Area 52 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Area 52 Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	15% 6% 2 5% 4 4 4 3 5% 7% 45%	109205
	Night	15% 6% 2 5% 3 3 3 3 4 6% 51%	109592
Transparency	Day	38% 4 10% 23% 19% 6%	112853
	Night	34% 4 9% 22% 23% 8%	114084
Seeing	Day	16% 9% 23% 26% 21% 6%	15606
	Night	16% 7% 24% 32% 18% 2	25583

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	19% 9% 2 5% 3 3 2 2 3 4 49%	4077
	11th to 20th	Day	12% 7% 2 5% 3 3 3 2 4 4 55%	4080
	21st to end	Day	24% 6% 2 4 2 2 2 2 3 4 48%	4488
February	1st to 10th	Day	26% 7% 2 5% 4 4 3 3 3 5% 39%	1800
	11th to 20th	Day	22% 8% 2 5% 4 3 3 2 4 5% 41%	1838
	21st to end	Day	28% 8% 2 5% 3 2 3 2 3 4 39%	1727
March	1st to 10th	Day	22% 10% 3 6% 3 3 3 3 3 5% 40%	3436
	11th to 20th	Day	19% 7% 3 7% 5% 4 4 4 5% 7% 37%	3480
	21st to end	Day	19% 8% 3 7% 4 3 3 2 5% 5% 41%	3814
April	1st to 10th	Day	16% 6% 3 6% 4 4 3 3 5% 6% 43%	2185
	11th to 20th	Day	19% 6% 2 5% 3 4 4 3 5% 6% 43%	2254
	21st to end	Day	18% 8% 3 5% 4 3 3 3 6% 6% 42%	2280
May	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 5% 6% 36%	3911
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 6% 8% 43%	4082
	21st to end	Day	17% 8% 3 8% 5% 5% 5% 4 5% 7% 32%	4477
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 43%	2470
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 5% 7% 9% 44%	2470
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 6% 10% 48%	2470
July	1st to 10th	Day	9% 4 2 6% 5% 5% 5% 4 8% 10% 40%	4120
	11th to 20th	Day	6% 3 1 4 4 4 4 3 6% 9% 57%	4120
	21st to end	Day	11% 8% 3 8% 5% 5% 5% 5% 7% 8% 33%	4465
August	1st to 10th	Day	3 2 1 4 3 4 4 4 5% 9% 62%	4104
	11th to 20th	Day	7% 3 2 5% 4 4 4 4 7% 10% 50%	3885
	21st to end	Day	5% 4 1 6% 5% 5% 6% 5% 6% 9% 48%	4180
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 6% 50%	2280
	11th to 20th	Day	12% 5% 2 5% 3 4 5% 5% 6% 7% 46%	2117
	21st to end	Day	9% 5% 2 5% 3 4 4 3 5% 6% 53%	2062
October	1st to 10th	Day	9% 4 2 5% 3 3 4 3 4 6% 57%	1900
	11th to 20th	Day	15% 4 2 4 3 2 3 2 4 5% 57%	1900
	21st to end	Day	19% 6% 2 5% 3 2 3 2 3 5% 51%	2090
November	1st to 10th	Day	16% 6% 2 5% 3 3 3 3 4 6% 50%	1958
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 4 6% 53%	2000
	21st to end	Day	20% 7% 2 5% 3 2 3 2 2 5% 49%	1866
December	1st to 10th	Day	20% 6% 1 4 3 3 3 2 4 4 49%	2940
	11th to 20th	Day	25% 7% 2 5% 3 3 3 2 3 5% 42%	3391
	21st to end	Day	23% 8% 2 6% 3 3 3 2 4 5% 39%	4488
January	1st to 10th	Night	17% 6% 2 4 2 2 3 3 4 5% 52%	5888
	11th to 20th	Night	13% 5% 1 3 2 2 2 2 3 4 64%	5900
	21st to end	Night	26% 7% 2 4 3 2 2 2 3 4 46%	6490
February	1st to 10th	Night	22% 7% 2 5% 3 3 3 3 4 4 45%	2600
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 5% 4 43%	2464
	21st to end	Night	21% 7% 2 4 3 2 3 3 3 4 49%	1727
