NAA Observatory
Clear Sky Chart
History

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

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	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	126747
	Night	14% 10% 4 8% 4 4 4 3 5% 6% 38%	123522
Transparency	Day	45% 5% 12% 23% 14% 2	130269
	Night	46% 5% 11% 22% 15% 2	128074
Seeing	Day	17% 11% 29% 24% 13% 5%	17181
	Night	18% 9% 30% 30% 12% 1	26977

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	12% 11% 6% 13% 7% 5% 6% 3 6% 6% 24%	3617
	11th to 20th	Day	10% 13% 6% 12% 5% 5% 4 6% 6% 4 28%	3620
	21st to end	Day	15% 18% 5% 9% 5% 3 3 3 5% 6% 28%	4120
February	1st to 10th	Day	16% 14% 4 9% 7% 6% 5% 4 4 6% 25%	1844
	11th to 20th	Day	14% 12% 4 9% 5% 4 4 3 4 5% 34%	1948
	21st to end	Day	15% 13% 5% 9% 5% 3 4 3 5% 6% 31%	1650
March	1st to 10th	Day	17% 12% 3 7% 4 3 3 2 4 5% 40%	4480
	11th to 20th	Day	9% 7% 3 7% 5% 4 5% 4 6% 7% 42%	4674
	21st to end	Day	19% 13% 6% 12% 6% 5% 5% 3 5% 5% 20%	5967
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 3 6% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 4 5% 4 6% 6% 35%	2374
	21st to end	Day	19% 8% 3 8% 5% 4 5% 4 5% 6% 32%	2500
May	1st to 10th	Day	17% 8% 2 6% 4 4 5% 5% 6% 8% 33%	5862
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 6% 7% 40%	6171
	21st to end	Day	14% 7% 2 7% 5% 4 4 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 5% 6% 8% 46%	2800
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	2800
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	2800
July	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 8% 10% 35%	6360
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 5% 8% 11% 40%	6360
	21st to end	Day	10% 6% 3 8% 5% 5% 5% 5% 7% 9% 37%	6924
August	1st to 10th	Day	8% 5% 3 6% 4 5% 4 4 7% 9% 44%	4469
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 8% 10% 41%	3850
	21st to end	Day	7% 5% 2 6% 4 4 4 4 7% 10% 47%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	10% 4 2 4 3 3 4 4 5% 7% 53%	2275
	21st to end	Day	10% 7% 2 6% 4 4 5% 4 6% 7% 45%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 4 6% 48%	2145
	11th to 20th	Day	17% 8% 3 8% 4 4 4 4 5% 7% 36%	1993
	21st to end	Day	19% 9% 2 7% 4 4 3 3 5% 7% 36%	1980
November	1st to 10th	Day	12% 9% 3 7% 4 3 4 4 5% 5% 44%	1780
	11th to 20th	Day	16% 12% 4 9% 5% 4 4 2 5% 7% 32%	1800
	21st to end	Day	18% 10% 4 9% 5% 4 3 3 4 5% 34%	1737
December	1st to 10th	Day	9% 10% 4 9% 5% 4 4 3 5% 5% 44%	2610
	11th to 20th	Day	12% 16% 6% 12% 5% 5% 6% 4 5% 6% 24%	3008
	21st to end	Day	24% 23% 7% 12% 6% 4 4 2 3 4 11%	3982
January	1st to 10th	Night	13% 13% 7% 12% 5% 4 4 3 4 4 30%	6348
	11th to 20th	Night	13% 17% 6% 13% 6% 5% 4 4 6% 4 22%	6360
	21st to end	Night	17% 14% 7% 8% 4 4 3 3 4 4 31%	6840
February	1st to 10th	Night	15% 14% 4 8% 6% 4 4 3 4 6% 31%	2555
	11th to 20th	Night	14% 12% 4 8% 5% 4 4 4 4 5% 36%	2338
