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 2025-02-06.

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%	108217
	Night	18% 11% 3 8% 5% 4 4 3 5% 6% 33%	110126
Transparency	Day	52% 5% 12% 20% 11% 1	111767
	Night	51% 4 12% 21% 11% 1	114740
Seeing	Day	28% 17% 31% 17% 6% 1	16796
	Night	26% 15% 33% 20% 6%	24636

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	14% 19% 5% 11% 5% 4 5% 3 5% 5% 25%	3157
	11th to 20th	Day	10% 15% 7% 12% 6% 6% 5% 3 4 4 28%	3249
	21st to end	Day	7% 8% 3 7% 6% 5% 6% 4 6% 7% 42%	4347
February	1st to 10th	Day	14% 12% 5% 8% 5% 5% 5% 4 4 6% 33%	1755
	11th to 20th	Day	16% 12% 4 10% 5% 5% 5% 3 5% 6% 29%	1663
	21st to end	Day	12% 13% 4 9% 7% 4 4 3 5% 6% 33%	1456
March	1st to 10th	Day	21% 14% 5% 7% 4 3 4 4 4 5% 29%	3338
	11th to 20th	Day	17% 20% 5% 11% 5% 5% 4 3 4 4 22%	3480
	21st to end	Day	28% 15% 4 7% 4 4 3 3 4 5% 25%	3816
April	1st to 10th	Day	21% 14% 4 8% 5% 4 3 3 5% 6% 27%	2232
	11th to 20th	Day	21% 11% 2 7% 4 4 4 4 4 6% 34%	2442
	21st to end	Day	25% 12% 3 6% 5% 3 3 3 4 5% 31%	2470
May	1st to 10th	Day	21% 17% 4 8% 5% 4 4 3 4 5% 25%	4190
	11th to 20th	Day	19% 10% 3 7% 5% 4 4 3 4 7% 34%	4398
	21st to end	Day	19% 8% 3 7% 6% 6% 5% 4 7% 7% 28%	4961
June	1st to 10th	Day	27% 10% 3 7% 5% 4 3 4 5% 6% 25%	2850
	11th to 20th	Day	20% 7% 2 6% 5% 4 5% 4 5% 7% 34%	2850
	21st to end	Day	29% 9% 3 7% 4 4 4 3 5% 6% 26%	2850
July	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 7% 32%	4760
	11th to 20th	Day	17% 6% 2 7% 5% 5% 6% 4 6% 8% 35%	4760
	21st to end	Day	14% 6% 3 7% 5% 5% 6% 5% 6% 8% 36%	4763
August	1st to 10th	Day	18% 7% 2 7% 5% 5% 6% 5% 6% 7% 31%	4105
	11th to 20th	Day	13% 7% 3 7% 5% 6% 5% 5% 7% 8% 33%	4045
	21st to end	Day	16% 5% 2 5% 4 5% 4 3 5% 7% 44%	4505
September	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 40%	2280
	11th to 20th	Day	17% 7% 3 6% 4 3 4 3 4 5% 44%	2129
	21st to end	Day	20% 6% 2 6% 4 5% 3 2 5% 6% 40%	2081
October	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 5% 7% 40%	1900
	11th to 20th	Day	21% 7% 3 7% 4 4 4 3 4 6% 36%	1900
	21st to end	Day	24% 8% 2 6% 5% 4 4 4 4 6% 32%	1999
November	1st to 10th	Day	20% 11% 3 6% 4 4 5% 4 4 6% 33%	1605
	11th to 20th	Day	20% 12% 4 8% 4 4 4 3 5% 6% 32%	1600
	21st to end	Day	21% 12% 3 8% 4 4 4 3 4 6% 31%	1600
December	1st to 10th	Day	17% 12% 3 7% 4 4 4 3 5% 5% 34%	2587
	11th to 20th	Day	19% 11% 4 9% 5% 4 3 3 3 6% 33%	2618
	21st to end	Day	15% 13% 6% 10% 5% 4 4 3 3 3 34%	3476
January	1st to 10th	Night	12% 15% 6% 10% 5% 4 4 4 6% 6% 28%	6808
	11th to 20th	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 27%	6633
	21st to end	Night	8% 11% 3 7% 4 4 4 3 5% 7% 44%	6606
February	1st to 10th	Night	14% 13% 3 8% 5% 4 4 5% 6% 6% 31%	2536
	11th to 20th	Night	17% 14% 5% 8% 6% 5% 4 3 3 5% 30%	2403
	21st to end	Night	14% 14% 4 7% 5% 5% 4 4 5% 6% 33%	1995
