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 2024-11-28.

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	16% 10% 3 8% 5% 5% 5% 4 5% 6% 32%	106208
	Night	16% 11% 4 8% 6% 5% 5% 4 5% 6% 32%	101918
Transparency	Day	53% 7% 12% 17% 10% 2	105969
	Night	55% 7% 12% 16% 8% 2	101803
Seeing	Day	25% 12% 21% 26% 13% 3	15046
	Night	27% 9% 14% 26% 20% 3	23482

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	25% 20% 5% 11% 7% 6% 5% 3 4 4 11%	2539
	11th to 20th	Day	24% 15% 4 8% 5% 4 4 4 5% 6% 23%	2516
	21st to end	Day	20% 14% 5% 12% 9% 7% 6% 4 5% 4 14%	2926
February	1st to 10th	Day	22% 16% 5% 11% 6% 5% 4 4 5% 6% 16%	1710
	11th to 20th	Day	23% 18% 4 9% 7% 5% 4 3 4 5% 16%	1665
	21st to end	Day	23% 22% 4 9% 5% 5% 4 3 4 3 17%	1484
March	1st to 10th	Day	10% 12% 5% 12% 9% 8% 7% 5% 7% 7% 18%	3227
	11th to 20th	Day	14% 13% 3 9% 6% 4 4 4 4 3 35%	3490
	21st to end	Day	16% 15% 5% 13% 9% 8% 6% 5% 6% 5% 12%	3943
April	1st to 10th	Day	23% 17% 5% 10% 7% 6% 6% 4 4 6% 13%	2280
	11th to 20th	Day	20% 14% 4 11% 6% 5% 4 4 5% 5% 21%	2260
	21st to end	Day	21% 15% 4 11% 6% 5% 5% 4 4 5% 20%	2489
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 4 3 4 5% 33%	4420
	11th to 20th	Day	12% 9% 4 9% 7% 6% 5% 4 6% 7% 31%	4377
	21st to end	Day	24% 11% 4 11% 6% 6% 6% 4 6% 6% 16%	4813
June	1st to 10th	Day	25% 10% 3 8% 6% 5% 4 4 6% 6% 24%	2660
	11th to 20th	Day	22% 11% 4 10% 7% 6% 5% 4 6% 5% 19%	2801
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	2850
July	1st to 10th	Day	6% 4 2 4 3 3 3 3 4 5% 63%	4713
	11th to 20th	Day	5% 3 1 4 3 3 3 2 4 5% 67%	4420
	21st to end	Day	10% 4 2 4 3 3 3 3 5% 6% 57%	4790
August	1st to 10th	Day	5% 3 2 4 3 3 3 3 5% 7% 63%	4264
	11th to 20th	Day	8% 6% 2 7% 4 5% 5% 4 7% 8% 44%	4026
	21st to end	Day	13% 6% 3 7% 4 4 4 3 5% 7% 45%	4314
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 7% 50%	2280
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 5% 7% 39%	2280
	21st to end	Day	14% 8% 3 7% 4 5% 4 3 5% 7% 39%	2007
October	1st to 10th	Day	14% 7% 2 6% 3 3 4 3 5% 9% 44%	3180
	11th to 20th	Day	19% 8% 3 8% 5% 6% 5% 5% 6% 8% 28%	3161
	21st to end	Day	19% 8% 3 10% 6% 5% 6% 5% 6% 7% 25%	3277
November	1st to 10th	Day	23% 14% 4 9% 7% 6% 5% 5% 6% 5% 15%	1631
	11th to 20th	Day	24% 13% 5% 9% 6% 5% 4 3 6% 7% 17%	1600
	21st to end	Day	22% 16% 3 10% 6% 4 5% 3 6% 5% 21%	1576
December	1st to 10th	Day	17% 14% 3 10% 7% 6% 5% 4 3 4 26%	1520
	11th to 20th	Day	18% 15% 5% 10% 7% 5% 6% 4 5% 6% 20%	1925
	21st to end	Day	19% 15% 5% 10% 7% 7% 6% 4 5% 5% 16%	2794
January	1st to 10th	Night	23% 20% 6% 13% 7% 6% 5% 3 4 3 10%	4406
	11th to 20th	Night	19% 15% 5% 9% 6% 5% 4 4 5% 7% 21%	4382
	21st to end	Night	21% 15% 5% 12% 7% 6% 6% 4 5% 5% 13%	4733
