Stargazer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stargazer Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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	20% 9% 3 6% 6% 3 5% 5% 5% 7% 32%	74809
	Night	23% 10% 4 7% 6% 3 4 4 5% 6% 29%	74908
Transparency	Day	53% 9% 14% 16% 7% 1	74500
	Night	57% 8% 13% 15% 6% 1	74735
Seeing	Day	24% 9% 20% 25% 17% 5%	14047
	Night	30% 8% 14% 24% 20% 4	22529

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	33% 14% 5% 7% 5% 3 4 4 3 5% 16%	1329
	11th to 20th	Day	28% 11% 4 8% 5% 4 5% 5% 5% 8% 17%	1375
	21st to end	Day	29% 12% 4 8% 8% 5% 6% 5% 5% 6% 13%	1616
February	1st to 10th	Day	31% 13% 5% 8% 6% 3 4 4 4 6% 17%	1620
	11th to 20th	Day	27% 13% 3 8% 6% 3 5% 5% 4 5% 21%	1575
	21st to end	Day	25% 15% 6% 6% 6% 3 5% 5% 5% 4 20%	1377
March	1st to 10th	Day	24% 11% 4 8% 6% 4 6% 5% 4 7% 20%	1842
	11th to 20th	Day	25% 13% 4 6% 5% 3 5% 4 4 5% 24%	1980
	21st to end	Day	23% 12% 4 9% 8% 4 6% 4 5% 7% 18%	2247
April	1st to 10th	Day	23% 12% 4 6% 7% 4 6% 6% 6% 7% 20%	2160
	11th to 20th	Day	22% 9% 4 8% 7% 4 6% 5% 5% 6% 24%	2173
	21st to end	Day	22% 9% 4 7% 6% 4 5% 4 6% 6% 26%	2389
May	1st to 10th	Day	15% 8% 3 7% 6% 3 6% 5% 6% 7% 34%	2520
	11th to 20th	Day	18% 7% 2 6% 6% 3 5% 5% 5% 9% 32%	2477
	21st to end	Day	22% 8% 4 7% 6% 3 4 4 4 6% 32%	2723
June	1st to 10th	Day	21% 7% 3 5% 6% 3 5% 5% 4 7% 33%	2632
	11th to 20th	Day	20% 8% 4 5% 6% 3 5% 5% 5% 7% 29%	2700
	21st to end	Day	11% 6% 3 6% 5% 3 4 5% 6% 8% 41%	2700
July	1st to 10th	Day	10% 6% 2 5% 4 3 3 4 4 6% 53%	2700
	11th to 20th	Day	9% 5% 3 4 4 3 4 4 5% 6% 54%	2649
	21st to end	Day	8% 3 2 4 2 2 2 3 3 6% 65%	2741
August	1st to 10th	Day	10% 5% 2 3 4 2 4 4 4 7% 57%	2482
	11th to 20th	Day	7% 4 2 4 4 2 4 5% 5% 7% 57%	2298
	21st to end	Day	10% 7% 2 5% 5% 3 4 4 6% 8% 45%	2483
September	1st to 10th	Day	11% 6% 3 5% 4 2 3 4 4 7% 50%	2160
	11th to 20th	Day	13% 7% 3 6% 5% 4 4 5% 5% 7% 41%	2160
	21st to end	Day	18% 8% 3 7% 5% 3 4 4 4 7% 36%	1903
October	1st to 10th	Day	20% 8% 4 6% 6% 3 5% 5% 5% 7% 30%	1800
	11th to 20th	Day	22% 11% 4 7% 7% 3 4 5% 5% 8% 25%	1800
	21st to end	Day	28% 10% 4 7% 6% 3 4 4 5% 6% 23%	1864
November	1st to 10th	Day	32% 9% 5% 7% 6% 3 5% 5% 5% 5% 18%	1505
	11th to 20th	Day	32% 10% 4 7% 6% 3 5% 4 4 6% 21%	1520
	21st to end	Day	31% 11% 4 6% 7% 3 4 5% 4 7% 18%	1520
December	1st to 10th	Day	26% 12% 4 7% 6% 4 5% 3 3 6% 23%	1520
	11th to 20th	Day	28% 13% 6% 8% 8% 4 4 5% 5% 5% 14%	1827
	21st to end	Day	24% 16% 5% 10% 7% 4 6% 5% 4 5% 14%	2442
January	1st to 10th	Night	34% 12% 4 8% 6% 3 4 3 4 5% 17%	2836
	11th to 20th	Night	32% 12% 4 7% 5% 4 4 4 4 6% 20%	2605
	21st to end	Night	29% 12% 4 7% 6% 4 5% 4 4 7% 19%	2739
February	1st to 10th	Night	30% 13% 4 7% 5% 3 4 4 4 5% 20%	2340
	11th to 20th	Night	27% 12% 4 8% 6% 3 4 4 4 5% 24%	2272
	21st to end	Night	26% 12% 4 6% 6% 4 4 5% 5% 5% 22%	1878
