Starry Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starry Hill Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	18% 11% 4 7% 6% 4 5% 5% 5% 6% 29%	75240
	Night	18% 11% 4 7% 7% 4 5% 5% 5% 6% 30%	74534
Transparency	Day	56% 7% 12% 16% 8% 1	74938
	Night	57% 7% 12% 15% 8% 2	74354
Seeing	Day	25% 12% 21% 26% 13% 3	13857
	Night	28% 9% 14% 26% 20% 3	22436

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	26% 15% 5% 9% 9% 4 6% 4 4 4 15%	1519
	11th to 20th	Day	23% 11% 3 7% 5% 4 5% 5% 6% 6% 23%	1464
	21st to end	Day	23% 12% 5% 8% 7% 3 5% 4 5% 5% 22%	1660
February	1st to 10th	Day	23% 15% 5% 10% 7% 4 5% 4 4 6% 17%	1620
	11th to 20th	Day	24% 17% 5% 7% 7% 3 5% 4 4 5% 17%	1575
	21st to end	Day	24% 20% 4 8% 7% 4 5% 4 4 4 17%	1399
March	1st to 10th	Day	18% 16% 5% 10% 8% 5% 5% 5% 5% 5% 17%	1850
	11th to 20th	Day	23% 15% 5% 10% 8% 4 5% 5% 4 4 17%	1980
	21st to end	Day	21% 17% 7% 11% 9% 5% 5% 6% 5% 5% 10%	2245
April	1st to 10th	Day	24% 15% 6% 8% 8% 4 6% 6% 4 6% 12%	2160
	11th to 20th	Day	22% 12% 5% 9% 7% 4 5% 5% 5% 5% 20%	2139
	21st to end	Day	22% 14% 4 9% 8% 3 5% 5% 4 5% 21%	2358
May	1st to 10th	Day	19% 11% 4 8% 6% 4 5% 5% 5% 5% 29%	2520
	11th to 20th	Day	19% 10% 5% 8% 7% 4 6% 6% 6% 6% 23%	2477
	21st to end	Day	24% 9% 4 9% 7% 4 5% 6% 6% 6% 21%	2723
June	1st to 10th	Day	26% 9% 3 7% 6% 4 5% 5% 6% 6% 23%	2520
	11th to 20th	Day	23% 11% 4 9% 7% 5% 6% 5% 6% 5% 18%	2653
	21st to end	Day	12% 8% 4 6% 6% 4 5% 5% 5% 7% 37%	2700
July	1st to 10th	Day	10% 7% 2 6% 5% 3 4 5% 5% 6% 47%	2687
	11th to 20th	Day	10% 4 3 5% 5% 3 5% 4 5% 8% 48%	2520
	21st to end	Day	8% 3 2 3 3 2 3 3 5% 7% 62%	2741
August	1st to 10th	Day	9% 5% 2 4 4 3 4 4 5% 8% 52%	2435
	11th to 20th	Day	8% 4 2 4 3 3 4 4 5% 7% 56%	2298
	21st to end	Day	10% 6% 2 6% 5% 3 5% 5% 6% 8% 44%	2458
September	1st to 10th	Day	10% 5% 2 5% 5% 3 4 4 5% 8% 50%	2160
	11th to 20th	Day	12% 7% 3 6% 6% 3 5% 5% 5% 7% 40%	2160
	21st to end	Day	15% 8% 3 6% 5% 4 5% 4 4 7% 39%	1902
October	1st to 10th	Day	19% 9% 3 7% 5% 3 4 4 5% 7% 33%	1800
	11th to 20th	Day	18% 10% 4 8% 6% 5% 4 5% 5% 7% 30%	1800
	21st to end	Day	24% 9% 4 8% 6% 4 4 4 4 6% 27%	1890
November	1st to 10th	Day	24% 13% 5% 7% 8% 5% 5% 6% 6% 5% 15%	1548
	11th to 20th	Day	25% 12% 6% 8% 6% 4 4 4 5% 7% 18%	1520
	21st to end	Day	24% 14% 4 9% 6% 3 5% 4 5% 4 22%	1520
December	1st to 10th	Day	17% 13% 4 8% 8% 4 6% 5% 3 4 26%	1520
	11th to 20th	Day	18% 14% 5% 9% 8% 3 6% 5% 5% 6% 20%	1925
	21st to end	Day	19% 14% 5% 9% 8% 6% 7% 6% 5% 5% 16%	2794
January	1st to 10th	Night	28% 14% 5% 9% 7% 4 5% 4 4 4 16%	2646
	11th to 20th	Night	21% 14% 4 7% 7% 4 5% 5% 5% 7% 21%	2574
	21st to end	Night	23% 12% 5% 8% 7% 4 4 5% 5% 6% 21%	2695
February	1st to 10th	Night	23% 13% 5% 9% 8% 4 5% 5% 4 6% 19%	2340
	11th to 20th	Night	21% 15% 5% 9% 8% 4 4 4 4 5% 22%	2272
	21st to end	Night	21% 17% 4 7% 7% 4 7% 5% 4 5% 18%	1856