March	1st to 10th	Night	22% 8% 2 6% 3 3 3 3 5% 5% 40%	3419
	11th to 20th	Night	17% 7% 3 5% 3 3 3 3 4 7% 45%	3480
	21st to end	Night	16% 6% 2 6% 4 4 3 3 5% 7% 43%	3817
April	1st to 10th	Night	14% 7% 1 5% 4 4 3 3 4 6% 50%	1995
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	1875
	21st to end	Night	14% 6% 3 5% 3 3 3 2 4 7% 49%	1900
May	1st to 10th	Night	22% 7% 3 6% 4 3 4 4 5% 6% 37%	2985
	11th to 20th	Night	16% 6% 3 6% 4 3 3 3 5% 7% 44%	2823
	21st to end	Night	15% 7% 2 6% 4 4 4 3 6% 8% 40%	3090
June	1st to 10th	Night	6% 4 2 5% 3 4 4 3 6% 9% 54%	1710
	11th to 20th	Night	5% 4 2 4 3 4 3 3 5% 8% 59%	1710
	21st to end	Night	4 3 1 5% 3 3 3 3 4 8% 62%	1710
July	1st to 10th	Night	8% 4 1 5% 4 4 4 4 7% 11% 49%	2850
	11th to 20th	Night	4 2 1 3 2 2 3 4 7% 10% 61%	2850
	21st to end	Night	9% 5% 3 7% 5% 5% 6% 4 7% 8% 41%	3098
August	1st to 10th	Night	2 2 1 3 2 2 3 2 5% 9% 69%	2836
	11th to 20th	Night	5% 2 1 4 3 4 4 4 7% 11% 56%	2947
	21st to end	Night	3 3 2 5% 4 4 5% 4 7% 9% 55%	3476
September	1st to 10th	Night	5% 5% 2 6% 3 3 3 3 5% 8% 57%	1900
	11th to 20th	Night	10% 4 2 5% 4 3 5% 3 6% 8% 50%	2059
	21st to end	Night	8% 3 2 4 4 3 4 3 5% 6% 59%	2099
October	1st to 10th	Night	8% 3 2 3 3 3 2 3 5% 6% 63%	2280
	11th to 20th	Night	12% 4 1 4 2 2 2 2 4 5% 61%	2280
	21st to end	Night	17% 6% 2 4 2 3 2 2 3 3 56%	2508
November	1st to 10th	Night	15% 5% 1 4 3 2 3 2 4 5% 55%	2340
	11th to 20th	Night	11% 5% 2 4 3 2 3 3 4 4 59%	2400
	21st to end	Night	18% 6% 2 4 3 3 3 2 3 5% 51%	2530
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 4 50%	4250
	11th to 20th	Night	22% 8% 2 5% 4 4 3 2 2 3 44%	4816
	21st to end	Night	20% 9% 2 5% 3 2 3 2 3 5% 45%	6490

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	42% 1 8% 28% 18% 3	4078
	11th to 20th	Day	35% 2 8% 25% 26% 5%	4080
	21st to end	Day	42% 2 4 17% 23% 11%	4488
February	1st to 10th	Day	49% 4 7% 17% 18% 5%	1800
	11th to 20th	Day	47% 3 6% 20% 18% 6%	1847
	21st to end	Day	50% 3 6% 12% 20% 8%	1727
March	1st to 10th	Day	47% 4 7% 14% 20% 8%	3436
	11th to 20th	Day	46% 6% 8% 18% 18% 5%	3480
	21st to end	Day	46% 4 8% 20% 19% 2	3814
April	1st to 10th	Day	41% 5% 8% 21% 19% 7%	2185
	11th to 20th	Day	42% 5% 8% 20% 18% 6%	2254
	21st to end	Day	42% 6% 10% 20% 20% 3	2280
May	1st to 10th	Day	48% 6% 9% 23% 11% 2	3946
	11th to 20th	Day	38% 7% 11% 24% 20% 1	4086
	21st to end	Day	50% 8% 12% 17% 10% 3	4489
June	1st to 10th	Day	36% 8% 14% 26% 15% 2	2470
	11th to 20th	Day	34% 9% 13% 26% 17% 2	2470
	21st to end	Day	29% 7% 13% 30% 19% 2	2470
July	1st to 10th	Day	37% 7% 21% 26% 9% 1	4056
	11th to 20th	Day	24% 4 13% 36% 22% 1	4051
	21st to end	Day	47% 5% 15% 25% 7%	4325
August	1st to 10th	Day	20% 5% 15% 27% 28% 5%	4090
	11th to 20th	Day	29% 5% 22% 33% 10% 1	3924
	21st to end	Day	31% 5% 16% 30% 15% 2	4179