March	1st to 10th	Night	21% 15% 4 9% 4 5% 5% 3 4 4 26%	3501
	11th to 20th	Night	23% 18% 4 8% 5% 4 4 4 4 5% 20%	3479
	21st to end	Night	29% 11% 3 7% 5% 4 3 3 5% 6% 25%	3816
April	1st to 10th	Night	24% 13% 3 8% 4 4 3 3 4 5% 28%	1943
	11th to 20th	Night	21% 11% 2 6% 4 4 5% 3 5% 5% 33%	1688
	21st to end	Night	23% 12% 3 7% 4 3 4 3 4 6% 31%	1710
May	1st to 10th	Night	23% 13% 3 5% 5% 5% 4 4 5% 7% 27%	2751
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 6% 8% 36%	2506
	21st to end	Night	15% 8% 3 9% 5% 5% 6% 4 7% 7% 31%	2639
June	1st to 10th	Night	27% 9% 2 6% 5% 4 4 3 4 6% 31%	1330
	11th to 20th	Night	20% 8% 3 5% 4 4 5% 4 5% 7% 36%	1330
	21st to end	Night	28% 8% 2 6% 5% 5% 4 3 5% 6% 28%	1330
July	1st to 10th	Night	14% 8% 3 8% 5% 4 5% 5% 6% 7% 36%	2210
	11th to 20th	Night	18% 7% 3 7% 5% 5% 5% 4 5% 8% 34%	2210
	21st to end	Night	12% 7% 3 6% 4 5% 4 3 5% 9% 41%	2758
August	1st to 10th	Night	18% 6% 2 9% 5% 6% 6% 5% 5% 7% 30%	2825
	11th to 20th	Night	11% 8% 3 8% 5% 5% 5% 5% 6% 8% 36%	2782
	21st to end	Night	14% 5% 2 5% 4 4 3 4 5% 7% 47%	3151
September	1st to 10th	Night	18% 5% 2 5% 3 4 3 3 5% 7% 45%	1900
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 7% 47%	2047
	21st to end	Night	18% 6% 2 6% 3 3 4 3 5% 6% 43%	2099
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 4 5% 6% 40%	2280
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 4 6% 39%	2280
	21st to end	Night	22% 9% 3 7% 4 4 3 3 4 6% 36%	2597
November	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 5% 35%	2692
	11th to 20th	Night	21% 11% 4 7% 4 5% 5% 3 5% 6% 30%	2800
	21st to end	Night	23% 11% 3 8% 4 4 4 3 5% 5% 30%	2800
December	1st to 10th	Night	14% 11% 3 8% 5% 3 4 3 5% 7% 38%	4598
	11th to 20th	Night	20% 12% 4 6% 4 3 3 3 4 5% 37%	5591
	21st to end	Night	20% 12% 5% 11% 5% 4 4 3 4 4 30%	7502

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	62% 4 11% 14% 9%	3158
	11th to 20th	Day	59% 5% 12% 14% 11% 1	3275
	21st to end	Day	42% 5% 11% 22% 19% 2	4347
February	1st to 10th	Day	51% 5% 10% 18% 12% 2	1755
	11th to 20th	Day	56% 5% 9% 19% 10% 1	1671
	21st to end	Day	50% 6% 8% 21% 12% 2	1456
March	1st to 10th	Day	58% 4 11% 19% 6% 3	3342
	11th to 20th	Day	66% 5% 8% 14% 6% 2	3480
	21st to end	Day	63% 3 5% 16% 10% 2	3816
April	1st to 10th	Day	59% 3 7% 16% 11% 3	2232
	11th to 20th	Day	51% 5% 9% 21% 12% 1	2442
	21st to end	Day	57% 4 8% 16% 12% 4	2470
May	1st to 10th	Day	61% 5% 7% 19% 8% 1	4204
	11th to 20th	Day	49% 7% 13% 23% 8% 1	4402
	21st to end	Day	52% 8% 10% 19% 11%	4961
June	1st to 10th	Day	59% 5% 10% 17% 8% 1	2850
	11th to 20th	Day	50% 5% 13% 20% 11% 1	2850
	21st to end	Day	59% 4 11% 16% 9%	2850
July	1st to 10th	Day	50% 4 18% 19% 10%	4685
	11th to 20th	Day	48% 4 14% 25% 8%	4679
	21st to end	Day	46% 5% 16% 24% 8%	4610
August	1st to 10th	Day	51% 5% 14% 22% 8%	4092
