ASTROTX Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASTROTX Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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	12% 5% 2 5% 3 3 3 3 5% 7% 51%	105420
	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	104079
Transparency	Day	32% 5% 9% 25% 22% 7%	109150
	Night	29% 4 9% 23% 24% 11%	108476
Seeing	Day	13% 8% 24% 28% 21% 6%	15387
	Night	13% 8% 29% 34% 15% 1	25373

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	13% 5% 2 5% 4 2 3 3 4 6% 53%	2937
	11th to 20th	Day	11% 5% 1 3 2 2 2 3 4 5% 61%	2940
	21st to end	Day	22% 10% 3 6% 2 2 2 2 2 4 46%	3144
February	1st to 10th	Day	18% 8% 3 7% 3 3 3 3 3 5% 45%	1807
	11th to 20th	Day	18% 8% 3 5% 4 3 2 2 4 5% 47%	1849
	21st to end	Day	22% 6% 1 4 2 2 2 2 3 4 51%	1609
March	1st to 10th	Day	16% 7% 2 4 3 3 4 2 5% 6% 47%	3436
	11th to 20th	Day	19% 7% 2 6% 5% 4 4 3 5% 6% 40%	3480
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 6% 42%	3814
April	1st to 10th	Day	13% 5% 2 6% 4 4 3 3 6% 7% 47%	2090
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 44%	2176
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 6% 47%	2392
May	1st to 10th	Day	21% 8% 3 7% 4 4 3 3 5% 6% 37%	4082
	11th to 20th	Day	15% 5% 2 6% 4 3 4 3 4 6% 47%	4082
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	4477
June	1st to 10th	Day	6% 4 1 5% 4 4 5% 4 7% 9% 51%	2470
	11th to 20th	Day	6% 3 2 5% 5% 5% 4 4 6% 9% 52%	2470
	21st to end	Day	5% 4 1 4 4 3 4 4 6% 9% 55%	2470
July	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 8% 53%	4120
	11th to 20th	Day	3 2 1 3 3 4 4 4 6% 10% 61%	4120
	21st to end	Day	6% 5% 2 6% 5% 5% 4 5% 7% 11% 42%	4465
August	1st to 10th	Day	2 1 1 2 2 2 3 3 6% 7% 71%	4104
	11th to 20th	Day	4 2 1 3 3 2 3 4 7% 10% 61%	4041
	21st to end	Day	4 2 1 3 3 4 5% 4 7% 11% 56%	4090
September	1st to 10th	Day	8% 5% 2 5% 3 2 4 4 5% 6% 55%	2090
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 5% 6% 49%	2090
	21st to end	Day	7% 4 1 4 3 3 3 3 4 7% 59%	2090
October	1st to 10th	Day	6% 3 1 4 3 4 3 3 5% 6% 63%	2028
	11th to 20th	Day	13% 4 1 3 2 2 3 2 4 5% 61%	1900
	21st to end	Day	14% 4 1 4 3 3 2 2 3 4 59%	2090
November	1st to 10th	Day	14% 5% 2 4 3 2 2 3 4 6% 55%	1958
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 7% 59%	1890
	21st to end	Day	16% 6% 2 4 3 3 2 2 5% 5% 53%	1800
December	1st to 10th	Day	18% 5% 2 4 3 3 3 3 4 5% 50%	2940
	11th to 20th	Day	19% 8% 2 5% 4 4 3 3 4 3 46%	3391
	21st to end	Day	20% 8% 2 5% 2 3 2 2 3 4 48%	4488
January	1st to 10th	Night	14% 5% 1 5% 3 3 3 2 3 5% 57%	4238
	11th to 20th	Night	12% 4 1 3 2 2 2 2 3 4 65%	4250
	21st to end	Night	20% 7% 3 5% 3 2 2 2 3 4 48%	4546
February	1st to 10th	Night	16% 5% 2 5% 3 3 4 3 3 5% 50%	2390
	11th to 20th	Night	17% 7% 2 5% 3 3 3 2 4 4 50%	2218
	21st to end	Night	19% 5% 2 4 3 2 2 2 4 5% 51%	1843