September	1st to 10th	Day	34% 6% 11% 20% 19% 9%	2280
	11th to 20th	Day	36% 7% 10% 22% 19% 6%	2117
	21st to end	Day	29% 6% 10% 22% 24% 10%	2062
October	1st to 10th	Day	17% 3 5% 11% 29% 34%	3160
	11th to 20th	Day	21% 2 3 18% 35% 21%	3140
	21st to end	Day	32% 2 4 28% 27% 7%	3428
November	1st to 10th	Day	36% 4 7% 22% 23% 8%	1958
	11th to 20th	Day	33% 4 6% 19% 28% 10%	2000
	21st to end	Day	42% 2 4 18% 26% 7%	1866
December	1st to 10th	Day	40% 2 7% 29% 18% 4	2940
	11th to 20th	Day	47% 2 4 18% 21% 7%	3389
	21st to end	Day	49% 3 5% 20% 19% 4	4488
January	1st to 10th	Night	36% 2 6% 27% 25% 3	5888
	11th to 20th	Night	27% 2 7% 21% 33% 10%	5900
	21st to end	Night	45% 2 6% 15% 23% 9%	6490
February	1st to 10th	Night	44% 4 7% 18% 20% 7%	2600
	11th to 20th	Night	45% 4 6% 18% 20% 7%	2465
	21st to end	Night	42% 3 6% 16% 22% 11%	1727
March	1st to 10th	Night	46% 4 7% 14% 17% 12%	3419
	11th to 20th	Night	40% 5% 8% 21% 16% 10%	3480
	21st to end	Night	40% 6% 8% 21% 23% 3	3817
April	1st to 10th	Night	37% 4 9% 20% 18% 12%	1995
	11th to 20th	Night	36% 6% 7% 20% 21% 10%	1875
	21st to end	Night	37% 4 8% 24% 23% 4	1900
May	1st to 10th	Night	46% 8% 9% 21% 12% 4	3015
	11th to 20th	Night	39% 5% 11% 24% 19% 2	2824
	21st to end	Night	40% 8% 14% 21% 13% 4	3099
June	1st to 10th	Night	25% 9% 13% 31% 18% 3	1710
	11th to 20th	Night	24% 5% 12% 31% 25% 3	1710
	21st to end	Night	22% 5% 12% 31% 25% 5%	1710
July	1st to 10th	Night	29% 6% 19% 31% 13% 1	2805
	11th to 20th	Night	19% 3 13% 38% 25% 2	2796
	21st to end	Night	40% 5% 15% 26% 14% 1	2999
August	1st to 10th	Night	14% 4 14% 28% 33% 7%	2826
	11th to 20th	Night	23% 4 20% 33% 18% 3	2974
	21st to end	Night	25% 6% 14% 26% 27% 2	3476
September	1st to 10th	Night	27% 6% 10% 22% 24% 10%	1900
	11th to 20th	Night	31% 6% 10% 22% 22% 8%	2059
	21st to end	Night	26% 5% 9% 21% 26% 13%	2099
October	1st to 10th	Night	15% 3 5% 14% 25% 38%	3800
	11th to 20th	Night	18% 2 3 15% 36% 26%	3779
	21st to end	Night	29% 2 4 23% 29% 13%	4120
November	1st to 10th	Night	31% 5% 8% 19% 27% 11%	2340
	11th to 20th	Night	30% 3 7% 19% 28% 13%	2400
	21st to end	Night	37% 3 6% 18% 27% 8%	2530
December	1st to 10th	Night	39% 2 6% 28% 20% 5%	4250
	11th to 20th	Night	47% 2 4 17% 19% 10%	4817
	21st to end	Night	45% 2 7% 21% 22% 4	6490

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	28% 14% 34% 20% 3	300
	11th to 20th	Day	24% 18% 39% 15% 4	300
	21st to end	Day	25% 19% 36% 15% 5%	330
February	1st to 10th	Day	33% 15% 31% 17% 4	300
	11th to 20th	Day	34% 22% 32% 10% 2	300
	21st to end	Day	41% 20% 31% 6% 2	225
March	1st to 10th	Day	29% 17% 33% 18% 4	270
	11th to 20th	Day	31% 21% 34% 12% 2	270
	21st to end	Day	26% 24% 34% 15% 2	297
April	1st to 10th	Day	22% 18% 36% 16% 7%	365
	11th to 20th	Day	26% 14% 32% 19% 7% 2	452
	21st to end	Day	22% 14% 32% 22% 9% 2	460