March	1st to 10th	Night	26% 10% 4 7% 7% 3 6% 4 5% 6% 23%	2116
	11th to 20th	Night	28% 13% 3 7% 6% 3 5% 5% 4 6% 22%	1979
	21st to end	Night	30% 12% 5% 7% 6% 2 5% 4 6% 6% 17%	2108
April	1st to 10th	Night	24% 12% 4 7% 7% 4 5% 6% 7% 6% 20%	1800
	11th to 20th	Night	21% 9% 4 8% 8% 4 5% 5% 5% 6% 26%	1735
	21st to end	Night	25% 11% 4 8% 6% 3 4 4 5% 6% 24%	1570
May	1st to 10th	Night	18% 8% 2 5% 5% 2 4 4 4 7% 39%	1440
	11th to 20th	Night	16% 7% 3 6% 6% 3 5% 5% 5% 8% 34%	1416
	21st to end	Night	21% 9% 4 6% 5% 3 6% 3 5% 7% 30%	1552
June	1st to 10th	Night	21% 6% 4 7% 7% 3 4 5% 4 7% 31%	1324
	11th to 20th	Night	22% 10% 3 6% 5% 3 6% 5% 5% 6% 28%	1260
	21st to end	Night	14% 6% 2 5% 5% 3 5% 4 6% 8% 43%	1260
July	1st to 10th	Night	11% 5% 2 5% 4 2 3 3 3 6% 55%	1260
	11th to 20th	Night	9% 4 2 3 4 2 3 4 5% 6% 58%	1309
	21st to end	Night	8% 3 1 3 2 2 3 3 3 5% 66%	1576
August	1st to 10th	Night	10% 4 1 3 3 2 3 3 6% 5% 60%	1454
	11th to 20th	Night	7% 4 1 3 3 2 2 3 4 5% 64%	1602
	21st to end	Night	10% 5% 2 5% 5% 3 4 4 5% 7% 51%	1872
September	1st to 10th	Night	10% 5% 2 5% 4 2 4 4 5% 6% 55%	1800
	11th to 20th	Night	17% 7% 3 5% 5% 3 5% 5% 4 6% 41%	1800
	21st to end	Night	17% 7% 3 6% 4 3 3 4 4 7% 41%	2055
October	1st to 10th	Night	17% 8% 3 8% 5% 3 4 4 4 6% 37%	2160
	11th to 20th	Night	23% 9% 3 7% 6% 3 4 4 5% 6% 32%	2160
	21st to end	Night	25% 10% 3 7% 5% 2 4 4 5% 6% 29%	2490
November	1st to 10th	Night	33% 10% 4 7% 6% 4 4 3 4 5% 20%	2572
	11th to 20th	Night	31% 11% 4 6% 5% 3 5% 5% 4 5% 21%	2660
	21st to end	Night	32% 9% 4 7% 6% 4 4 4 5% 5% 20%	2660
December	1st to 10th	Night	27% 12% 4 7% 5% 3 4 4 4 6% 24%	2660
	11th to 20th	Night	28% 14% 4 10% 7% 3 4 4 5% 6% 15%	3374
	21st to end	Night	27% 15% 5% 9% 7% 4 5% 4 4 5% 13%	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	72% 8% 6% 10% 3 1	1329
	11th to 20th	Day	66% 10% 9% 9% 5% 1	1375
	21st to end	Day	69% 8% 11% 8% 3	1616
February	1st to 10th	Day	69% 8% 9% 8% 3 2	1620
	11th to 20th	Day	66% 8% 9% 9% 7% 1	1584
	21st to end	Day	66% 9% 9% 10% 5% 1	1377
March	1st to 10th	Day	65% 8% 12% 11% 4	1842
	11th to 20th	Day	66% 7% 10% 13% 4 1	1980
	21st to end	Day	68% 10% 11% 11% 1	2247
April	1st to 10th	Day	64% 11% 14% 9% 1	2160
	11th to 20th	Day	63% 8% 9% 13% 6% 1	2173
	21st to end	Day	61% 9% 11% 12% 6% 1	2389
May	1st to 10th	Day	51% 10% 15% 17% 6% 1	2520
	11th to 20th	Day	50% 12% 19% 14% 5%	2480
	21st to end	Day	56% 12% 15% 16% 2	2724
June	1st to 10th	Day	54% 9% 17% 13% 6% 1	2632
	11th to 20th	Day	57% 12% 16% 12% 3	2700
	21st to end	Day	43% 10% 19% 20% 7% 1	2700
July	1st to 10th	Day	35% 9% 17% 26% 12% 2	2626
	11th to 20th	Day	32% 7% 16% 31% 13% 1	2568
	21st to end	Day	24% 6% 16% 37% 15% 3	2589
August	1st to 10th	Day	30% 9% 20% 27% 12% 2	2465
	11th to 20th	Day	26% 9% 21% 30% 11% 3	2298
	21st to end	Day	37% 8% 17% 28% 9% 1	2483