March	1st to 10th	Night	18% 14% 5% 8% 6% 4 7% 5% 5% 6% 22%	2106
	11th to 20th	Night	20% 11% 5% 9% 8% 4 6% 5% 5% 6% 21%	1979
	21st to end	Night	22% 16% 5% 8% 8% 4 6% 5% 5% 4 16%	2111
April	1st to 10th	Night	18% 15% 5% 9% 9% 5% 5% 5% 5% 5% 20%	1800
	11th to 20th	Night	18% 12% 5% 8% 8% 4 5% 5% 5% 6% 25%	1771
	21st to end	Night	20% 12% 4 8% 7% 4 6% 4 5% 6% 24%	1602
May	1st to 10th	Night	17% 9% 4 6% 6% 3 5% 5% 5% 5% 34%	1440
	11th to 20th	Night	14% 8% 4 8% 7% 5% 6% 6% 7% 6% 29%	1416
	21st to end	Night	19% 9% 4 6% 6% 3 5% 6% 7% 6% 30%	1552
June	1st to 10th	Night	18% 6% 4 6% 7% 4 6% 5% 6% 8% 31%	1440
	11th to 20th	Night	18% 10% 4 7% 7% 4 4 5% 6% 8% 27%	1301
	21st to end	Night	10% 7% 2 5% 5% 3 5% 4 4 8% 48%	1260
July	1st to 10th	Night	7% 5% 3 4 4 2 4 4 6% 7% 55%	1268
	11th to 20th	Night	7% 4 2 4 4 2 3 3 5% 5% 60%	1440
	21st to end	Night	4 3 1 3 3 2 3 3 4 6% 69%	1576
August	1st to 10th	Night	6% 4 1 4 3 3 3 4 4 6% 62%	1502
	11th to 20th	Night	6% 4 1 3 2 1 3 3 4 7% 66%	1602
	21st to end	Night	8% 5% 2 5% 5% 2 4 4 4 6% 55%	1898
September	1st to 10th	Night	8% 5% 2 4 4 2 5% 4 4 5% 57%	1800
	11th to 20th	Night	13% 7% 3 6% 6% 3 4 5% 4 7% 43%	1800
	21st to end	Night	12% 6% 2 7% 6% 3 5% 4 5% 7% 43%	2053
October	1st to 10th	Night	16% 9% 4 6% 6% 3 4 3 5% 5% 39%	2160
	11th to 20th	Night	18% 9% 4 7% 6% 4 5% 5% 5% 6% 32%	2160
	21st to end	Night	19% 9% 3 7% 6% 3 4 4 4 7% 32%	2466
November	1st to 10th	Night	26% 10% 5% 7% 6% 4 5% 5% 6% 6% 20%	2526
	11th to 20th	Night	25% 11% 4 7% 8% 4 6% 5% 4 5% 21%	2660
	21st to end	Night	21% 13% 4 7% 7% 4 6% 5% 5% 5% 23%	2660
December	1st to 10th	Night	19% 13% 5% 8% 7% 4 6% 5% 5% 5% 25%	2660
	11th to 20th	Night	20% 14% 5% 9% 8% 4 5% 6% 6% 5% 20%	3280
	21st to end	Night	19% 13% 5% 10% 9% 5% 6% 5% 6% 6% 16%	4862

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	75% 5% 5% 8% 5% 1	1519
	11th to 20th	Day	62% 8% 12% 8% 7% 4	1464
	21st to end	Day	64% 7% 8% 12% 8% 1	1660
February	1st to 10th	Day	70% 7% 8% 10% 5%	1620
	11th to 20th	Day	69% 8% 8% 10% 5% 1	1584
	21st to end	Day	72% 5% 10% 9% 4	1399
March	1st to 10th	Day	69% 6% 10% 11% 3	1850
	11th to 20th	Day	72% 5% 8% 12% 2 1	1980
	21st to end	Day	77% 7% 9% 6%	2245
April	1st to 10th	Day	72% 7% 12% 8% 1	2160
	11th to 20th	Day	66% 5% 11% 11% 6% 1	2139
	21st to end	Day	67% 8% 10% 11% 4 1	2358
May	1st to 10th	Day	57% 7% 15% 16% 5%	2520
	11th to 20th	Day	60% 9% 15% 12% 4	2480
	21st to end	Day	64% 9% 12% 13% 2	2724
June	1st to 10th	Day	62% 7% 15% 12% 5%	2520
	11th to 20th	Day	66% 7% 14% 9% 3	2653
	21st to end	Day	45% 9% 18% 18% 9% 2	2700
July	1st to 10th	Day	38% 8% 13% 24% 15% 3	2613
	11th to 20th	Day	33% 8% 11% 28% 19% 1	2445
	21st to end	Day	22% 6% 12% 35% 20% 5%	2589
August	1st to 10th	Day	31% 8% 15% 28% 16% 2	2418
	11th to 20th	Day	28% 6% 16% 32% 15% 4	2298