	21st to end	Night	17% 11% 3 9% 4 3 4 3 5% 5% 36%	1980
March	1st to 10th	Night	16% 10% 4 7% 4 4 4 3 4 5% 40%	5373
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 3 5% 7% 47%	5314
	21st to end	Night	18% 11% 4 10% 5% 5% 5% 3 4 6% 28%	4962
April	1st to 10th	Night	16% 10% 2 7% 3 4 4 3 5% 8% 38%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 3 3 3 5% 6% 39%	1975
	21st to end	Night	18% 8% 3 8% 4 4 4 3 6% 7% 35%	1899
May	1st to 10th	Night	17% 8% 2 7% 5% 4 5% 3 6% 6% 36%	4033
	11th to 20th	Night	11% 7% 2 5% 4 4 5% 4 6% 8% 43%	3734
	21st to end	Night	15% 9% 3 7% 3 4 5% 4 5% 7% 40%	3942
June	1st to 10th	Night	10% 4 3 6% 3 5% 5% 3 6% 7% 49%	1600
	11th to 20th	Night	8% 5% 2 5% 4 5% 5% 4 7% 9% 46%	1600
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	1600
July	1st to 10th	Night	10% 6% 3 8% 5% 5% 5% 4 7% 8% 40%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 8% 50%	3617
	21st to end	Night	8% 6% 2 7% 4 5% 5% 4 6% 8% 43%	3947
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 7% 9% 48%	2687
	11th to 20th	Night	8% 4 2 6% 4 4 4 4 6% 8% 50%	3195
	21st to end	Night	7% 5% 1 5% 4 4 4 4 5% 9% 53%	3586
September	1st to 10th	Night	9% 6% 2 5% 3 3 4 3 5% 7% 53%	2000
	11th to 20th	Night	8% 4 2 5% 3 3 4 3 5% 7% 56%	2105
	21st to end	Night	10% 5% 2 7% 4 4 4 4 6% 7% 47%	2200
October	1st to 10th	Night	10% 5% 2 6% 3 4 3 3 5% 6% 52%	2251
	11th to 20th	Night	17% 7% 2 6% 4 4 4 2 4 6% 45%	2405
	21st to end	Night	19% 10% 2 7% 4 3 4 3 5% 6% 37%	2860
November	1st to 10th	Night	13% 7% 2 6% 4 4 3 3 4 5% 48%	2563
	11th to 20th	Night	16% 10% 3 8% 4 4 4 3 5% 7% 36%	2600
	21st to end	Night	16% 11% 4 8% 5% 4 3 3 5% 5% 36%	2660
December	1st to 10th	Night	12% 9% 3 7% 4 4 4 3 5% 6% 42%	4580
	11th to 20th	Night	17% 16% 4 9% 5% 3 4 3 4 5% 30%	5199
	21st to end	Night	24% 19% 5% 9% 5% 4 4 3 4 5% 16%	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	60% 4 12% 16% 7% 1	3618
	11th to 20th	Day	57% 4 10% 10% 14% 5%	3620
	21st to end	Day	58% 3 12% 18% 8%	4120
February	1st to 10th	Day	59% 6% 11% 13% 8% 2	1844
	11th to 20th	Day	53% 4 10% 22% 9% 2	1958
	21st to end	Day	54% 6% 10% 20% 8% 2	1650
March	1st to 10th	Day	48% 4 8% 25% 12% 3	4480
	11th to 20th	Day	39% 5% 12% 33% 11% 1	4674
	21st to end	Day	65% 6% 10% 12% 6% 1	5969
April	1st to 10th	Day	54% 6% 11% 18% 11%	2400
	11th to 20th	Day	47% 7% 10% 20% 12% 3	2374
	21st to end	Day	51% 5% 13% 18% 11% 2	2500
May	1st to 10th	Day	46% 7% 11% 22% 12% 2	5897
	11th to 20th	Day	42% 6% 11% 25% 15% 1	6175
	21st to end	Day	42% 6% 11% 24% 14% 3	6949
June	1st to 10th	Day	35% 8% 11% 26% 18% 2	2800
	11th to 20th	Day	36% 6% 10% 25% 19% 3	2800
	21st to end	Day	40% 9% 14% 21% 14% 3	2800
July	1st to 10th	Day	42% 5% 13% 24% 13% 1	6291
	11th to 20th	Day	36% 6% 15% 27% 15% 1	6285
	21st to end	Day	40% 6% 17% 21% 14% 2	6728
