AJP Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AJP Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 10% 3 8% 5% 4 4 4 5% 6% 32%	132103
	Night	18% 11% 3 8% 4 4 4 3 5% 6% 33%	131210
Transparency	Day	52% 4 11% 21% 11% 1	135607
	Night	52% 4 11% 21% 11% 1	135793
Seeing	Day	28% 17% 31% 17% 6% 1	18604
	Night	26% 15% 33% 20% 6%	26351

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% 18% 5% 11% 5% 4 4 3 5% 4 27%	4093
	11th to 20th	Day	14% 16% 6% 13% 6% 6% 4 3 4 4 25%	4238
	21st to end	Day	6% 9% 4 8% 6% 5% 5% 4 6% 8% 40%	5634
February	1st to 10th	Day	13% 12% 5% 9% 6% 5% 5% 4 5% 6% 33%	1890
	11th to 20th	Day	16% 12% 4 10% 5% 5% 5% 3 5% 5% 31%	1843
	21st to end	Day	11% 13% 5% 9% 6% 4 4 3 5% 6% 34%	1604
March	1st to 10th	Day	22% 14% 4 10% 4 3 3 4 4 5% 27%	4903
	11th to 20th	Day	17% 17% 4 10% 5% 5% 4 3 4 5% 25%	5090
	21st to end	Day	26% 14% 4 7% 4 4 3 3 4 6% 24%	5587
April	1st to 10th	Day	21% 13% 4 8% 5% 4 3 3 4 6% 27%	2467
	11th to 20th	Day	21% 12% 3 7% 4 4 4 3 4 6% 33%	2702
	21st to end	Day	24% 12% 3 7% 5% 3 3 3 4 5% 31%	2730
May	1st to 10th	Day	24% 15% 3 8% 5% 4 4 3 4 5% 24%	6144
	11th to 20th	Day	23% 11% 3 7% 4 4 4 3 4 6% 31%	6458
	21st to end	Day	22% 10% 3 6% 5% 5% 4 4 6% 7% 30%	7287
June	1st to 10th	Day	27% 9% 3 7% 5% 4 4 4 5% 6% 26%	3000
	11th to 20th	Day	20% 8% 2 6% 5% 5% 5% 4 6% 7% 34%	3000
	21st to end	Day	29% 9% 3 7% 4 4 4 3 5% 7% 27%	3000
July	1st to 10th	Day	15% 7% 3 8% 5% 5% 6% 5% 7% 8% 28%	6820
	11th to 20th	Day	15% 5% 2 7% 4 5% 5% 4 6% 8% 39%	6820
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 8% 38%	6847
August	1st to 10th	Day	18% 7% 2 7% 5% 5% 6% 5% 6% 7% 33%	4235
	11th to 20th	Day	13% 7% 3 7% 5% 6% 5% 5% 7% 8% 34%	4175
	21st to end	Day	16% 5% 2 5% 4 5% 4 3 6% 7% 44%	4647
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 41%	2400
	11th to 20th	Day	17% 7% 3 6% 4 3 4 3 4 5% 45%	2241
	21st to end	Day	19% 6% 2 6% 4 4 4 3 5% 6% 41%	2190
October	1st to 10th	Day	15% 8% 3 6% 4 3 4 4 5% 7% 42%	2000
	11th to 20th	Day	21% 8% 3 7% 4 4 4 3 5% 6% 35%	2000
	21st to end	Day	23% 8% 2 7% 5% 4 4 4 4 7% 31%	2104
November	1st to 10th	Day	20% 11% 3 6% 4 4 5% 4 4 6% 33%	1687
	11th to 20th	Day	20% 12% 4 8% 4 4 4 3 5% 6% 33%	1680
	21st to end	Day	20% 12% 3 8% 4 4 4 3 4 6% 31%	1680
December	1st to 10th	Day	17% 12% 3 7% 4 4 4 3 5% 5% 34%	2666
	11th to 20th	Day	19% 11% 4 9% 5% 4 3 3 3 6% 32%	2688
	21st to end	Day	15% 12% 6% 10% 5% 4 4 3 3 4 34%	3553
January	1st to 10th	Night	13% 16% 5% 9% 4 4 4 3 5% 6% 30%	8831
	11th to 20th	Night	18% 15% 5% 11% 6% 5% 4 3 4 5% 25%	8651
	21st to end	Night	6% 11% 4 8% 4 4 5% 3 5% 7% 43%	8544
February	1st to 10th	Night	13% 13% 4 9% 5% 5% 4 4 6% 6% 31%	2730
	11th to 20th	Night	16% 14% 5% 7% 5% 5% 4 3 3 5% 31%	2663
	21st to end	Night	14% 15% 3 7% 4 5% 4 4 5% 5% 34%	2198
