Astro-Physics
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro-Physics clear sky chart from 2005-11-06 to 2025-05-11.

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% 4 8% 5% 4 4 4 5% 7% 37%	113810
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	115338
Transparency	Day	44% 6% 12% 23% 13% 1	117351
	Night	45% 5% 11% 22% 14% 2	119940
Seeing	Day	17% 11% 29% 25% 14% 5%	16478
	Night	18% 9% 31% 30% 11% 1	26474

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	9% 12% 8% 16% 6% 6% 6% 4 5% 5% 24%	3617
	11th to 20th	Day	9% 11% 7% 12% 4 4 5% 7% 7% 6% 30%	3620
	21st to end	Day	13% 15% 8% 9% 4 3 4 3 4 6% 31%	4187
February	1st to 10th	Day	12% 15% 5% 9% 6% 5% 6% 3 5% 7% 27%	1800
	11th to 20th	Day	13% 11% 3 8% 6% 6% 4 3 4 6% 36%	1917
	21st to end	Day	13% 15% 4 10% 5% 4 4 3 5% 6% 32%	1650
March	1st to 10th	Day	15% 9% 3 7% 4 3 4 3 4 5% 44%	4480
	11th to 20th	Day	11% 7% 3 7% 4 4 4 3 6% 8% 42%	4750
	21st to end	Day	20% 12% 5% 11% 6% 5% 5% 3 5% 5% 23%	5938
April	1st to 10th	Day	18% 13% 3 8% 5% 5% 4 4 5% 5% 31%	2400
	11th to 20th	Day	16% 10% 4 7% 5% 4 4 4 6% 6% 34%	2374
	21st to end	Day	19% 9% 3 8% 5% 4 4 3 6% 6% 33%	2470
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 7% 35%	4268
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 3 5% 7% 43%	4400
	21st to end	Day	12% 7% 2 6% 5% 4 5% 5% 6% 9% 39%	4824
June	1st to 10th	Day	11% 5% 3 6% 5% 5% 4 4 5% 8% 44%	2660
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 45%	2660
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 5% 7% 10% 36%	2660
July	1st to 10th	Day	11% 9% 2 8% 5% 5% 6% 4 7% 9% 33%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 50%	4440
	21st to end	Day	9% 6% 3 7% 5% 6% 5% 4 6% 8% 41%	4813
August	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 42%	4363
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	3746
	21st to end	Day	9% 6% 2 5% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 5% 8% 47%	2280
	11th to 20th	Day	13% 3 1 4 3 3 4 4 5% 9% 51%	2199
	21st to end	Day	12% 6% 2 7% 4 4 4 4 5% 7% 44%	2090
October	1st to 10th	Day	13% 7% 2 7% 3 4 3 3 5% 6% 48%	1971
	11th to 20th	Day	19% 10% 3 5% 4 4 4 3 5% 6% 36%	1900
	21st to end	Day	20% 10% 3 7% 4 4 3 3 4 5% 37%	1906
November	1st to 10th	Day	13% 9% 3 6% 4 3 4 4 4 6% 44%	1762
	11th to 20th	Day	14% 13% 4 10% 6% 4 4 3 4 6% 32%	1800
	21st to end	Day	16% 14% 4 9% 5% 3 2 2 4 5% 35%	1645
December	1st to 10th	Day	8% 9% 4 9% 5% 4 4 4 5% 7% 41%	2610
	11th to 20th	Day	12% 15% 5% 12% 6% 5% 5% 4 5% 7% 23%	3008
	21st to end	Day	20% 22% 9% 12% 5% 4 4 2 4 5% 12%	3982
January	1st to 10th	Night	9% 12% 7% 15% 6% 4 3 3 3 4 32%	6348
	11th to 20th	Night	11% 14% 7% 13% 6% 6% 4 6% 6% 4 22%	6360
	21st to end	Night	14% 15% 6% 10% 5% 5% 4 3 4 5% 30%	6789
February	1st to 10th	Night	13% 15% 4 9% 6% 6% 3 3 4 5% 33%	2600
	11th to 20th	Night	13% 11% 4 7% 5% 5% 4 3 4 6% 38%	2368