March	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 5% 6% 42%	3419
	11th to 20th	Night	17% 6% 2 5% 3 4 4 4 5% 7% 46%	3480
	21st to end	Night	14% 5% 2 6% 3 3 4 3 5% 6% 48%	3817
April	1st to 10th	Night	11% 4 1 4 3 3 3 3 6% 8% 53%	2090
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 5% 50%	1951
	21st to end	Night	12% 6% 1 4 3 3 3 3 5% 7% 54%	1783
May	1st to 10th	Night	21% 8% 2 6% 4 3 3 3 6% 6% 37%	2823
	11th to 20th	Night	12% 6% 2 6% 4 4 3 3 4 6% 49%	2823
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 8% 46%	3090
June	1st to 10th	Night	4 3 1 4 3 3 3 4 7% 9% 58%	1710
	11th to 20th	Night	4 2 1 3 3 3 3 3 5% 10% 63%	1710
	21st to end	Night	3 3 1 4 4 3 3 4 7% 10% 58%	1710
July	1st to 10th	Night	4 3 1 4 3 4 4 3 6% 9% 59%	2850
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 9% 68%	2850
	21st to end	Night	4 4 2 5% 3 4 4 4 7% 10% 53%	3098
August	1st to 10th	Night	1 1 1 2 1 1 2 2 5% 8% 76%	2836
	11th to 20th	Night	3 2 1 3 1 2 2 3 6% 9% 68%	2798
	21st to end	Night	3 2 1 3 2 3 4 4 7% 9% 63%	3564
September	1st to 10th	Night	7% 5% 2 5% 3 3 4 3 5% 8% 56%	2090
	11th to 20th	Night	11% 3 2 5% 3 3 4 3 5% 7% 54%	2090
	21st to end	Night	6% 3 1 3 2 2 3 3 4 8% 64%	2090
October	1st to 10th	Night	6% 3 1 3 2 3 4 3 5% 6% 65%	2133
	11th to 20th	Night	12% 4 1 3 2 2 2 2 3 5% 64%	2280
	21st to end	Night	15% 5% 1 3 2 2 2 1 3 4 61%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 2 3 2 3 4 60%	2340
	11th to 20th	Night	7% 5% 2 4 3 3 3 2 5% 6% 59%	2505
	21st to end	Night	15% 5% 2 4 3 2 3 3 4 5% 54%	2600
December	1st to 10th	Night	16% 7% 2 5% 3 2 3 2 3 5% 53%	4250
	11th to 20th	Night	18% 7% 2 6% 3 3 3 2 3 4 50%	4816
	21st to end	Night	16% 6% 2 6% 3 3 2 2 3 5% 52%	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	31% 4 7% 23% 30% 5%	2938
	11th to 20th	Day	28% 2 9% 22% 32% 8%	2940
	21st to end	Day	45% 3 3 19% 22% 8%	3144
February	1st to 10th	Day	44% 3 5% 18% 20% 9%	1807
	11th to 20th	Day	42% 3 7% 19% 21% 8%	1859
	21st to end	Day	39% 3 5% 15% 24% 13%	1609
March	1st to 10th	Day	36% 5% 7% 17% 27% 9%	3436
	11th to 20th	Day	44% 6% 9% 21% 16% 4	3480
	21st to end	Day	40% 6% 11% 19% 18% 5%	3814
April	1st to 10th	Day	37% 4 10% 20% 18% 10%	2090
	11th to 20th	Day	39% 5% 9% 18% 21% 8%	2176
	21st to end	Day	36% 6% 9% 21% 24% 5%	2392
May	1st to 10th	Day	48% 5% 8% 20% 14% 4	4120
	11th to 20th	Day	38% 5% 9% 25% 22% 1	4086
	21st to end	Day	44% 8% 12% 20% 11% 5%	4489
June	1st to 10th	Day	26% 9% 16% 29% 17% 3	2470
	11th to 20th	Day	28% 6% 13% 30% 19% 5%	2470
	21st to end	Day	25% 6% 12% 30% 23% 3	2470
July	1st to 10th	Day	29% 6% 16% 38% 12%	4056
	11th to 20th	Day	18% 6% 11% 42% 21% 2	4051
	21st to end	Day	33% 8% 14% 32% 13%	4325
August	1st to 10th	Day	12% 5% 12% 31% 36% 5%	4090
	11th to 20th	Day	18% 5% 15% 42% 19% 2	4080
	21st to end	Day	22% 8% 18% 36% 14% 2	4089