May	1st to 10th	Day	18% 11% 33% 29% 9% 1	544
	11th to 20th	Day	14% 6% 31% 32% 15% 2	675
	21st to end	Day	12% 7% 26% 33% 17% 4	734
June	1st to 10th	Day	11% 4 16% 33% 28% 9%	680
	11th to 20th	Day	8% 3 17% 37% 29% 7%	680
	21st to end	Day	5% 3 18% 35% 27% 12%	680
July	1st to 10th	Day	6% 2 12% 32% 37% 10%	671
	11th to 20th	Day	4 1 10% 30% 41% 14%	668
	21st to end	Day	6% 1 10% 30% 38% 14%	739
August	1st to 10th	Day	4 1 9% 33% 42% 11%	676
	11th to 20th	Day	12% 1 8% 30% 36% 11%	574
	21st to end	Day	9% 1 7% 24% 41% 19%	539
September	1st to 10th	Day	9% 3 10% 26% 40% 13%	490
	11th to 20th	Day	8% 3 15% 27% 36% 11%	327
	21st to end	Day	12% 5% 16% 32% 28% 8%	300
October	1st to 10th	Day	5% 2 27% 38% 23% 4	300
	11th to 20th	Day	13% 10% 33% 27% 14% 3	300
	21st to end	Day	27% 11% 26% 25% 9% 2	330
November	1st to 10th	Day	21% 12% 29% 24% 12% 2	300
	11th to 20th	Day	17% 14% 36% 25% 8% 1	300
	21st to end	Day	26% 16% 27% 22% 7% 2	300
December	1st to 10th	Day	24% 16% 35% 18% 5% 2	300
	11th to 20th	Day	29% 17% 31% 17% 6%	300
	21st to end	Day	25% 21% 36% 12% 5% 1	330
January	1st to 10th	Night	23% 11% 32% 29% 5%	1000
	11th to 20th	Night	22% 13% 34% 26% 5% 1	1000
	21st to end	Night	23% 11% 33% 28% 5%	1100
February	1st to 10th	Night	26% 13% 30% 26% 5%	1000
	11th to 20th	Night	25% 16% 33% 22% 3	910
	21st to end	Night	26% 14% 32% 24% 4	471
March	1st to 10th	Night	25% 11% 31% 29% 5%	570
	11th to 20th	Night	20% 14% 31% 29% 6%	570
	21st to end	Night	20% 11% 36% 29% 5%	627
April	1st to 10th	Night	17% 12% 36% 28% 7%	570
	11th to 20th	Night	15% 13% 36% 26% 11% 1	563
	21st to end	Night	15% 10% 34% 29% 11%	570
May	1st to 10th	Night	15% 7% 29% 38% 11% 1	570
	11th to 20th	Night	14% 5% 28% 34% 17% 2	561
	21st to end	Night	11% 3 25% 34% 25% 2	617
June	1st to 10th	Night	5% 3 11% 42% 36% 3	570
	11th to 20th	Night	6% 2 14% 43% 33% 2	570
	21st to end	Night	4 2 11% 37% 40% 6%	570
July	1st to 10th	Night	6% 8% 31% 49% 6%	555
	11th to 20th	Night	4 1 5% 32% 48% 9%	552
	21st to end	Night	5% 1 7% 31% 48% 8%	624
August	1st to 10th	Night	4 1 7% 35% 46% 7%	567
	11th to 20th	Night	8% 1 9% 34% 40% 8%	564
	21st to end	Night	5% 1 7% 34% 44% 8%	627
September	1st to 10th	Night	7% 2 9% 35% 41% 7%	570
	11th to 20th	Night	11% 2 12% 36% 36% 4	570
	21st to end	Night	9% 2 14% 36% 35% 4	579
October	1st to 10th	Night	9% 2 14% 41% 30% 4	760
	11th to 20th	Night	14% 5% 25% 37% 18% 1	760
	21st to end	Night	20% 7% 24% 29% 17% 2	836
November	1st to 10th	Night	17% 5% 26% 37% 15% 1	780
	11th to 20th	Night	13% 9% 29% 37% 11%	800
	21st to end	Night	21% 11% 28% 28% 12% 1	930
December	1st to 10th	Night	22% 9% 30% 32% 7%	1000
	11th to 20th	Night	27% 9% 26% 32% 6%	1000
	21st to end	Night	23% 11% 30% 28% 7%	1100

Page generated at 2025-02-18 02:26:30. Version ac07

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