All-Star Telescope Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the All-Star Telescope Observing Site clear sky chart from 2005-11-06 to 2024-11-02.

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	9% 9% 3 8% 5% 5% 5% 4 6% 8% 36%	101927
	Night	7% 10% 4 9% 6% 5% 5% 5% 6% 8% 35%	97141
Transparency	Day	43% 6% 16% 23% 10% 2	101672
	Night	44% 6% 15% 23% 10% 2	97042
Seeing	Day	13% 8% 27% 31% 16% 5%	15372
	Night	11% 6% 30% 35% 14% 2	24368

By Month

Percentage Cloud Cover NW (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	3 11% 7% 11% 7% 4 4 4 7% 8% 35%	2219
	11th to 20th	Day	3 10% 4 11% 7% 4 6% 5% 8% 8% 34%	2206
	21st to end	Day	4 8% 4 8% 5% 4 6% 4 7% 10% 40%	2794
February	1st to 10th	Day	6% 12% 5% 11% 8% 5% 5% 4 7% 9% 28%	1520
	11th to 20th	Day	6% 12% 4 11% 7% 7% 6% 5% 7% 7% 27%	1635
	21st to end	Day	3 12% 6% 11% 8% 6% 6% 5% 7% 7% 30%	1522
March	1st to 10th	Day	3 11% 6% 12% 8% 6% 6% 4 5% 7% 30%	3138
	11th to 20th	Day	6% 10% 4 7% 4 4 5% 4 6% 10% 42%	3444
	21st to end	Day	8% 23% 6% 13% 5% 4 5% 3 5% 5% 25%	3903
April	1st to 10th	Day	7% 16% 4 10% 7% 5% 6% 5% 7% 7% 26%	2334
	11th to 20th	Day	12% 14% 5% 9% 5% 4 5% 4 6% 7% 27%	2440
	21st to end	Day	12% 13% 4 9% 5% 4 5% 4 6% 8% 30%	2470
May	1st to 10th	Day	18% 14% 4 7% 5% 5% 4 4 5% 6% 28%	4406
	11th to 20th	Day	11% 10% 4 9% 6% 6% 6% 5% 7% 8% 29%	4694
	21st to end	Day	17% 10% 4 10% 7% 6% 6% 4 5% 6% 23%	5161
June	1st to 10th	Day	16% 8% 3 8% 6% 6% 5% 5% 8% 8% 28%	2850
	11th to 20th	Day	20% 9% 3 8% 7% 6% 6% 5% 7% 7% 23%	2850
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 6% 7% 10% 36%	2850
July	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 7% 8% 47%	4740
	11th to 20th	Day	9% 3 1 4 3 5% 5% 5% 8% 11% 46%	4740
	21st to end	Day	6% 4 2 5% 4 4 4 5% 7% 9% 51%	5055
August	1st to 10th	Day	10% 5% 2 7% 4 5% 6% 4 8% 8% 41%	4401
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	4099
	21st to end	Day	11% 4 2 5% 4 4 4 4 6% 8% 49%	4521
September	1st to 10th	Day	12% 6% 2 5% 4 3 4 4 5% 7% 47%	2367
	11th to 20th	Day	15% 8% 2 5% 4 3 4 3 5% 7% 44%	2090
	21st to end	Day	9% 8% 2 7% 4 4 5% 5% 6% 7% 43%	2090
October	1st to 10th	Day	12% 9% 3 7% 4 4 6% 4 5% 6% 39%	2009
	11th to 20th	Day	9% 9% 3 8% 4 5% 5% 4 7% 7% 37%	1743
	21st to end	Day	6% 10% 4 10% 7% 5% 5% 4 6% 8% 34%	1881
November	1st to 10th	Day	9% 16% 5% 9% 5% 3 4 4 6% 7% 31%	1550
	11th to 20th	Day	6% 13% 4 9% 5% 5% 4 4 6% 8% 35%	1421
	21st to end	Day	5% 14% 4 8% 6% 5% 5% 4 7% 9% 34%	1330
December	1st to 10th	Day	6% 10% 4 12% 7% 7% 5% 5% 6% 5% 32%	1330
	11th to 20th	Day	4 10% 5% 12% 8% 6% 5% 5% 7% 9% 31%	1682
	21st to end	Day	3 9% 4 9% 6% 4 6% 4 6% 9% 41%	2442
January	1st to 10th	Night	3 12% 6% 13% 7% 6% 6% 5% 7% 8% 26%	4726
	11th to 20th	Night	3 9% 4 13% 6% 5% 7% 6% 6% 7% 33%	4699
	21st to end	Night	3 11% 4 9% 6% 6% 6% 5% 7% 8% 35%	4885
February	1st to 10th	Night	6% 11% 5% 14% 7% 7% 5% 4 6% 6% 30%	2660