	11th to 20th	Day	47% 6% 15% 25% 8%	4084
	21st to end	Day	39% 4 14% 25% 17%	4505
September	1st to 10th	Day	41% 7% 14% 25% 13%	2280
	11th to 20th	Day	43% 3 11% 26% 15% 1	2129
	21st to end	Day	45% 4 11% 25% 14% 1	2081
October	1st to 10th	Day	45% 4 14% 27% 10% 1	3160
	11th to 20th	Day	50% 3 10% 27% 10%	3140
	21st to end	Day	44% 3 16% 20% 16%	3275
November	1st to 10th	Day	52% 5% 10% 22% 10% 1	1605
	11th to 20th	Day	54% 5% 13% 15% 13% 1	1600
	21st to end	Day	56% 4 15% 17% 8%	1600
December	1st to 10th	Day	53% 2 14% 23% 8%	2587
	11th to 20th	Day	56% 4 9% 17% 12% 2	2618
	21st to end	Day	57% 3 9% 16% 12% 2	3476
January	1st to 10th	Night	56% 4 14% 20% 6% 1	6808
	11th to 20th	Night	58% 4 11% 18% 8% 1	6682
	21st to end	Night	41% 3 10% 26% 17% 3	6606
February	1st to 10th	Night	53% 4 12% 16% 13% 2	2536
	11th to 20th	Night	57% 4 8% 17% 12% 2	2406
	21st to end	Night	52% 4 7% 19% 14% 3	1995
March	1st to 10th	Night	62% 4 9% 16% 8% 2	3510
	11th to 20th	Night	63% 8% 7% 12% 8% 1	3480
	21st to end	Night	62% 3 5% 18% 11% 2	3816
April	1st to 10th	Night	60% 3 7% 14% 12% 3	1943
	11th to 20th	Night	52% 4 9% 19% 13% 2	1688
	21st to end	Night	55% 5% 8% 15% 14% 3	1710
May	1st to 10th	Night	56% 6% 11% 16% 8% 1	2757
	11th to 20th	Night	44% 8% 15% 25% 7% 1	2507
	21st to end	Night	47% 10% 16% 18% 8%	2640
June	1st to 10th	Night	55% 5% 11% 20% 9% 1	1330
	11th to 20th	Night	48% 5% 12% 24% 8% 2	1330
	21st to end	Night	57% 6% 11% 17% 7% 1	1330
July	1st to 10th	Night	47% 5% 17% 20% 12%	2175
	11th to 20th	Night	48% 5% 15% 26% 6%	2168
	21st to end	Night	41% 6% 17% 26% 10%	2671
August	1st to 10th	Night	53% 7% 12% 21% 6%	2813
	11th to 20th	Night	45% 5% 18% 26% 7%	2809
	21st to end	Night	37% 4 18% 25% 14% 2	3150
September	1st to 10th	Night	39% 5% 13% 29% 13% 1	1900
	11th to 20th	Night	38% 4 14% 28% 14% 1	2047
	21st to end	Night	41% 5% 11% 28% 14% 1	2099
October	1st to 10th	Night	48% 3 12% 27% 9%	3800
	11th to 20th	Night	45% 3 11% 31% 11%	3779
	21st to end	Night	44% 3 16% 23% 14%	4271
November	1st to 10th	Night	51% 4 12% 22% 10% 1	2692
	11th to 20th	Night	54% 5% 11% 18% 11% 1	2800
	21st to end	Night	56% 6% 13% 16% 9%	2800
December	1st to 10th	Night	48% 2 12% 27% 10%	4598
	11th to 20th	Night	51% 3 8% 22% 14% 1	5592
	21st to end	Night	59% 3 9% 18% 10% 1	7502

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% 34% 13% 1	200
	11th to 20th	Day	36% 20% 23% 15% 6% 1	200
	21st to end	Day	23% 18% 35% 18% 6%	386
February	1st to 10th	Day	29% 22% 28% 14% 4 2	385
	11th to 20th	Day	27% 24% 31% 13% 5% 1	366
	21st to end	Day	25% 20% 35% 16% 3	328
March	1st to 10th	Day	33% 20% 32% 11% 2 1	460
	11th to 20th	Day	30% 18% 29% 17% 4 2	460
	21st to end	Day	34% 19% 28% 13% 4 1	506
April	1st to 10th	Day	38% 20% 28% 11% 3	460
	11th to 20th	Day	31% 24% 27% 13% 4 1	452
	21st to end	Day	35% 17% 26% 14% 6% 1	460