February	1st to 10th	Night	23% 14% 5% 10% 6% 5% 5% 3 4 6% 18%	2470
	11th to 20th	Night	20% 15% 4 11% 6% 5% 4 3 4 5% 22%	2402
	21st to end	Night	20% 19% 4 9% 6% 6% 6% 4 5% 4 18%	1969
March	1st to 10th	Night	10% 12% 5% 12% 8% 8% 7% 6% 7% 6% 18%	3643
	11th to 20th	Night	12% 9% 3 9% 5% 5% 4 3 5% 6% 39%	3489
	21st to end	Night	21% 17% 5% 11% 7% 6% 6% 5% 5% 4 14%	3710
April	1st to 10th	Night	17% 16% 4 11% 8% 6% 4 4 5% 5% 20%	1900
	11th to 20th	Night	17% 13% 4 10% 6% 5% 5% 4 5% 5% 27%	1870
	21st to end	Night	19% 12% 3 9% 6% 5% 6% 3 5% 6% 23%	1691
May	1st to 10th	Night	19% 11% 3 8% 5% 4 4 3 4 4 35%	2540
	11th to 20th	Night	9% 9% 3 8% 6% 5% 5% 4 6% 7% 39%	2516
	21st to end	Night	15% 11% 4 9% 7% 6% 6% 5% 6% 6% 25%	2762
June	1st to 10th	Night	18% 8% 3 7% 6% 6% 5% 5% 5% 8% 31%	1520
	11th to 20th	Night	17% 11% 4 8% 6% 5% 3 4 6% 8% 29%	1373
	21st to end	Night	10% 7% 2 5% 5% 4 4 4 5% 8% 47%	1330
July	1st to 10th	Night	4 3 1 3 2 2 2 2 4 4 72%	2242
	11th to 20th	Night	4 3 1 3 2 2 2 3 3 4 72%	2540
	21st to end	Night	6% 4 1 3 2 2 3 2 4 5% 68%	2762
August	1st to 10th	Night	3 2 1 3 3 2 2 2 4 5% 72%	2655
	11th to 20th	Night	8% 6% 2 5% 4 4 5% 4 5% 6% 52%	2812
	21st to end	Night	13% 6% 2 6% 4 3 4 3 4 5% 50%	3360
September	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 4 5% 57%	1900
	11th to 20th	Night	13% 7% 3 7% 6% 4 4 4 4 7% 43%	1900
	21st to end	Night	11% 6% 2 8% 5% 3 5% 3 5% 7% 44%	2168
October	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	3800
	11th to 20th	Night	16% 9% 4 8% 5% 5% 4 4 5% 6% 34%	3777
	21st to end	Night	14% 8% 3 9% 6% 6% 7% 6% 7% 8% 28%	4273
November	1st to 10th	Night	25% 10% 4 8% 6% 5% 5% 4 6% 6% 20%	2663
	11th to 20th	Night	25% 12% 3 9% 7% 6% 5% 4 5% 5% 20%	2800
	21st to end	Night	21% 14% 4 9% 5% 5% 6% 4 5% 6% 22%	2758
December	1st to 10th	Night	19% 14% 4 8% 7% 5% 6% 4 5% 5% 24%	2660
	11th to 20th	Night	20% 14% 5% 10% 7% 5% 5% 4 6% 5% 20%	3280
	21st to end	Night	19% 14% 5% 11% 7% 6% 5% 4 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	79% 6% 6% 6% 3 1	2539
	11th to 20th	Day	62% 7% 11% 13% 5% 2	2540
	21st to end	Day	76% 6% 6% 8% 4	2926
February	1st to 10th	Day	69% 8% 8% 10% 5%	1710
	11th to 20th	Day	69% 8% 8% 10% 5% 1	1674
	21st to end	Day	72% 5% 10% 9% 4	1484
March	1st to 10th	Day	63% 7% 15% 11% 4	3227
	11th to 20th	Day	56% 5% 10% 27% 2 1	3490
	21st to end	Day	71% 8% 12% 8% 1	3943
April	1st to 10th	Day	72% 7% 12% 8% 1	2280
	11th to 20th	Day	65% 6% 11% 11% 7% 1	2260
	21st to end	Day	68% 8% 10% 11% 3 1	2489
May	1st to 10th	Day	54% 6% 14% 18% 9%	4420
	11th to 20th	Day	50% 9% 21% 16% 3	4380
	21st to end	Day	68% 9% 14% 8% 1	4814
June	1st to 10th	Day	61% 7% 15% 12% 5%	2660
	11th to 20th	Day	65% 7% 14% 10% 4	2801