	11th to 20th	Night	4 13% 5% 12% 7% 6% 5% 5% 7% 7% 30%	2431
	21st to end	Night	2 12% 6% 10% 6% 6% 5% 5% 6% 8% 34%	1930
March	1st to 10th	Night	2 9% 5% 15% 7% 7% 8% 6% 7% 8% 26%	3736
	11th to 20th	Night	6% 9% 3 6% 5% 5% 5% 5% 7% 9% 41%	3535
	21st to end	Night	7% 20% 6% 15% 7% 5% 5% 4 5% 6% 20%	3749
April	1st to 10th	Night	8% 15% 4 8% 7% 6% 5% 5% 7% 8% 30%	1845
	11th to 20th	Night	10% 13% 4 10% 7% 5% 5% 4 6% 8% 27%	1691
	21st to end	Night	11% 10% 4 9% 6% 6% 5% 4 8% 7% 30%	1710
May	1st to 10th	Night	22% 12% 3 8% 5% 5% 3 3 5% 5% 30%	2554
	11th to 20th	Night	10% 8% 4 7% 5% 5% 6% 4 8% 10% 33%	2199
	21st to end	Night	15% 11% 4 10% 7% 7% 6% 5% 6% 6% 22%	2414
June	1st to 10th	Night	18% 7% 3 8% 5% 4 5% 4 7% 7% 32%	1330
	11th to 20th	Night	19% 10% 3 7% 6% 6% 6% 4 7% 7% 26%	1330
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 8% 7% 41%	1330
July	1st to 10th	Night	8% 5% 2 6% 5% 6% 5% 6% 7% 8% 42%	2220
	11th to 20th	Night	6% 3 1 4 5% 5% 5% 5% 8% 10% 48%	2220
	21st to end	Night	5% 5% 2 4 3 3 5% 4 6% 10% 52%	2494
August	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	2529
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 4 7% 11% 41%	2737
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 7% 49%	3146
September	1st to 10th	Night	10% 7% 2 5% 3 4 4 4 5% 9% 47%	1810
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 8% 43%	2090
	21st to end	Night	10% 7% 2 6% 4 4 4 4 7% 8% 45%	2090
October	1st to 10th	Night	11% 8% 3 6% 5% 5% 4 4 5% 7% 41%	2171
	11th to 20th	Night	8% 9% 3 8% 4 5% 5% 5% 7% 7% 39%	2437
	21st to end	Night	7% 10% 4 9% 5% 5% 5% 5% 6% 8% 35%	2717
November	1st to 10th	Night	8% 14% 4 9% 5% 5% 5% 4 5% 7% 34%	2543
	11th to 20th	Night	4 12% 4 9% 6% 5% 5% 4 7% 9% 34%	2759
	21st to end	Night	4 11% 4 9% 6% 6% 6% 4 7% 9% 35%	2850
December	1st to 10th	Night	5% 11% 4 11% 7% 5% 5% 4 6% 7% 34%	2850
	11th to 20th	Night	2 11% 5% 11% 8% 6% 6% 5% 6% 9% 32%	3510
	21st to end	Night	3 9% 3 8% 5% 5% 5% 4 6% 8% 44%	5214

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	45% 5% 12% 24% 11% 2	2219
	11th to 20th	Day	45% 6% 10% 16% 19% 4	2227
	21st to end	Day	40% 6% 14% 32% 8% 1	2794
February	1st to 10th	Day	51% 6% 14% 17% 9% 2	1520
	11th to 20th	Day	52% 6% 14% 17% 9% 2	1644
	21st to end	Day	50% 7% 13% 19% 10% 1	1522
March	1st to 10th	Day	50% 7% 14% 17% 11% 1	3138
	11th to 20th	Day	36% 6% 17% 25% 14% 1	3445
	21st to end	Day	60% 6% 12% 16% 6%	3903
April	1st to 10th	Day	52% 7% 16% 18% 7%	2334
	11th to 20th	Day	53% 6% 11% 21% 9% 1	2440
	21st to end	Day	49% 6% 15% 21% 7% 1	2470
May	1st to 10th	Day	55% 5% 13% 22% 5%	4406
	11th to 20th	Day	46% 8% 14% 22% 9%	4698
	21st to end	Day	57% 6% 14% 16% 6% 1	5162
June	1st to 10th	Day	49% 7% 16% 19% 7% 1	2850
	11th to 20th	Day	56% 7% 14% 16% 5% 1	2850
	21st to end	Day	38% 8% 21% 24% 7% 1	2850
July	1st to 10th	Day	31% 7% 16% 28% 14% 3	4666
	11th to 20th	Day	29% 7% 20% 35% 9% 1	4659