August	1st to 10th	Day	35% 7% 16% 29% 13% 1	4454
	11th to 20th	Day	34% 7% 12% 31% 15% 1	3886
	21st to end	Day	31% 6% 13% 28% 20% 3	4311
September	1st to 10th	Day	34% 6% 12% 25% 18% 4	2400
	11th to 20th	Day	30% 6% 11% 29% 22% 3	2275
	21st to end	Day	37% 7% 9% 25% 21% 2	2200
October	1st to 10th	Day	27% 3 7% 29% 32% 2	3496
	11th to 20th	Day	34% 4 12% 25% 24% 1	3221
	21st to end	Day	42% 4 9% 27% 15% 3	3180
November	1st to 10th	Day	40% 6% 13% 24% 15% 2	1780
	11th to 20th	Day	52% 5% 11% 20% 11% 1	1800
	21st to end	Day	54% 4 10% 22% 9%	1737
December	1st to 10th	Day	43% 3 17% 27% 9%	2610
	11th to 20th	Day	61% 4 11% 17% 6% 1	3005
	21st to end	Day	79% 3 6% 8% 4	3982
January	1st to 10th	Night	57% 5% 9% 19% 10%	6348
	11th to 20th	Night	64% 4 9% 9% 12% 1	6360
	21st to end	Night	57% 4 12% 16% 11% 1	6840
February	1st to 10th	Night	55% 5% 11% 17% 11% 2	2555
	11th to 20th	Night	50% 5% 10% 21% 14% 1	2339
	21st to end	Night	51% 4 12% 19% 12% 3	1980
March	1st to 10th	Night	48% 4 9% 24% 11% 4	5373
	11th to 20th	Night	38% 4 12% 29% 14% 3	5314
	21st to end	Night	58% 5% 9% 19% 8% 1	4962
April	1st to 10th	Night	45% 5% 12% 24% 13% 2	2000
	11th to 20th	Night	46% 5% 9% 21% 15% 4	1975
	21st to end	Night	47% 7% 9% 22% 12% 3	1899
May	1st to 10th	Night	47% 5% 12% 17% 14% 4	4060
	11th to 20th	Night	36% 8% 14% 25% 13% 4	3735
	21st to end	Night	42% 6% 11% 21% 16% 4	3950
June	1st to 10th	Night	33% 7% 11% 26% 20% 3	1600
	11th to 20th	Night	33% 6% 12% 25% 22% 2	1600
	21st to end	Night	38% 7% 15% 21% 17% 3	1600
July	1st to 10th	Night	41% 5% 13% 24% 14% 3	3580
	11th to 20th	Night	31% 5% 14% 28% 20% 2	3572
	21st to end	Night	37% 6% 15% 23% 17% 3	3832
August	1st to 10th	Night	31% 7% 17% 27% 17% 1	2678
	11th to 20th	Night	30% 6% 14% 29% 19% 1	3225
	21st to end	Night	29% 5% 12% 33% 18% 3	3586
September	1st to 10th	Night	30% 6% 10% 27% 23% 3	2000
	11th to 20th	Night	28% 5% 8% 30% 27% 2	2105
	21st to end	Night	36% 7% 10% 28% 17% 3	2200
October	1st to 10th	Night	22% 4 7% 37% 29% 2	3680
	11th to 20th	Night	36% 3 7% 23% 29% 2	3916
	21st to end	Night	40% 3 9% 25% 19% 3	4610
November	1st to 10th	Night	39% 5% 9% 26% 19% 3	2563
	11th to 20th	Night	48% 4 12% 21% 12% 2	2600
	21st to end	Night	51% 3 11% 22% 11%	2660
December	1st to 10th	Night	43% 4 14% 27% 11% 2	4580
	11th to 20th	Night	57% 3 13% 16% 10% 1	5201
	21st to end	Night	71% 4 8% 11% 6%	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	24% 15% 35% 21% 4 1	300
	11th to 20th	Day	24% 17% 39% 15% 4 1	300
	21st to end	Day	23% 21% 31% 20% 4 1	330
February	1st to 10th	Day	23% 20% 40% 14% 3	300
	11th to 20th	Day	25% 20% 34% 18% 2	300
	21st to end	Day	24% 21% 39% 12% 2 1	249
March	1st to 10th	Day	27% 20% 37% 15% 1	300
	11th to 20th	Day	29% 17% 35% 15% 4 1	323
	21st to end	Day	29% 15% 31% 18% 5% 1	550
April	1st to 10th	Day	26% 15% 31% 21% 6% 1	500
	11th to 20th	Day	23% 21% 25% 18% 11% 2	492