	21st to end	Day	38% 6% 14% 29% 11% 2	2458
September	1st to 10th	Day	33% 6% 14% 27% 15% 5%	2160
	11th to 20th	Day	43% 9% 13% 17% 15% 4	2160
	21st to end	Day	46% 7% 15% 17% 13% 2	1902
October	1st to 10th	Day	52% 6% 10% 20% 10% 1	1800
	11th to 20th	Day	54% 7% 14% 17% 7% 1	1800
	21st to end	Day	61% 6% 8% 13% 11% 1	1890
November	1st to 10th	Day	69% 7% 9% 10% 3 1	1548
	11th to 20th	Day	68% 7% 9% 8% 6% 3	1520
	21st to end	Day	66% 6% 5% 10% 11% 2	1520
December	1st to 10th	Day	63% 7% 9% 11% 8% 2	1520
	11th to 20th	Day	67% 6% 12% 11% 4 1	1928
	21st to end	Day	69% 8% 11% 9% 3	2794
January	1st to 10th	Night	74% 6% 6% 7% 5% 2	2646
	11th to 20th	Night	64% 6% 11% 10% 6% 3	2574
	21st to end	Night	64% 6% 7% 12% 9% 2	2695
February	1st to 10th	Night	69% 6% 9% 9% 5% 1	2340
	11th to 20th	Night	66% 7% 9% 10% 6% 2	2274
	21st to end	Night	69% 6% 10% 10% 5%	1856
March	1st to 10th	Night	63% 5% 14% 14% 4	2106
	11th to 20th	Night	65% 6% 11% 13% 4 1	1980
	21st to end	Night	72% 9% 10% 8%	2111
April	1st to 10th	Night	68% 9% 11% 10% 2	1800
	11th to 20th	Night	61% 7% 13% 12% 5% 1	1771
	21st to end	Night	63% 8% 12% 12% 5% 1	1602
May	1st to 10th	Night	53% 8% 14% 18% 7% 1	1440
	11th to 20th	Night	55% 10% 15% 14% 4 2	1416
	21st to end	Night	55% 11% 16% 15% 3	1552
June	1st to 10th	Night	53% 10% 17% 16% 4	1440
	11th to 20th	Night	58% 10% 17% 12% 3	1301
	21st to end	Night	38% 9% 20% 21% 9% 2	1260
July	1st to 10th	Night	31% 8% 17% 24% 16% 4	1233
	11th to 20th	Night	29% 7% 12% 31% 19% 2	1392
	21st to end	Night	19% 6% 11% 36% 20% 7%	1488
August	1st to 10th	Night	26% 7% 15% 31% 19% 4	1494
	11th to 20th	Night	22% 7% 18% 33% 16% 4	1602
	21st to end	Night	32% 6% 15% 32% 13% 1	1898
September	1st to 10th	Night	31% 7% 15% 27% 16% 5%	1800
	11th to 20th	Night	43% 8% 10% 20% 15% 5%	1800
	21st to end	Night	42% 8% 14% 19% 14% 3	2053
October	1st to 10th	Night	47% 7% 13% 21% 10% 1	2160
	11th to 20th	Night	54% 6% 11% 19% 8% 2	2160
	21st to end	Night	54% 7% 11% 16% 12%	2466
November	1st to 10th	Night	65% 8% 11% 11% 4 1	2526
	11th to 20th	Night	66% 8% 10% 9% 6% 2	2660
	21st to end	Night	64% 8% 6% 10% 11% 1	2660
December	1st to 10th	Night	63% 7% 9% 12% 7% 2	2660
	11th to 20th	Night	67% 8% 11% 10% 4	3276
	21st to end	Night	71% 8% 10% 8% 3	4862

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	41% 12% 17% 15% 11% 5%	180
	11th to 20th	Day	30% 22% 16% 20% 10% 1	166
	21st to end	Day	34% 15% 18% 20% 11% 2	252
February	1st to 10th	Day	43% 12% 19% 16% 9% 1	340
	11th to 20th	Day	44% 14% 19% 17% 7%	335
	21st to end	Day	38% 16% 21% 19% 5% 1	310
March	1st to 10th	Day	32% 19% 25% 18% 6% 1	420
	11th to 20th	Day	32% 15% 20% 19% 13%	420
	21st to end	Day	35% 15% 26% 17% 6% 1	462
April	1st to 10th	Day	36% 16% 20% 18% 9% 2	420
	11th to 20th	Day	30% 20% 24% 16% 6% 4	412
	21st to end	Day	31% 15% 25% 21% 7% 2	420