September	1st to 10th	Day	28% 6% 10% 24% 21% 11%	2090
	11th to 20th	Day	35% 6% 10% 19% 23% 7%	2090
	21st to end	Day	24% 5% 8% 23% 28% 12%	2090
October	1st to 10th	Day	14% 3 6% 15% 33% 30%	3366
	11th to 20th	Day	17% 2 4 22% 32% 22%	3140
	21st to end	Day	24% 4 5% 28% 29% 10%	3428
November	1st to 10th	Day	32% 3 8% 22% 25% 10%	1958
	11th to 20th	Day	27% 4 9% 19% 29% 12%	1890
	21st to end	Day	35% 3 6% 21% 25% 10%	1800
December	1st to 10th	Day	37% 2 5% 27% 24% 5%	2940
	11th to 20th	Day	44% 3 4 12% 26% 10%	3389
	21st to end	Day	42% 3 4 24% 21% 8%	4488
January	1st to 10th	Night	30% 4 6% 21% 30% 9%	4238
	11th to 20th	Night	25% 3 8% 20% 30% 14%	4250
	21st to end	Night	42% 3 5% 16% 25% 9%	4546
February	1st to 10th	Night	38% 4 7% 18% 21% 13%	2390
	11th to 20th	Night	38% 3 7% 16% 25% 10%	2218
	21st to end	Night	37% 3 7% 15% 22% 16%	1843
March	1st to 10th	Night	42% 5% 7% 14% 21% 11%	3419
	11th to 20th	Night	38% 5% 12% 19% 20% 6%	3480
	21st to end	Night	37% 5% 9% 19% 26% 5%	3817
April	1st to 10th	Night	29% 5% 10% 23% 19% 14%	2090
	11th to 20th	Night	35% 5% 9% 20% 18% 14%	1951
	21st to end	Night	30% 5% 9% 22% 28% 5%	1783
May	1st to 10th	Night	45% 8% 9% 19% 13% 6%	2850
	11th to 20th	Night	35% 6% 12% 21% 24% 2	2824
	21st to end	Night	34% 8% 15% 25% 13% 6%	3099
June	1st to 10th	Night	21% 7% 16% 31% 20% 5%	1710
	11th to 20th	Night	18% 5% 13% 35% 21% 8%	1710
	21st to end	Night	20% 7% 13% 29% 26% 5%	1710
July	1st to 10th	Night	22% 5% 16% 42% 14% 2	2805
	11th to 20th	Night	14% 4 12% 43% 24% 4	2796
	21st to end	Night	25% 7% 15% 37% 14% 1	2999
August	1st to 10th	Night	8% 3 10% 32% 41% 6%	2826
	11th to 20th	Night	14% 4 14% 41% 22% 5%	2825
	21st to end	Night	17% 6% 21% 34% 20% 2	3564
September	1st to 10th	Night	27% 5% 11% 22% 23% 12%	2090
	11th to 20th	Night	30% 5% 12% 21% 23% 10%	2090
	21st to end	Night	21% 3 9% 22% 29% 16%	2090
October	1st to 10th	Night	12% 2 6% 15% 28% 37%	3562
	11th to 20th	Night	16% 2 4 20% 31% 27%	3779
	21st to end	Night	25% 3 3 23% 29% 16%	4120
November	1st to 10th	Night	29% 4 6% 20% 26% 15%	2340
	11th to 20th	Night	26% 3 7% 19% 28% 16%	2505
	21st to end	Night	34% 3 7% 17% 27% 11%	2600
December	1st to 10th	Night	36% 3 5% 20% 27% 9%	4250
	11th to 20th	Night	40% 2 5% 14% 24% 14%	4817
	21st to end	Night	36% 3 5% 20% 26% 10%	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	20% 14% 39% 22% 5%	300
	11th to 20th	Day	18% 20% 38% 18% 4 1	300
	21st to end	Day	21% 16% 39% 18% 6%	300
February	1st to 10th	Day	25% 19% 34% 20% 2	270
	11th to 20th	Day	25% 23% 37% 11% 3 1	270
	21st to end	Day	32% 22% 36% 7% 3	225
March	1st to 10th	Day	24% 17% 39% 17% 2	270
	11th to 20th	Day	27% 17% 39% 13% 3	270
	21st to end	Day	20% 20% 42% 15% 4	297
April	1st to 10th	Day	19% 19% 39% 19% 4	270
	11th to 20th	Day	22% 16% 35% 21% 6%	374
	21st to end	Day	18% 17% 36% 23% 5% 1	572