	21st to end	Night	16% 13% 3 8% 3 4 3 3 5% 6% 36%	1980
March	1st to 10th	Night	16% 9% 3 7% 4 3 3 3 4 5% 42%	5373
	11th to 20th	Night	8% 7% 2 6% 5% 4 4 3 5% 8% 48%	5241
	21st to end	Night	21% 13% 5% 9% 5% 4 4 3 4 5% 28%	4990
April	1st to 10th	Night	18% 9% 4 6% 4 4 3 4 4 5% 38%	2000
	11th to 20th	Night	18% 9% 3 7% 4 4 3 3 4 6% 38%	1975
	21st to end	Night	17% 8% 2 7% 4 4 4 4 5% 6% 37%	1930
May	1st to 10th	Night	16% 7% 2 7% 6% 4 5% 3 5% 7% 37%	2847
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 5% 6% 7% 40%	2511
	21st to end	Night	10% 8% 2 6% 5% 4 4 4 6% 8% 43%	2743
June	1st to 10th	Night	8% 4 1 6% 5% 4 6% 5% 6% 9% 46%	1520
	11th to 20th	Night	10% 6% 1 5% 3 3 5% 4 7% 9% 47%	1520
	21st to end	Night	10% 6% 3 6% 5% 4 5% 4 5% 7% 45%	1520
July	1st to 10th	Night	15% 6% 2 7% 5% 4 4 4 5% 7% 40%	2530
	11th to 20th	Night	5% 4 2 4 3 4 4 3 6% 9% 54%	2530
	21st to end	Night	6% 6% 3 7% 5% 5% 5% 4 7% 8% 45%	2750
August	1st to 10th	Night	7% 4 2 6% 4 5% 4 3 6% 8% 52%	2573
	11th to 20th	Night	9% 6% 2 5% 4 3 4 4 6% 7% 50%	3071
	21st to end	Night	8% 4 2 5% 4 4 4 4 5% 9% 51%	3476
September	1st to 10th	Night	9% 5% 3 6% 3 3 3 3 5% 5% 56%	1900
	11th to 20th	Night	11% 4 2 4 4 4 4 4 5% 6% 53%	1961
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 7% 49%	2090
October	1st to 10th	Night	12% 6% 3 5% 3 3 4 3 4 7% 50%	2206
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 44%	2280
	21st to end	Night	20% 9% 3 6% 3 3 4 4 5% 6% 37%	2691
November	1st to 10th	Night	12% 8% 3 6% 3 4 3 3 5% 5% 48%	2536
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 4 5% 6% 36%	2600
	21st to end	Night	17% 11% 4 8% 5% 3 4 2 4 6% 36%	2754
December	1st to 10th	Night	9% 9% 3 7% 5% 5% 4 4 5% 7% 41%	4580
	11th to 20th	Night	16% 18% 4 9% 5% 4 4 3 4 5% 29%	5199
	21st to end	Night	20% 20% 7% 11% 5% 4 5% 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	61% 5% 11% 17% 6%	3618
	11th to 20th	Day	52% 5% 13% 11% 17% 3	3620
	21st to end	Day	54% 4 14% 17% 10%	4187
February	1st to 10th	Day	56% 6% 13% 15% 10% 1	1800
	11th to 20th	Day	50% 4 11% 24% 9% 2	1927
	21st to end	Day	53% 6% 9% 21% 10% 2	1650
March	1st to 10th	Day	43% 5% 12% 27% 10% 4	4480
	11th to 20th	Day	38% 5% 13% 31% 11% 1	4750
	21st to end	Day	61% 7% 11% 13% 7% 1	5940
April	1st to 10th	Day	55% 5% 10% 21% 9%	2400
	11th to 20th	Day	49% 7% 10% 22% 10% 2	2374
	21st to end	Day	51% 6% 12% 17% 12% 1	2470
May	1st to 10th	Day	46% 8% 11% 20% 14% 1	4306
	11th to 20th	Day	40% 7% 11% 28% 14%	4404
	21st to end	Day	38% 9% 14% 21% 16% 4	4837
June	1st to 10th	Day	36% 8% 14% 24% 16% 1	2660
	11th to 20th	Day	34% 8% 13% 24% 19% 2	2660
	21st to end	Day	40% 8% 15% 22% 13% 2	2660
July	1st to 10th	Day	44% 7% 14% 24% 9% 2	4371
	11th to 20th	Day	28% 7% 17% 29% 18% 1	4365
	21st to end	Day	38% 8% 15% 17% 19% 3	4662
August	1st to 10th	Day	36% 7% 16% 28% 12% 1	4348