	21st to end	Day	45% 8% 18% 19% 8% 1	2850
July	1st to 10th	Day	24% 6% 8% 20% 21% 21%	4639
	11th to 20th	Day	20% 5% 7% 23% 37% 8%	4345
	21st to end	Day	28% 5% 12% 33% 20% 3	4639
August	1st to 10th	Day	22% 5% 10% 34% 25% 3	4247
	11th to 20th	Day	39% 6% 19% 24% 9% 2	4065
	21st to end	Day	41% 5% 14% 23% 15% 2	4313
September	1st to 10th	Day	33% 6% 14% 26% 15% 5%	2280
	11th to 20th	Day	43% 9% 13% 17% 14% 4	2280
	21st to end	Day	45% 7% 16% 18% 12% 2	2007
October	1st to 10th	Day	40% 5% 12% 31% 12% 1	3180
	11th to 20th	Day	56% 6% 13% 16% 9% 1	3161
	21st to end	Day	60% 5% 12% 11% 10% 1	3277
November	1st to 10th	Day	70% 6% 10% 11% 3 1	1631
	11th to 20th	Day	69% 7% 8% 7% 6% 2	1600
	21st to end	Day	66% 6% 6% 10% 10% 2	1576
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	80% 6% 5% 5% 3 1	4406
	11th to 20th	Night	62% 6% 11% 15% 4 2	4420
	21st to end	Night	72% 8% 7% 7% 5% 1	4733
February	1st to 10th	Night	69% 7% 10% 9% 5% 1	2470
	11th to 20th	Night	66% 7% 9% 10% 6% 2	2404
	21st to end	Night	69% 6% 11% 9% 5%	1969
March	1st to 10th	Night	64% 7% 14% 11% 4	3643
	11th to 20th	Night	50% 6% 17% 24% 2 1	3490
	21st to end	Night	74% 8% 10% 8%	3710
April	1st to 10th	Night	68% 9% 12% 10% 2	1900
	11th to 20th	Night	60% 7% 14% 11% 7% 1	1870
	21st to end	Night	64% 8% 12% 11% 4 1	1691
May	1st to 10th	Night	54% 7% 11% 20% 8%	2540
	11th to 20th	Night	47% 11% 20% 17% 4 1	2516
	21st to end	Night	63% 13% 14% 9% 2	2762
June	1st to 10th	Night	53% 9% 17% 16% 4	1520
	11th to 20th	Night	57% 10% 17% 14% 3	1373
	21st to end	Night	39% 9% 20% 21% 8% 2	1330
July	1st to 10th	Night	19% 5% 10% 26% 16% 23%	2207
	11th to 20th	Night	19% 4 7% 26% 36% 8%	2492
	21st to end	Night	21% 5% 13% 34% 22% 5%	2674
August	1st to 10th	Night	17% 5% 10% 40% 25% 3	2647
	11th to 20th	Night	35% 6% 24% 23% 9% 3	2839
	21st to end	Night	38% 6% 15% 28% 13% 1	3360
September	1st to 10th	Night	30% 7% 15% 27% 16% 5%	1900
	11th to 20th	Night	44% 7% 11% 20% 14% 5%	1900
	21st to end	Night	41% 8% 16% 19% 14% 3	2168
October	1st to 10th	Night	39% 6% 16% 28% 11% 1	3800
	11th to 20th	Night	52% 5% 13% 21% 8% 1	3777
	21st to end	Night	57% 7% 11% 15% 10%	4273
November	1st to 10th	Night	65% 8% 11% 11% 4 1	2663
	11th to 20th	Night	67% 8% 10% 8% 5% 2	2800
	21st to end	Night	64% 8% 7% 10% 10% 1	2758
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	31% 21% 15% 22% 11% 1	180
	21st to end	Day	35% 18% 18% 18% 10% 2	278
February	1st to 10th	Day	40% 12% 19% 17% 10% 2	370
	11th to 20th	Day	42% 14% 19% 17% 7% 1	365
	21st to end	Day	39% 16% 20% 19% 5% 1	341
March	1st to 10th	Day	30% 20% 25% 18% 6% 1	460
	11th to 20th	Day	30% 15% 20% 19% 14% 2	460
	21st to end	Day	33% 16% 26% 18% 6% 1	506
April	1st to 10th	Day	35% 17% 20% 18% 8% 2	460