March	1st to 10th	Night	23% 15% 4 9% 4 4 4 3 4 4 26%	5156
	11th to 20th	Night	21% 15% 4 8% 5% 5% 5% 4 5% 6% 23%	5089
	21st to end	Night	29% 12% 3 7% 4 4 3 3 4 5% 26%	5587
April	1st to 10th	Night	24% 13% 3 9% 5% 5% 3 3 4 5% 27%	2148
	11th to 20th	Night	22% 11% 3 6% 4 4 4 3 4 6% 32%	1868
	21st to end	Night	22% 12% 3 7% 4 4 4 3 4 6% 31%	1890
May	1st to 10th	Night	25% 14% 3 6% 5% 4 4 3 5% 6% 26%	4020
	11th to 20th	Night	25% 8% 3 7% 4 4 4 2 5% 6% 33%	3676
	21st to end	Night	18% 8% 2 8% 4 5% 5% 4 6% 6% 34%	3881
June	1st to 10th	Night	27% 9% 2 6% 5% 4 4 3 4 6% 31%	1400
	11th to 20th	Night	20% 8% 3 5% 4 4 5% 4 5% 7% 36%	1400
	21st to end	Night	28% 8% 2 7% 5% 4 4 3 5% 6% 28%	1400
July	1st to 10th	Night	17% 8% 3 8% 4 4 6% 5% 6% 7% 32%	3160
	11th to 20th	Night	14% 6% 3 7% 4 5% 5% 4 6% 9% 37%	3160
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	3965
August	1st to 10th	Night	17% 6% 2 8% 5% 6% 6% 5% 5% 7% 31%	2915
	11th to 20th	Night	11% 8% 3 7% 5% 5% 5% 5% 6% 8% 37%	2872
	21st to end	Night	14% 5% 2 6% 4 4 4 4 5% 7% 47%	3251
September	1st to 10th	Night	18% 5% 2 5% 3 4 3 3 5% 7% 47%	2000
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 5% 7% 48%	2155
	21st to end	Night	18% 6% 3 6% 4 3 4 3 5% 5% 44%	2210
October	1st to 10th	Night	19% 6% 2 6% 3 3 4 4 5% 6% 42%	2400
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 4 6% 38%	2400
	21st to end	Night	22% 9% 3 7% 4 4 3 3 4 6% 35%	2734
November	1st to 10th	Night	19% 11% 3 7% 4 4 4 3 5% 5% 34%	2830
	11th to 20th	Night	21% 11% 4 7% 4 4 5% 3 4 6% 30%	2940
	21st to end	Night	22% 11% 3 8% 4 4 4 3 5% 5% 31%	2940
December	1st to 10th	Night	14% 11% 3 8% 5% 3 4 3 4 6% 38%	4738
	11th to 20th	Night	19% 12% 4 6% 4 3 3 3 4 5% 36%	5741
	21st to end	Night	19% 12% 4 11% 5% 4 4 3 4 4 30%	7667

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	61% 3 10% 13% 12%	4094
	11th to 20th	Day	63% 4 11% 12% 10% 1	4264
	21st to end	Day	43% 4 11% 22% 19% 1	5634
February	1st to 10th	Day	52% 5% 10% 18% 12% 2	1890
	11th to 20th	Day	55% 5% 9% 20% 10% 1	1851
	21st to end	Day	50% 5% 8% 21% 13% 2	1604
March	1st to 10th	Day	61% 3 9% 14% 11% 2	4907
	11th to 20th	Day	61% 4 12% 15% 6% 2	5090
	21st to end	Day	63% 2 4 18% 11% 2	5587
April	1st to 10th	Day	59% 3 7% 16% 12% 3	2467
	11th to 20th	Day	53% 5% 8% 21% 12% 1	2702
	21st to end	Day	57% 4 8% 16% 12% 4	2730
May	1st to 10th	Day	64% 4 7% 15% 11%	6155
	11th to 20th	Day	55% 5% 12% 21% 6% 1	6462
	21st to end	Day	53% 6% 10% 20% 11%	7287
June	1st to 10th	Day	58% 5% 10% 18% 9% 1	3000
	11th to 20th	Day	50% 5% 12% 20% 12% 1	3000
	21st to end	Day	59% 4 11% 16% 10% 1	3000
July	1st to 10th	Day	52% 3 17% 20% 8%	6745
	11th to 20th	Day	43% 4 15% 29% 9%	6739
	21st to end	Day	43% 4 16% 30% 7%	6651
August	1st to 10th	Day	50% 5% 14% 23% 9%	4222
	11th to 20th	Day	46% 6% 15% 25% 8%	4214