	21st to end	Day	26% 6% 16% 28% 19% 4	4895
August	1st to 10th	Day	38% 7% 18% 31% 6% 1	4384
	11th to 20th	Day	34% 7% 25% 27% 6% 1	4139
	21st to end	Day	32% 5% 21% 32% 9% 1	4520
September	1st to 10th	Day	35% 5% 20% 24% 12% 4	2367
	11th to 20th	Day	40% 5% 15% 28% 11% 1	2090
	21st to end	Day	37% 6% 16% 24% 13% 3	2090
October	1st to 10th	Day	43% 7% 14% 26% 9% 2	2009
	11th to 20th	Day	43% 6% 17% 24% 9% 2	1743
	21st to end	Day	46% 5% 13% 24% 11% 1	1881
November	1st to 10th	Day	52% 6% 12% 19% 9% 2	1550
	11th to 20th	Day	46% 5% 16% 22% 10% 1	1421
	21st to end	Day	44% 6% 17% 21% 9% 2	1330
December	1st to 10th	Day	51% 5% 12% 17% 13% 2	1330
	11th to 20th	Day	48% 6% 19% 19% 6% 2	1684
	21st to end	Day	38% 5% 12% 22% 18% 6%	2442
January	1st to 10th	Night	52% 6% 12% 19% 8% 2	4726
	11th to 20th	Night	47% 6% 11% 16% 14% 5%	4740
	21st to end	Night	44% 5% 15% 24% 11% 1	4885
February	1st to 10th	Night	53% 4 12% 18% 10% 2	2660
	11th to 20th	Night	50% 6% 12% 19% 11% 2	2432
	21st to end	Night	46% 5% 10% 22% 14% 2	1930
March	1st to 10th	Night	50% 9% 11% 21% 8% 1	3736
	11th to 20th	Night	36% 10% 18% 22% 13% 1	3535
	21st to end	Night	61% 9% 10% 15% 5% 1	3749
April	1st to 10th	Night	50% 6% 13% 20% 11% 1	1845
	11th to 20th	Night	54% 5% 12% 20% 8% 1	1691
	21st to end	Night	50% 6% 14% 21% 7% 2	1710
May	1st to 10th	Night	55% 6% 11% 22% 6%	2554
	11th to 20th	Night	41% 7% 15% 25% 11%	2199
	21st to end	Night	56% 9% 13% 17% 4 1	2414
June	1st to 10th	Night	47% 7% 16% 20% 9% 1	1330
	11th to 20th	Night	55% 6% 15% 15% 6% 2	1330
	21st to end	Night	39% 7% 21% 25% 7% 2	1330
July	1st to 10th	Night	36% 8% 14% 23% 16% 2	2185
	11th to 20th	Night	27% 7% 22% 33% 7% 4	2178
	21st to end	Night	27% 6% 14% 33% 18% 2	2415
August	1st to 10th	Night	36% 6% 17% 32% 9%	2521
	11th to 20th	Night	34% 8% 26% 26% 6%	2763
	21st to end	Night	33% 6% 21% 29% 9% 1	3146
September	1st to 10th	Night	34% 4 17% 26% 14% 5%	1810
	11th to 20th	Night	38% 6% 16% 26% 11% 4	2090
	21st to end	Night	35% 5% 17% 26% 13% 3	2090
October	1st to 10th	Night	42% 5% 15% 25% 12% 2	2171
	11th to 20th	Night	40% 6% 17% 24% 11% 2	2437
	21st to end	Night	44% 6% 15% 25% 10% 1	2717
November	1st to 10th	Night	47% 7% 13% 20% 11% 2	2543
	11th to 20th	Night	44% 6% 17% 20% 12% 1	2759
	21st to end	Night	43% 7% 17% 20% 9% 3	2850
December	1st to 10th	Night	48% 6% 12% 21% 10% 3	2850
	11th to 20th	Night	47% 7% 17% 22% 6% 1	3507
	21st to end	Night	37% 4 13% 25% 14% 6%	5214

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	5% 7% 30% 34% 16% 7%	180
	11th to 20th	Day	9% 8% 34% 28% 18% 3	180
	21st to end	Day	9% 8% 30% 30% 20% 4	198
February	1st to 10th	Day	12% 9% 34% 27% 11% 7%	180
	11th to 20th	Day	11% 6% 34% 33% 13% 3	256
	21st to end	Day	6% 9% 28% 34% 19% 4	334
March	1st to 10th	Day	8% 9% 35% 31% 13% 4	460
	11th to 20th	Day	8% 9% 32% 35% 11% 5%	460
	21st to end	Day	15% 9% 33% 25% 11% 6%	506
April	1st to 10th	Day	12% 13% 34% 28% 12% 2	460