	11th to 20th	Day	29% 22% 24% 17% 6% 4	452
	21st to end	Day	31% 16% 24% 21% 7% 2	460
May	1st to 10th	Day	23% 13% 28% 24% 11% 2	463
	11th to 20th	Day	29% 12% 23% 23% 10% 4	488
	21st to end	Day	27% 12% 21% 27% 11% 2	529
June	1st to 10th	Day	26% 11% 23% 26% 12% 3	490
	11th to 20th	Day	26% 12% 28% 24% 6% 3	631
	21st to end	Day	13% 9% 25% 34% 15% 4	680
July	1st to 10th	Day	14% 7% 22% 38% 16% 3	657
	11th to 20th	Day	8% 7% 24% 38% 20% 3	484
	21st to end	Day	10% 3 20% 40% 22% 5%	531
August	1st to 10th	Day	11% 6% 17% 41% 18% 6%	487
	11th to 20th	Day	7% 4 17% 42% 21% 8%	477
	21st to end	Day	12% 11% 23% 32% 14% 7%	539
September	1st to 10th	Day	15% 6% 23% 36% 14% 5%	490
	11th to 20th	Day	17% 10% 22% 30% 20% 2	490
	21st to end	Day	18% 13% 23% 28% 16% 3	490
October	1st to 10th	Day	23% 14% 19% 22% 20% 3	490
	11th to 20th	Day	28% 14% 18% 20% 15% 5%	490
	21st to end	Day	33% 12% 19% 20% 11% 5%	443
November	1st to 10th	Day	33% 18% 17% 19% 10% 3	233
	11th to 20th	Day	35% 18% 13% 16% 14% 6%	200
	21st to end	Day	29% 16% 18% 16% 12% 8%	194
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	33% 12% 14% 23% 15% 3	760
	21st to end	Night	33% 11% 13% 25% 14% 3	836
February	1st to 10th	Night	35% 11% 15% 26% 11% 1	760
	11th to 20th	Night	34% 10% 13% 27% 14% 1	744
	21st to end	Night	36% 12% 17% 24% 11% 1	628
March	1st to 10th	Night	29% 14% 16% 24% 14% 1	760
	11th to 20th	Night	32% 14% 15% 24% 14% 1	760
	21st to end	Night	35% 11% 21% 22% 9% 1	836
April	1st to 10th	Night	31% 13% 16% 23% 15% 2	760
	11th to 20th	Night	27% 10% 21% 25% 15% 2	742
	21st to end	Night	29% 11% 14% 30% 13% 2	551
May	1st to 10th	Night	24% 9% 15% 31% 19% 1	380
	11th to 20th	Night	20% 9% 12% 28% 28% 2	374
	21st to end	Night	25% 7% 15% 24% 26% 3	411
June	1st to 10th	Night	20% 9% 17% 25% 26% 3	380
	11th to 20th	Night	23% 8% 18% 31% 20% 1	380
	21st to end	Night	14% 5% 14% 32% 32% 3	380
July	1st to 10th	Night	12% 3 10% 33% 39% 4	370
	11th to 20th	Night	10% 3 8% 38% 38% 3	368
	21st to end	Night	6% 2 10% 30% 47% 5%	416
August	1st to 10th	Night	11% 3 8% 25% 44% 9%	452
	11th to 20th	Night	9% 3 5% 26% 45% 12%	564
	21st to end	Night	13% 6% 9% 32% 36% 4	750
September	1st to 10th	Night	12% 4 10% 33% 35% 6%	760
	11th to 20th	Night	16% 6% 12% 24% 35% 7%	760
	21st to end	Night	18% 7% 13% 31% 28% 4	760
October	1st to 10th	Night	21% 7% 12% 30% 26% 4	760
	11th to 20th	Night	26% 9% 14% 25% 21% 4	760
	21st to end	Night	28% 10% 18% 26% 14% 4	836
November	1st to 10th	Night	34% 11% 17% 28% 9% 1	780
	11th to 20th	Night	37% 12% 15% 21% 12% 2	800
	21st to end	Night	34% 9% 17% 20% 14% 4	788
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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