25% 27% 10% 2	315
	11th to 20th	Day	32% 26% 28% 11% 3	496
	21st to end	Day	33% 24% 24% 16% 3	414
March	1st to 10th	Day	30% 25% 27% 14% 4	500
	11th to 20th	Day	34% 25% 24% 12% 4 1	500
	21st to end	Day	35% 21% 24% 13% 5% 2	550
April	1st to 10th	Day	38% 20% 23% 12% 4 2	500
	11th to 20th	Day	35% 25% 21% 14% 5% 1	492
	21st to end	Day	33% 22% 28% 11% 4 2	500
May	1st to 10th	Day	36% 19% 25% 13% 4 2	503
	11th to 20th	Day	25% 20% 32% 15% 6% 2	527
	21st to end	Day	24% 16% 31% 23% 6% 1	573
June	1st to 10th	Day	23% 16% 34% 17% 9% 1	530
	11th to 20th	Day	22% 17% 31% 20% 8% 2	530
	21st to end	Day	21% 16% 34% 19% 8% 2	530
July	1st to 10th	Day	19% 14% 33% 23% 10% 3	525
	11th to 20th	Day	17% 10% 33% 27% 10% 2	524
	21st to end	Day	18% 13% 33% 26% 8% 2	576
August	1st to 10th	Day	13% 14% 29% 26% 15% 3	526
	11th to 20th	Day	18% 13% 28% 21% 13% 7%	519
	21st to end	Day	14% 14% 30% 24% 13% 5%	583
September	1st to 10th	Day	16% 12% 30% 22% 15% 5%	530
	11th to 20th	Day	17% 12% 25% 22% 15% 8%	530
	21st to end	Day	19% 16% 25% 24% 10% 6%	530
October	1st to 10th	Day	29% 15% 20% 20% 12% 5%	502
	11th to 20th	Day	27% 16% 25% 21% 10% 2	433
	21st to end	Day	38% 15% 23% 17% 7%	429
November	1st to 10th	Day	26% 18% 22% 25% 9% 1	395
	11th to 20th	Day	33% 19% 23% 19% 7% 1	400
	21st to end	Day	37% 18% 27% 13% 4 1	325
December	1st to 10th	Day	30% 24% 26% 15% 6%	200
	11th to 20th	Day	41% 20% 28% 9% 3	200
	21st to end	Day	32% 18% 29% 16% 5%	220
January	1st to 10th	Night	25% 22% 27% 23% 3	800
	11th to 20th	Night	29% 18% 25% 24% 3	800
	21st to end	Night	20% 24% 31% 21% 4	880
February	1st to 10th	Night	24% 21% 33% 20% 2	800
	11th to 20th	Night	25% 24% 32% 17% 3	783
	21st to end	Night	24% 21% 28% 23% 3	660
March	1st to 10th	Night	27% 19% 29% 22% 4	800
	11th to 20th	Night	27% 21% 23% 23% 6%	800
	21st to end	Night	28% 18% 25% 23% 6% 1	880
April	1st to 10th	Night	31% 18% 27% 21% 4	800
	11th to 20th	Night	26% 20% 25% 23% 6%	766
	21st to end	Night	28% 17% 28% 21% 5%	600
May	1st to 10th	Night	31% 13% 25% 24% 8%	600
	11th to 20th	Night	21% 13% 26% 31% 9% 1	461
	21st to end	Night	24% 7% 20% 37% 11% 1	433
June	1st to 10th	Night	26% 9% 24% 32% 9% 1	400
	11th to 20th	Night	23% 10% 23% 32% 13%	400
	21st to end	Night	19% 8% 24% 38% 12% 1	400
July	1st to 10th	Night	16% 5% 22% 43% 14%	390
	11th to 20th	Night	18% 4 21% 36% 20% 2	388
	21st to end	Night	16% 5% 19% 42% 18% 1	438
August	1st to 10th	Night	19% 3 16% 42% 21%	463
	11th to 20th	Night	19% 6% 24% 30% 18% 2	601
	21st to end	Night	17% 5% 20% 34% 21% 3	880
September	1st to 10th	Night	15% 9% 21% 33% 21% 2	800
	11th to 20th	Night	18% 9% 18% 30% 21% 4	800
	21st to end	Night	21% 10% 19% 32% 16% 3	800
October	1st to 10th	Night	25% 10% 20% 27% 16% 2	773
	11th to 20th	Night	23% 16% 24% 25% 12% 1	760
	21st to end	Night	30% 16% 24% 23% 7%	836
November	1st to 10th	Night	21% 16% 24% 29% 8% 1	780
	11th to 20th	Night	28% 18% 23% 24% 7% 1	800
	21st to end	Night	27% 18% 24% 26% 5% 1	800
December	1st to 10th	Night	30% 19% 26% 21% 5%	800
	11th to 20th	Night	32% 17% 25% 21% 4 1	800
	21st to end	Night	30% 21% 25% 21% 3	880

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