	21st to end	Day	40% 4 14% 26% 17%	4647
September	1st to 10th	Day	40% 7% 14% 26% 13%	2400
	11th to 20th	Day	43% 3 12% 26% 15% 1	2241
	21st to end	Day	44% 4 11% 26% 14% 1	2190
October	1st to 10th	Day	44% 4 14% 28% 10%	3260
	11th to 20th	Day	50% 3 10% 27% 10%	3240
	21st to end	Day	45% 3 16% 20% 16%	3380
November	1st to 10th	Day	53% 5% 10% 22% 10% 1	1687
	11th to 20th	Day	54% 4 12% 16% 12% 1	1680
	21st to end	Day	56% 4 14% 18% 8%	1680
December	1st to 10th	Day	53% 2 14% 23% 8%	2666
	11th to 20th	Day	56% 4 9% 17% 12% 2	2688
	21st to end	Day	57% 3 9% 16% 12% 2	3553
January	1st to 10th	Night	55% 3 11% 22% 7%	8831
	11th to 20th	Night	62% 4 9% 16% 8% 1	8700
	21st to end	Night	43% 2 10% 24% 18% 3	8544
February	1st to 10th	Night	52% 4 11% 17% 13% 2	2730
	11th to 20th	Night	57% 4 9% 18% 12% 1	2666
	21st to end	Night	52% 4 7% 20% 15% 3	2198
March	1st to 10th	Night	63% 3 6% 11% 13% 3	5165
	11th to 20th	Night	61% 6% 11% 12% 9% 1	5090
	21st to end	Night	62% 2 4 17% 13% 2	5587
April	1st to 10th	Night	61% 3 7% 15% 11% 3	2148
	11th to 20th	Night	53% 4 9% 20% 12% 2	1868
	21st to end	Night	55% 4 9% 15% 14% 3	1890
May	1st to 10th	Night	59% 4 9% 16% 11% 1	4026
	11th to 20th	Night	52% 6% 11% 21% 8% 2	3677
	21st to end	Night	48% 7% 16% 18% 10%	3882
June	1st to 10th	Night	54% 5% 10% 21% 9% 1	1400
	11th to 20th	Night	48% 5% 12% 24% 9% 2	1400
	21st to end	Night	58% 6% 11% 17% 8% 1	1400
July	1st to 10th	Night	51% 4 15% 19% 11%	3125
	11th to 20th	Night	45% 4 17% 30% 4	3118
	21st to end	Night	40% 4 19% 26% 10%	3847
August	1st to 10th	Night	52% 7% 12% 22% 7%	2903
	11th to 20th	Night	44% 5% 18% 26% 7%	2899
	21st to end	Night	38% 4 18% 25% 14% 2	3250
September	1st to 10th	Night	39% 5% 12% 30% 13% 1	2000
	11th to 20th	Night	37% 4 14% 29% 14% 1	2155
	21st to end	Night	42% 4 10% 28% 15% 1	2210
October	1st to 10th	Night	47% 3 12% 27% 10% 1	3920
	11th to 20th	Night	45% 3 11% 30% 11%	3899
	21st to end	Night	44% 3 16% 23% 14%	4408
November	1st to 10th	Night	52% 4 11% 22% 10% 1	2830
	11th to 20th	Night	55% 5% 11% 18% 10% 1	2940
	21st to end	Night	55% 5% 12% 17% 10%	2940
December	1st to 10th	Night	48% 2 12% 27% 10%	4738
	11th to 20th	Night	52% 3 8% 22% 14% 1	5742
	21st to end	Night	59% 3 9% 18% 10% 1	7667

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	34% 20% 32% 13% 1	220
	11th to 20th	Day	38% 20% 22% 14% 5%	220
	21st to end	Day	24% 18% 35% 18% 6%	417
February	1st to 10th	Day	27% 21% 28% 16% 5% 3	430
	11th to 20th	Day	27% 23% 31% 12% 5% 1	426
	21st to end	Day	25% 21% 34% 16% 3	380
March	1st to 10th	Day	35% 21% 31% 11% 2 1	540
	11th to 20th	Day	32% 20% 27% 16% 4 1	540
	21st to end	Day	35% 21% 27% 13% 3 1	594
April	1st to 10th	Day	39% 21% 27% 9% 2	540
	11th to 20th	Day	35% 22% 26% 13% 4 1	532
	21st to end	Day	35% 17% 26% 15% 6% 1	540