September	1st to 10th	Day	35% 8% 16% 24% 13% 4	2160
	11th to 20th	Day	43% 8% 14% 17% 16% 1	2160
	21st to end	Day	45% 9% 16% 17% 10% 2	1903
October	1st to 10th	Day	50% 10% 16% 17% 7%	1800
	11th to 20th	Day	56% 9% 15% 15% 5% 1	1800
	21st to end	Day	63% 8% 10% 12% 7%	1864
November	1st to 10th	Day	67% 8% 13% 8% 4	1505
	11th to 20th	Day	66% 9% 8% 10% 5% 2	1520
	21st to end	Day	68% 6% 9% 10% 8% 1	1520
December	1st to 10th	Day	64% 7% 10% 11% 8% 1	1520
	11th to 20th	Day	69% 10% 10% 8% 3 1	1829
	21st to end	Day	71% 11% 10% 6% 2	2442
January	1st to 10th	Night	73% 7% 7% 10% 3 1	2836
	11th to 20th	Night	67% 8% 11% 9% 5% 1	2605
	21st to end	Night	65% 7% 11% 13% 4 1	2739
February	1st to 10th	Night	68% 6% 11% 9% 4 2	2340
	11th to 20th	Night	65% 7% 11% 10% 6% 1	2274
	21st to end	Night	64% 7% 10% 14% 4 1	1878
March	1st to 10th	Night	63% 7% 12% 13% 5%	2116
	11th to 20th	Night	65% 7% 11% 13% 4 1	1980
	21st to end	Night	68% 8% 14% 9% 1	2108
April	1st to 10th	Night	63% 10% 13% 12% 2	1800
	11th to 20th	Night	60% 8% 12% 12% 6% 1	1735
	21st to end	Night	63% 8% 11% 12% 6% 1	1570
May	1st to 10th	Night	46% 7% 19% 20% 8% 1	1440
	11th to 20th	Night	47% 10% 21% 15% 7%	1416
	21st to end	Night	54% 10% 17% 18% 2	1552
June	1st to 10th	Night	53% 7% 17% 17% 4 1	1324
	11th to 20th	Night	56% 8% 19% 14% 2	1260
	21st to end	Night	39% 8% 23% 22% 5% 3	1260
July	1st to 10th	Night	34% 7% 16% 26% 13% 4	1225
	11th to 20th	Night	28% 8% 17% 29% 16% 1	1267
	21st to end	Night	21% 6% 14% 33% 20% 4	1488
August	1st to 10th	Night	24% 7% 19% 30% 16% 3	1446
	11th to 20th	Night	22% 7% 20% 32% 15% 4	1602
	21st to end	Night	32% 7% 19% 31% 10% 1	1872
September	1st to 10th	Night	31% 7% 16% 30% 12% 4	1800
	11th to 20th	Night	45% 8% 9% 19% 15% 3	1800
	21st to end	Night	45% 7% 13% 23% 10% 1	2055
October	1st to 10th	Night	49% 6% 13% 24% 7% 1	2160
	11th to 20th	Night	54% 6% 16% 16% 7% 1	2160
	21st to end	Night	57% 9% 11% 17% 7%	2490
November	1st to 10th	Night	67% 7% 10% 11% 3 1	2572
	11th to 20th	Night	66% 7% 10% 10% 5% 2	2660
	21st to end	Night	67% 6% 10% 9% 7% 1	2660
December	1st to 10th	Night	61% 8% 10% 13% 7% 1	2660
	11th to 20th	Night	70% 10% 11% 6% 3 1	3371
	21st to end	Night	71% 13% 7% 7% 2	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	37% 13% 16% 16% 12% 7%	180
	11th to 20th	Day	36% 14% 16% 15% 13% 6%	162
	21st to end	Day	43% 9% 14% 17% 13% 3	208
February	1st to 10th	Day	42% 12% 19% 17% 9% 1	340
	11th to 20th	Day	39% 12% 19% 19% 10% 1	335
	21st to end	Day	34% 14% 21% 20% 10% 1	301
March	1st to 10th	Day	32% 14% 24% 19% 10% 1	420
	11th to 20th	Day	34% 14% 15% 20% 15% 3	420
	21st to end	Day	34% 13% 23% 20% 10% 1	462
April	1st to 10th	Day	30% 14% 24% 19% 12% 3	420
	11th to 20th	Day	23% 16% 24% 21% 11% 5%	412
	21st to end	Day	25% 13% 24% 23% 13% 2	420
May	1st to 10th	Day	18% 8% 25% 32% 15% 2	423