	21st to end	Day	24% 14% 31% 22% 7% 1	500
May	1st to 10th	Day	23% 13% 31% 22% 9% 2	503
	11th to 20th	Day	19% 11% 31% 24% 11% 4	655
	21st to end	Day	17% 11% 31% 22% 14% 5%	789
June	1st to 10th	Day	12% 10% 30% 29% 14% 5%	730
	11th to 20th	Day	12% 7% 31% 30% 16% 3	730
	21st to end	Day	14% 8% 32% 29% 12% 5%	730
July	1st to 10th	Day	8% 4 23% 32% 21% 12%	721
	11th to 20th	Day	12% 5% 26% 30% 18% 9%	718
	21st to end	Day	7% 5% 29% 33% 19% 7%	794
August	1st to 10th	Day	7% 3 22% 34% 23% 10%	671
	11th to 20th	Day	11% 4 20% 29% 24% 12%	519
	21st to end	Day	7% 5% 19% 32% 26% 10%	583
September	1st to 10th	Day	12% 7% 21% 31% 22% 6%	530
	11th to 20th	Day	10% 4 17% 33% 27% 10%	530
	21st to end	Day	13% 11% 24% 26% 19% 7%	530
October	1st to 10th	Day	15% 9% 25% 30% 16% 5%	475
	11th to 20th	Day	18% 21% 32% 21% 6% 1	330
	21st to end	Day	29% 14% 34% 17% 6%	363
November	1st to 10th	Day	18% 16% 34% 22% 6% 3	306
	11th to 20th	Day	25% 18% 35% 14% 6% 1	300
	21st to end	Day	25% 14% 36% 20% 5%	300
December	1st to 10th	Day	19% 20% 40% 13% 7%	300
	11th to 20th	Day	22% 18% 39% 16% 4 1	300
	21st to end	Day	30% 18% 35% 15% 2	330
January	1st to 10th	Night	20% 14% 38% 23% 5%	1000
	11th to 20th	Night	23% 15% 35% 22% 4 1	1000
	21st to end	Night	22% 12% 35% 28% 4	1100
February	1st to 10th	Night	23% 14% 35% 25% 4	955
	11th to 20th	Night	21% 14% 35% 25% 5%	783
	21st to end	Night	21% 14% 38% 22% 3	660
March	1st to 10th	Night	21% 14% 35% 23% 6%	800
	11th to 20th	Night	17% 12% 37% 26% 7% 1	776
	21st to end	Night	25% 11% 33% 24% 7% 1	660
April	1st to 10th	Night	21% 13% 35% 25% 6% 1	600
	11th to 20th	Night	22% 13% 31% 27% 7%	593
	21st to end	Night	22% 9% 33% 29% 7%	600
May	1st to 10th	Night	19% 7% 35% 30% 9% 1	600
	11th to 20th	Night	17% 7% 30% 33% 12% 1	591
	21st to end	Night	14% 6% 27% 35% 17% 1	650
June	1st to 10th	Night	11% 4 28% 42% 14% 1	600
	11th to 20th	Night	10% 5% 29% 44% 12% 1	600
	21st to end	Night	11% 4 29% 38% 17% 1	600
July	1st to 10th	Night	8% 3 16% 44% 26% 3	585
	11th to 20th	Night	7% 3 18% 42% 26% 2	582
	21st to end	Night	6% 2 17% 46% 26% 2	657
August	1st to 10th	Night	6% 3 13% 47% 29% 3	597
	11th to 20th	Night	7% 2 14% 42% 31% 4	594
	21st to end	Night	8% 2 15% 42% 30% 3	660
September	1st to 10th	Night	12% 3 19% 37% 27% 3	600
	11th to 20th	Night	9% 4 19% 37% 28% 4	705
	21st to end	Night	12% 7% 22% 34% 22% 3	800
October	1st to 10th	Night	12% 6% 26% 36% 19% 2	800
	11th to 20th	Night	22% 10% 30% 27% 10% 1	800
	21st to end	Night	25% 12% 29% 25% 9%	880
November	1st to 10th	Night	17% 10% 37% 24% 10% 1	789
	11th to 20th	Night	21% 15% 33% 25% 7%	800
	21st to end	Night	20% 13% 37% 24% 6%	860
December	1st to 10th	Night	24% 12% 34% 25% 5%	1000
	11th to 20th	Night	24% 12% 34% 26% 5%	1000
	21st to end	Night	31% 13% 32% 21% 3	1100

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