May	1st to 10th	Day	36% 21% 25% 13% 4 1	593
	11th to 20th	Day	31% 16% 29% 16% 7% 1	774
	21st to end	Day	31% 16% 33% 16% 4	845
June	1st to 10th	Day	35% 14% 29% 14% 7% 1	730
	11th to 20th	Day	27% 16% 31% 20% 5% 1	730
	21st to end	Day	32% 13% 30% 20% 5%	730
July	1st to 10th	Day	23% 14% 35% 20% 7% 1	720
	11th to 20th	Day	17% 12% 39% 23% 8% 1	718
	21st to end	Day	18% 13% 38% 23% 7% 1	705
August	1st to 10th	Day	15% 13% 39% 23% 9% 2	526
	11th to 20th	Day	18% 11% 30% 29% 10% 1	519
	21st to end	Day	19% 14% 31% 25% 10% 2	583
September	1st to 10th	Day	17% 18% 33% 22% 10% 2	530
	11th to 20th	Day	22% 14% 32% 23% 8% 2	530
	21st to end	Day	24% 14% 35% 17% 6% 4	524
October	1st to 10th	Day	20% 15% 29% 21% 12% 2	420
	11th to 20th	Day	30% 15% 27% 18% 7% 2	420
	21st to end	Day	32% 20% 27% 14% 5% 2	462
November	1st to 10th	Day	34% 16% 30% 13% 5% 1	425
	11th to 20th	Day	33% 19% 27% 16% 5% 1	430
	21st to end	Day	33% 21% 29% 12% 4 1	430
December	1st to 10th	Day	31% 18% 32% 16% 4	419
	11th to 20th	Day	39% 22% 29% 9% 2	220
	21st to end	Day	33% 19% 29% 15% 4	242
January	1st to 10th	Night	26% 18% 34% 18% 3	950
	11th to 20th	Night	26% 18% 31% 19% 4	917
	21st to end	Night	22% 19% 35% 19% 5%	924
February	1st to 10th	Night	23% 20% 35% 17% 5% 1	840
	11th to 20th	Night	28% 19% 35% 14% 3	823
	21st to end	Night	24% 17% 38% 17% 4	692
March	1st to 10th	Night	28% 18% 35% 17% 2	840
	11th to 20th	Night	28% 17% 32% 18% 5%	840
	21st to end	Night	27% 16% 36% 16% 4	924
April	1st to 10th	Night	33% 19% 28% 17% 3	688
	11th to 20th	Night	31% 12% 35% 15% 6%	624
	21st to end	Night	32% 15% 28% 20% 4 1	630
May	1st to 10th	Night	33% 16% 27% 19% 4	630
	11th to 20th	Night	26% 15% 34% 18% 7%	622
	21st to end	Night	21% 20% 35% 18% 4	618
June	1st to 10th	Night	31% 8% 31% 25% 5%	400
	11th to 20th	Night	24% 13% 31% 26% 7%	400
	21st to end	Night	33% 8% 29% 25% 5%	400
July	1st to 10th	Night	20% 10% 42% 22% 7%	390
	11th to 20th	Night	15% 8% 37% 31% 8%	388
	21st to end	Night	19% 8% 36% 28% 9%	574
August	1st to 10th	Night	17% 7% 38% 28% 10%	598
	11th to 20th	Night	16% 7% 34% 32% 10% 1	592
	21st to end	Night	17% 11% 29% 30% 12% 1	660
September	1st to 10th	Night	20% 10% 37% 27% 7%	600
	11th to 20th	Night	20% 10% 34% 26% 10% 1	755
	21st to end	Night	21% 13% 33% 23% 9% 1	800
October	1st to 10th	Night	22% 13% 33% 23% 8%	800
	11th to 20th	Night	23% 15% 34% 19% 8% 1	800
	21st to end	Night	30% 17% 32% 17% 4	880
November	1st to 10th	Night	30% 15% 31% 19% 4 1	907
	11th to 20th	Night	31% 16% 30% 16% 6% 1	950
	21st to end	Night	31% 19% 27% 19% 4 1	950
December	1st to 10th	Night	29% 17% 34% 16% 4	950
	11th to 20th	Night	31% 20% 32% 14% 3	950
	21st to end	Night	31% 19% 28% 17% 4 1	1045

Page generated at 2026-05-29 02:53:26. Version ac07

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