May	1st to 10th	Day	15% 13% 37% 28% 6% 1	653
	11th to 20th	Day	16% 5% 35% 30% 12% 2	675
	21st to end	Day	15% 6% 31% 33% 13% 3	734
June	1st to 10th	Day	6% 3 19% 39% 26% 7%	680
	11th to 20th	Day	7% 2 19% 40% 26% 6%	680
	21st to end	Day	5% 3 15% 37% 31% 10%	680
July	1st to 10th	Day	6% 3 11% 32% 37% 11%	671
	11th to 20th	Day	2 1 7% 36% 43% 11%	668
	21st to end	Day	5% 1 5% 33% 43% 12%	739
August	1st to 10th	Day	3 1 8% 33% 44% 12%	676
	11th to 20th	Day	8% 1 5% 37% 40% 9%	665
	21st to end	Day	5% 1 5% 29% 43% 16%	488
September	1st to 10th	Day	8% 1 8% 26% 39% 18%	300
	11th to 20th	Day	3 2 9% 30% 42% 15%	300
	21st to end	Day	10% 2 12% 32% 30% 14%	300
October	1st to 10th	Day	4 2 22% 31% 33% 8%	300
	11th to 20th	Day	12% 7% 28% 33% 17% 3	300
	21st to end	Day	24% 10% 22% 26% 17% 1	330
November	1st to 10th	Day	20% 10% 34% 25% 8% 3	300
	11th to 20th	Day	13% 12% 31% 30% 12% 2	300
	21st to end	Day	18% 16% 36% 22% 7% 1	300
December	1st to 10th	Day	22% 12% 33% 23% 8% 2	300
	11th to 20th	Day	23% 13% 38% 20% 5%	300
	21st to end	Day	21% 15% 40% 19% 5%	330
January	1st to 10th	Night	16% 11% 35% 31% 7%	1000
	11th to 20th	Night	19% 14% 32% 30% 5%	1000
	21st to end	Night	17% 12% 34% 31% 5%	1051
February	1st to 10th	Night	20% 13% 36% 27% 4	870
	11th to 20th	Night	20% 14% 36% 26% 3	744
	21st to end	Night	24% 17% 38% 19% 3	587
March	1st to 10th	Night	19% 13% 36% 26% 6%	570
	11th to 20th	Night	19% 11% 38% 27% 5%	570
	21st to end	Night	14% 14% 42% 26% 4	627
April	1st to 10th	Night	12% 14% 42% 27% 4	570
	11th to 20th	Night	18% 14% 38% 24% 7%	563
	21st to end	Night	15% 11% 39% 26% 8% 1	570
May	1st to 10th	Night	11% 10% 41% 31% 6% 1	570
	11th to 20th	Night	11% 5% 38% 32% 13% 1	561
	21st to end	Night	10% 7% 32% 36% 13% 2	617
June	1st to 10th	Night	6% 2 19% 46% 24% 3	570
	11th to 20th	Night	5% 2 27% 50% 16% 1	570
	21st to end	Night	3 2 20% 45% 27% 4	570
July	1st to 10th	Night	4 1 16% 42% 33% 5%	555
	11th to 20th	Night	3 1 11% 48% 34% 2	552
	21st to end	Night	2 1 7% 51% 35% 4	624
August	1st to 10th	Night	2 1 6% 51% 37% 4	567
	11th to 20th	Night	6% 1 8% 45% 34% 6%	564
	21st to end	Night	4 1 6% 43% 40% 6%	627
September	1st to 10th	Night	8% 2 11% 40% 35% 5%	570
	11th to 20th	Night	13% 1 8% 42% 34% 2	570
	21st to end	Night	8% 1 16% 40% 30% 5%	570
October	1st to 10th	Night	7% 2 23% 38% 27% 3	613
	11th to 20th	Night	14% 5% 26% 38% 16% 1	760
	21st to end	Night	19% 8% 27% 32% 14% 1	836
November	1st to 10th	Night	12% 5% 32% 34% 15% 1	780
	11th to 20th	Night	10% 10% 34% 34% 12% 1	905
	21st to end	Night	18% 11% 34% 28% 9% 1	1000
December	1st to 10th	Night	17% 9% 37% 30% 7%	1000
	11th to 20th	Night	21% 9% 33% 30% 6%	1000
	21st to end	Night	19% 10% 35% 29% 6%	1100

Page generated at 2025-01-19 02:06:44. Version ac07

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