	11th to 20th	Day	25% 7% 18% 27% 19% 5%	448
	21st to end	Day	23% 8% 20% 27% 16% 6%	485
June	1st to 10th	Day	20% 9% 27% 27% 15% 2	562
	11th to 20th	Day	22% 9% 27% 27% 12% 3	630
	21st to end	Day	11% 7% 23% 31% 23% 5%	630
July	1st to 10th	Day	13% 5% 19% 35% 24% 4	621
	11th to 20th	Day	9% 7% 20% 36% 23% 5%	567
	21st to end	Day	8% 2 15% 37% 30% 9%	487
August	1st to 10th	Day	12% 3 12% 33% 30% 10%	447
	11th to 20th	Day	6% 2 14% 36% 30% 12%	437
	21st to end	Day	12% 7% 20% 31% 22% 8%	495
September	1st to 10th	Day	16% 6% 20% 34% 19% 4	450
	11th to 20th	Day	15% 8% 20% 30% 24% 4	450
	21st to end	Day	22% 8% 24% 24% 18% 5%	450
October	1st to 10th	Day	25% 10% 17% 25% 16% 7%	450
	11th to 20th	Day	32% 8% 20% 18% 17% 6%	450
	21st to end	Day	35% 12% 18% 18% 13% 5%	379
November	1st to 10th	Day	39% 11% 21% 18% 9% 2	188
	11th to 20th	Day	36% 13% 14% 14% 12% 10%	180
	21st to end	Day	36% 12% 18% 12% 11% 11%	180
December	1st to 10th	Day	45% 12% 10% 11% 11% 11%	180
	11th to 20th	Day	42% 14% 17% 18% 9% 1	180
	21st to end	Day	42% 9% 18% 22% 7% 3	198
January	1st to 10th	Night	45% 12% 14% 16% 12% 2	850
	11th to 20th	Night	41% 11% 10% 18% 17% 2	755
	21st to end	Night	38% 8% 16% 21% 15% 3	792
February	1st to 10th	Night	40% 9% 13% 26% 10% 2	720
	11th to 20th	Night	36% 11% 13% 25% 15% 1	704
	21st to end	Night	34% 10% 16% 24% 13% 3	592
March	1st to 10th	Night	34% 13% 15% 22% 13% 2	720
	11th to 20th	Night	38% 11% 15% 21% 13% 2	720
	21st to end	Night	36% 12% 18% 23% 10% 1	792
April	1st to 10th	Night	31% 10% 17% 24% 15% 3	720
	11th to 20th	Night	27% 10% 20% 25% 15% 3	667
	21st to end	Night	30% 9% 17% 29% 12% 2	490
May	1st to 10th	Night	20% 6% 17% 32% 24% 2	360
	11th to 20th	Night	21% 5% 13% 25% 31% 5%	354
	21st to end	Night	25% 6% 13% 26% 26% 5%	389
June	1st to 10th	Night	23% 6% 18% 29% 22% 3	360
	11th to 20th	Night	24% 8% 13% 34% 20% 1	360
	21st to end	Night	16% 4 12% 35% 29% 5%	360
July	1st to 10th	Night	13% 3 8% 34% 39% 3	350
	11th to 20th	Night	9% 3 13% 35% 35% 5%	348
	21st to end	Night	7% 2 8% 32% 42% 8%	394
August	1st to 10th	Night	11% 2 6% 24% 44% 12%	380
	11th to 20th	Night	9% 1 5% 23% 47% 15%	534
	21st to end	Night	14% 4 9% 30% 37% 6%	684
September	1st to 10th	Night	12% 2 11% 37% 31% 6%	720
	11th to 20th	Night	20% 5% 11% 22% 32% 10%	720
	21st to end	Night	22% 6% 13% 26% 27% 5%	720
October	1st to 10th	Night	23% 6% 12% 25% 27% 7%	720
	11th to 20th	Night	30% 9% 13% 22% 20% 5%	720
	21st to end	Night	32% 9% 16% 24% 14% 4	792
November	1st to 10th	Night	38% 11% 17% 23% 10%	740
	11th to 20th	Night	39% 11% 17% 18% 13% 3	760
	21st to end	Night	39% 9% 16% 19% 12% 5%	760
December	1st to 10th	Night	35% 8% 14% 24% 15% 4	760
	11th to 20th	Night	42% 12% 15% 21% 9% 1	787
	21st to end	Night	42% 13% 18% 17% 9%	935

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