May	1st to 10th	Day	35% 20% 25% 15% 5% 1	508
	11th to 20th	Day	30% 14% 29% 17% 8% 2	674
	21st to end	Day	30% 17% 34% 16% 4	735
June	1st to 10th	Day	35% 14% 29% 14% 7% 1	680
	11th to 20th	Day	27% 15% 31% 20% 5% 1	680
	21st to end	Day	32% 13% 29% 20% 5%	680
July	1st to 10th	Day	22% 14% 35% 20% 8% 1	670
	11th to 20th	Day	18% 12% 39% 23% 8% 1	668
	21st to end	Day	18% 13% 36% 24% 8% 1	655
August	1st to 10th	Day	16% 13% 39% 22% 8% 1	486
	11th to 20th	Day	19% 11% 31% 28% 9% 1	479
	21st to end	Day	20% 13% 29% 25% 10% 2	539
September	1st to 10th	Day	16% 18% 33% 22% 9% 1	490
	11th to 20th	Day	23% 14% 32% 21% 8% 2	490
	21st to end	Day	24% 13% 34% 17% 7% 4	485
October	1st to 10th	Day	20% 16% 29% 20% 13% 2	390
	11th to 20th	Day	30% 16% 28% 16% 7% 2	390
	21st to end	Day	32% 20% 28% 14% 5% 1	429
November	1st to 10th	Day	33% 16% 31% 14% 5% 1	395
	11th to 20th	Day	32% 19% 28% 16% 5% 1	400
	21st to end	Day	34% 21% 29% 12% 4 1	400
December	1st to 10th	Day	31% 16% 32% 16% 4 1	390
	11th to 20th	Day	40% 20% 30% 9% 2	200
	21st to end	Day	33% 19% 30% 15% 4	220
January	1st to 10th	Night	26% 18% 34% 18% 3	900
	11th to 20th	Night	25% 18% 31% 20% 5%	870
	21st to end	Night	21% 19% 36% 19% 6%	880
February	1st to 10th	Night	24% 20% 35% 16% 5% 1	781
	11th to 20th	Night	28% 20% 35% 13% 4 1	743
	21st to end	Night	24% 17% 37% 18% 4	628
March	1st to 10th	Night	27% 18% 36% 18% 2	760
	11th to 20th	Night	27% 18% 32% 17% 6%	760
	21st to end	Night	27% 16% 35% 17% 4	836
April	1st to 10th	Night	32% 19% 28% 18% 3	622
	11th to 20th	Night	30% 13% 35% 16% 7%	564
	21st to end	Night	32% 16% 28% 19% 4 1	570
May	1st to 10th	Night	32% 16% 27% 20% 5%	570
	11th to 20th	Night	25% 14% 33% 20% 8%	562
	21st to end	Night	20% 21% 36% 18% 4	558
June	1st to 10th	Night	31% 8% 30% 26% 5%	380
	11th to 20th	Night	24% 12% 31% 26% 7%	380
	21st to end	Night	33% 7% 29% 26% 6%	380
July	1st to 10th	Night	19% 10% 42% 21% 7%	370
	11th to 20th	Night	16% 8% 36% 32% 8%	368
	21st to end	Night	20% 8% 35% 28% 9%	545
August	1st to 10th	Night	17% 8% 38% 28% 9%	568
	11th to 20th	Night	17% 7% 34% 32% 10% 1	562
	21st to end	Night	17% 11% 29% 29% 12% 1	627
September	1st to 10th	Night	20% 11% 37% 26% 6%	570
	11th to 20th	Night	21% 10% 34% 26% 10% 1	717
	21st to end	Night	21% 13% 33% 24% 9% 1	760
October	1st to 10th	Night	23% 12% 32% 23% 9%	760
	11th to 20th	Night	23% 16% 34% 18% 8%	760
	21st to end	Night	29% 17% 33% 17% 4	836
November	1st to 10th	Night	29% 14% 32% 20% 4 1	859
	11th to 20th	Night	32% 16% 29% 16% 6% 1	900
	21st to end	Night	32% 19% 26% 19% 3	900
December	1st to 10th	Night	29% 17% 34% 16% 4	900
	11th to 20th	Night	31% 20% 32% 14% 3	900
	21st to end	Night	32% 18% 28% 17% 4 1	990

Page generated at 2025-02-03 04:05:55. Version ac07

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