May	1st to 10th	Day	23% 13% 27% 25% 11% 1	423
	11th to 20th	Day	30% 11% 22% 23% 10% 4	448
	21st to end	Day	26% 11% 21% 28% 12% 2	485
June	1st to 10th	Day	26% 11% 24% 25% 12% 3	450
	11th to 20th	Day	27% 12% 28% 24% 7% 2	583
	21st to end	Day	13% 9% 25% 34% 16% 3	630
July	1st to 10th	Day	15% 7% 20% 39% 16% 3	608
	11th to 20th	Day	9% 8% 25% 36% 20% 2	444
	21st to end	Day	9% 3 19% 41% 23% 6%	487
August	1st to 10th	Day	12% 7% 16% 40% 19% 6%	447
	11th to 20th	Day	7% 3 17% 42% 21% 9%	437
	21st to end	Day	11% 11% 24% 33% 15% 6%	495
September	1st to 10th	Day	16% 6% 24% 35% 14% 4	450
	11th to 20th	Day	16% 9% 21% 31% 21% 2	450
	21st to end	Day	19% 13% 22% 27% 16% 3	450
October	1st to 10th	Day	24% 14% 18% 22% 20% 3	450
	11th to 20th	Day	27% 14% 17% 21% 15% 5%	450
	21st to end	Day	34% 11% 18% 20% 11% 5%	405
November	1st to 10th	Day	32% 20% 16% 19% 10% 2	210
	11th to 20th	Day	33% 18% 12% 16% 15% 7%	180
	21st to end	Day	29% 15% 19% 15% 12% 9%	180
December	1st to 10th	Day	38% 15% 10% 15% 14% 8%	180
	11th to 20th	Day	39% 13% 22% 17% 7% 2	180
	21st to end	Day	38% 14% 16% 19% 13% 1	198
January	1st to 10th	Night	43% 13% 13% 16% 13% 2	760
	11th to 20th	Night	34% 12% 14% 22% 15% 3	736
	21st to end	Night	33% 10% 12% 26% 15% 3	792
February	1st to 10th	Night	36% 12% 15% 27% 10% 1	720
	11th to 20th	Night	35% 10% 13% 27% 14% 1	704
	21st to end	Night	35% 10% 17% 25% 11% 1	592
March	1st to 10th	Night	30% 14% 16% 24% 14% 2	720
	11th to 20th	Night	33% 13% 15% 25% 13% 1	720
	21st to end	Night	35% 11% 20% 22% 10% 1	792
April	1st to 10th	Night	31% 12% 16% 23% 16% 2	720
	11th to 20th	Night	28% 9% 20% 25% 15% 2	703
	21st to end	Night	29% 10% 14% 31% 14% 2	522
May	1st to 10th	Night	24% 8% 15% 32% 20% 1	360
	11th to 20th	Night	21% 8% 12% 28% 29% 2	354
	21st to end	Night	25% 7% 14% 24% 26% 3	389
June	1st to 10th	Night	20% 9% 16% 25% 26% 3	360
	11th to 20th	Night	24% 8% 16% 31% 20% 1	360
	21st to end	Night	14% 5% 14% 31% 33% 3	360
July	1st to 10th	Night	12% 3 9% 32% 39% 4	350
	11th to 20th	Night	10% 3 8% 38% 37% 3	348
	21st to end	Night	6% 2 10% 28% 48% 5%	394
August	1st to 10th	Night	12% 3 8% 24% 43% 10%	428
	11th to 20th	Night	8% 3 6% 26% 46% 12%	534
	21st to end	Night	12% 6% 9% 32% 37% 4	710
September	1st to 10th	Night	13% 4 10% 34% 34% 6%	720
	11th to 20th	Night	16% 6% 11% 24% 35% 7%	720
	21st to end	Night	19% 7% 12% 31% 28% 4	720
October	1st to 10th	Night	22% 7% 12% 30% 26% 4	720
	11th to 20th	Night	26% 9% 14% 25% 22% 5%	720
	21st to end	Night	28% 10% 17% 26% 15% 4	792
November	1st to 10th	Night	34% 11% 17% 28% 9% 1	740
	11th to 20th	Night	36% 12% 15% 22% 12% 2	760
	21st to end	Night	34% 9% 17% 20% 15% 4	760
December	1st to 10th	Night	31% 10% 16% 27% 14% 3	760
	11th to 20th	Night	38% 11% 16% 24% 9% 1	760
	21st to end	Night	44% 13% 16% 19% 8%	836

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