	11th to 20th	Day	34% 7% 13% 32% 13% 1	3782
	21st to end	Day	34% 6% 13% 28% 17% 1	4179
September	1st to 10th	Day	35% 7% 10% 27% 19% 2	2280
	11th to 20th	Day	31% 6% 12% 31% 19% 2	2199
	21st to end	Day	38% 6% 9% 28% 17% 1	2090
October	1st to 10th	Day	24% 3 7% 38% 26% 1	3270
	11th to 20th	Day	34% 4 11% 27% 23% 1	3140
	21st to end	Day	43% 4 9% 28% 14% 3	3118
November	1st to 10th	Day	41% 5% 15% 24% 14% 1	1762
	11th to 20th	Day	54% 4 12% 19% 9% 1	1800
	21st to end	Day	52% 5% 13% 23% 7%	1645
December	1st to 10th	Day	43% 4 20% 25% 7% 1	2610
	11th to 20th	Day	60% 4 12% 16% 6% 1	3005
	21st to end	Day	76% 4 7% 9% 5%	3982
January	1st to 10th	Night	56% 4 9% 20% 10%	6348
	11th to 20th	Night	62% 5% 9% 9% 14% 1	6360
	21st to end	Night	58% 3 12% 16% 10% 1	6789
February	1st to 10th	Night	54% 4 11% 18% 12% 1	2600
	11th to 20th	Night	48% 5% 11% 22% 13% 1	2369
	21st to end	Night	49% 5% 12% 19% 11% 4	1980
March	1st to 10th	Night	45% 5% 12% 22% 11% 6%	5373
	11th to 20th	Night	34% 6% 11% 34% 13% 3	5241
	21st to end	Night	59% 5% 9% 17% 9% 1	4990
April	1st to 10th	Night	46% 6% 11% 21% 14% 2	2000
	11th to 20th	Night	47% 4 7% 21% 17% 3	1975
	21st to end	Night	45% 7% 12% 23% 11% 2	1930
May	1st to 10th	Night	43% 9% 12% 18% 17% 2	2874
	11th to 20th	Night	38% 10% 13% 25% 13% 1	2512
	21st to end	Night	35% 9% 14% 18% 21% 4	2751
June	1st to 10th	Night	31% 9% 16% 24% 19% 2	1520
	11th to 20th	Night	32% 7% 11% 23% 24% 3	1520
	21st to end	Night	37% 6% 14% 22% 19% 2	1520
July	1st to 10th	Night	43% 6% 16% 20% 12% 3	2490
	11th to 20th	Night	25% 6% 16% 33% 18% 1	2482
	21st to end	Night	33% 8% 15% 20% 21% 3	2662
August	1st to 10th	Night	30% 5% 16% 29% 18% 2	2564
	11th to 20th	Night	32% 6% 14% 30% 17%	3101
	21st to end	Night	30% 6% 13% 30% 18% 3	3476
September	1st to 10th	Night	30% 6% 11% 26% 24% 3	1900
	11th to 20th	Night	30% 6% 9% 32% 20% 3	1961
	21st to end	Night	34% 6% 10% 30% 18% 1	2090
October	1st to 10th	Night	25% 4 7% 36% 28% 1	3687
	11th to 20th	Night	34% 4 9% 26% 24% 3	3779
	21st to end	Night	42% 5% 9% 25% 16% 3	4429
November	1st to 10th	Night	38% 5% 10% 27% 18% 3	2536
	11th to 20th	Night	49% 5% 13% 20% 12% 1	2600
	21st to end	Night	51% 5% 12% 22% 10% 1	2754
December	1st to 10th	Night	43% 5% 15% 26% 9% 1	4580
	11th to 20th	Night	58% 5% 11% 16% 9% 1	5201
	21st to end	Night	70% 5% 9% 11% 5%	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	18% 16% 32% 26% 8%	300
	11th to 20th	Day	19% 16% 39% 22% 3 1	300
	21st to end	Day	22% 13% 36% 24% 4 1	330
February	1st to 10th	Day	17% 20% 40% 18% 5%	300
	11th to 20th	Day	20% 20% 35% 20% 5%	300
	21st to end	Day	23% 20% 39% 16% 2	249
March	1st to 10th	Day	19% 21% 40% 15% 5%	300
	11th to 20th	Day	24% 20% 34% 15% 6% 1	300
	21st to end	Day	28% 16% 30% 19% 6% 1	534
April	1st to 10th	Day	26% 14% 34% 22% 5%	500
	11th to 20th	Day	22% 22% 26% 20% 9% 3	492
	21st to end	Day	24% 13% 31% 21% 9% 1	500