	11th to 20th	Day	15% 12% 28% 28% 15% 2	452
	21st to end	Day	17% 13% 26% 30% 12% 3	460
May	1st to 10th	Day	16% 13% 25% 28% 15% 3	520
	11th to 20th	Day	15% 7% 24% 32% 18% 4	675
	21st to end	Day	16% 7% 21% 29% 21% 6%	734
June	1st to 10th	Day	16% 11% 27% 31% 11% 4	680
	11th to 20th	Day	21% 10% 24% 30% 13% 3	680
	21st to end	Day	10% 6% 24% 34% 20% 6%	680
July	1st to 10th	Day	12% 5% 20% 36% 21% 6%	671
	11th to 20th	Day	15% 4 21% 32% 23% 4	668
	21st to end	Day	7% 4 24% 36% 21% 8%	692
August	1st to 10th	Day	10% 5% 27% 30% 21% 7%	487
	11th to 20th	Day	9% 5% 22% 39% 20% 5%	477
	21st to end	Day	11% 7% 24% 31% 20% 7%	539
September	1st to 10th	Day	14% 8% 29% 27% 16% 6%	490
	11th to 20th	Day	19% 6% 24% 29% 16% 5%	490
	21st to end	Day	15% 12% 32% 27% 11% 3	490
October	1st to 10th	Day	15% 9% 27% 30% 13% 6%	409
	11th to 20th	Day	12% 12% 30% 28% 14% 3	300
	21st to end	Day	15% 12% 34% 28% 9% 3	330
November	1st to 10th	Day	18% 7% 30% 31% 10% 4	273
	11th to 20th	Day	15% 13% 32% 28% 10% 2	223
	21st to end	Day	9% 12% 31% 23% 17% 7%	180
December	1st to 10th	Day	9% 7% 31% 32% 16% 4	180
	11th to 20th	Day	8% 10% 39% 28% 11% 3	180
	21st to end	Day	7% 5% 30% 31% 22% 4	198
January	1st to 10th	Night	8% 8% 37% 34% 12% 1	950
	11th to 20th	Night	9% 7% 38% 35% 11% 1	950
	21st to end	Night	12% 9% 34% 34% 10% 2	846
February	1st to 10th	Night	10% 10% 29% 33% 16% 3	760
	11th to 20th	Night	8% 6% 35% 36% 14% 1	744
	21st to end	Night	6% 7% 29% 33% 20% 5%	628
March	1st to 10th	Night	8% 5% 33% 37% 14% 3	760
	11th to 20th	Night	10% 6% 32% 37% 13% 2	760
	21st to end	Night	13% 5% 29% 36% 14% 3	836
April	1st to 10th	Night	12% 8% 30% 33% 15% 2	705
	11th to 20th	Night	16% 7% 31% 35% 11%	563
	21st to end	Night	16% 9% 27% 37% 10% 1	570
May	1st to 10th	Night	18% 8% 29% 31% 14%	451
	11th to 20th	Night	12% 5% 28% 41% 15%	374
	21st to end	Night	20% 4 23% 36% 16% 1	411
June	1st to 10th	Night	18% 8% 23% 36% 14% 1	380
	11th to 20th	Night	19% 4 29% 33% 14% 1	380
	21st to end	Night	10% 4 22% 41% 22% 2	380
July	1st to 10th	Night	11% 4 18% 40% 26% 1	370
	11th to 20th	Night	12% 2 22% 43% 20% 1	368
	21st to end	Night	8% 3 18% 41% 28% 2	416
August	1st to 10th	Night	10% 3 14% 41% 29% 2	378
	11th to 20th	Night	10% 4 17% 43% 25% 1	514
	21st to end	Night	10% 3 22% 42% 22% 1	627
September	1st to 10th	Night	13% 3 24% 39% 18% 3	665
	11th to 20th	Night	17% 4 21% 36% 20% 2	760
	21st to end	Night	14% 5% 31% 35% 13% 2	760
October	1st to 10th	Night	16% 5% 28% 34% 14% 3	760
	11th to 20th	Night	11% 5% 33% 36% 12% 2	760
	21st to end	Night	11% 9% 34% 34% 12% 1	836
November	1st to 10th	Night	15% 7% 35% 30% 11% 2	861
	11th to 20th	Night	9% 10% 41% 29% 9% 2	950
	21st to end	Night	7% 7% 39% 35% 10% 2	950
December	1st to 10th	Night	11% 8% 32% 33% 13% 3	950
	11th to 20th	Night	7% 7% 39% 36% 11% 1	950
	21st to end	Night	11% 5% 34% 36% 12% 2	1045

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