May	1st to 10th	Day	18% 16% 31% 23% 9% 3	533
	11th to 20th	Day	22% 11% 30% 24% 12% 2	676
	21st to end	Day	16% 10% 28% 23% 16% 6%	734
June	1st to 10th	Day	10% 8% 32% 28% 18% 5%	680
	11th to 20th	Day	12% 6% 33% 29% 16% 5%	680
	21st to end	Day	14% 9% 27% 32% 15% 4	680
July	1st to 10th	Day	10% 2 19% 30% 25% 13%	671
	11th to 20th	Day	10% 5% 26% 30% 20% 9%	668
	21st to end	Day	8% 4 22% 37% 21% 8%	739
August	1st to 10th	Day	9% 3 21% 32% 25% 11%	676
	11th to 20th	Day	10% 5% 20% 27% 22% 17%	496
	21st to end	Day	11% 4 17% 32% 26% 10%	539
September	1st to 10th	Day	14% 8% 22% 26% 23% 7%	490
	11th to 20th	Day	11% 3 19% 32% 23% 11%	490
	21st to end	Day	15% 10% 26% 26% 16% 7%	490
October	1st to 10th	Day	15% 12% 27% 25% 18% 4	371
	11th to 20th	Day	22% 21% 29% 19% 8%	300
	21st to end	Day	28% 15% 29% 22% 5% 1	330
November	1st to 10th	Day	17% 16% 36% 20% 9% 2	300
	11th to 20th	Day	21% 17% 39% 15% 8%	300
	21st to end	Day	29% 9% 41% 17% 5%	300
December	1st to 10th	Day	16% 19% 41% 17% 5% 2	300
	11th to 20th	Day	19% 15% 38% 20% 8% 1	300
	21st to end	Day	27% 15% 38% 17% 2	330
January	1st to 10th	Night	19% 13% 37% 27% 5%	1000
	11th to 20th	Night	21% 13% 37% 23% 5% 1	1000
	21st to end	Night	20% 11% 36% 27% 6%	1100
February	1st to 10th	Night	22% 11% 39% 23% 5%	1000
	11th to 20th	Night	19% 14% 36% 26% 5%	813
	21st to end	Night	22% 13% 39% 21% 3	660
March	1st to 10th	Night	21% 11% 37% 25% 6%	800
	11th to 20th	Night	16% 13% 35% 29% 7%	718
	21st to end	Night	24% 13% 29% 27% 6% 1	660
April	1st to 10th	Night	23% 13% 35% 25% 4	600
	11th to 20th	Night	23% 12% 32% 26% 7%	593
	21st to end	Night	22% 9% 33% 30% 7%	600
May	1st to 10th	Night	18% 10% 34% 31% 8% 1	600
	11th to 20th	Night	18% 6% 32% 33% 10%	563
	21st to end	Night	14% 5% 29% 36% 15%	617
June	1st to 10th	Night	8% 5% 29% 44% 14%	570
	11th to 20th	Night	12% 4 32% 39% 12% 1	570
	21st to end	Night	11% 5% 29% 39% 16% 1	570
July	1st to 10th	Night	8% 2 17% 41% 29% 3	555
	11th to 20th	Night	8% 3 21% 42% 24% 2	552
	21st to end	Night	7% 2 18% 47% 24% 1	624
August	1st to 10th	Night	5% 3 15% 46% 28% 3	567
	11th to 20th	Night	8% 3 14% 40% 30% 5%	564
	21st to end	Night	10% 2 16% 42% 27% 3	627
September	1st to 10th	Night	9% 5% 21% 36% 25% 4	570
	11th to 20th	Night	11% 4 21% 34% 27% 3	631
	21st to end	Night	13% 8% 26% 32% 19% 2	760
October	1st to 10th	Night	13% 6% 31% 33% 15% 1	760
	11th to 20th	Night	21% 12% 30% 28% 9%	760
	21st to end	Night	23% 11% 33% 24% 8%	836
November	1st to 10th	Night	17% 10% 39% 24% 10% 1	780
	11th to 20th	Night	20% 15% 35% 24% 6%	800
	21st to end	Night	22% 13% 35% 24% 6%	954
December	1st to 10th	Night	22% 12% 39% 22% 6%	1000
	11th to 20th	Night	23% 11% 35% 26% 5% 1	1000
	21st to end	Night	29% 12% 34% 23% 3	1100

Page generated at 2025-05-08 08:33:58. Version ac07

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