Hilltop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hilltop Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	24% 11% 3 7% 4 4 4 3 5% 6% 27%	105580
	Night	27% 14% 4 8% 4 4 3 3 4 5% 26%	102118
Transparency	Day	55% 8% 12% 15% 8% 1	105332
	Night	60% 7% 11% 15% 7% 1	102012
Seeing	Day	27% 12% 24% 24% 11% 3	15421
	Night	33% 9% 18% 24% 14% 2	23681

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	40% 15% 5% 10% 6% 4 3 2 3 3 9%	2219
	11th to 20th	Day	35% 15% 4 7% 4 3 3 3 3 4 19%	2330
	21st to end	Day	50% 13% 3 7% 4 3 2 2 3 3 10%	3050
February	1st to 10th	Day	37% 13% 3 8% 4 5% 4 3 4 5% 14%	1710
	11th to 20th	Day	36% 14% 4 8% 4 4 3 3 3 5% 18%	1665
	21st to end	Day	31% 18% 4 8% 5% 5% 3 3 4 5% 14%	1419
March	1st to 10th	Day	23% 18% 4 11% 6% 6% 5% 4 5% 6% 13%	3354
	11th to 20th	Day	26% 10% 3 8% 4 4 4 3 4 6% 28%	3490
	21st to end	Day	28% 16% 4 9% 6% 6% 4 3 4 4 14%	3848
April	1st to 10th	Day	33% 12% 4 9% 6% 5% 5% 3 4 6% 13%	2280
	11th to 20th	Day	29% 12% 4 9% 5% 5% 4 4 4 6% 19%	2345
	21st to end	Day	26% 12% 3 9% 6% 5% 5% 4 5% 7% 19%	2470
May	1st to 10th	Day	21% 9% 3 7% 4 3 4 3 4 5% 36%	4344
	11th to 20th	Day	15% 10% 3 7% 5% 4 5% 4 7% 8% 31%	4377
	21st to end	Day	28% 10% 3 9% 5% 5% 5% 4 6% 7% 19%	4813
June	1st to 10th	Day	26% 11% 3 8% 4 4 4 3 6% 7% 23%	2793
	11th to 20th	Day	24% 12% 4 8% 5% 5% 5% 4 6% 7% 20%	2850
	21st to end	Day	18% 11% 3 8% 5% 4 4 4 5% 8% 31%	2850
July	1st to 10th	Day	9% 9% 2 5% 3 3 3 3 4 7% 53%	4740
	11th to 20th	Day	9% 10% 3 5% 4 3 3 2 4 7% 49%	4682
	21st to end	Day	15% 11% 3 6% 3 3 3 3 5% 8% 38%	4790
August	1st to 10th	Day	8% 11% 3 7% 4 4 4 4 5% 10% 39%	4128
	11th to 20th	Day	14% 11% 4 9% 5% 5% 6% 5% 6% 7% 29%	4026
	21st to end	Day	18% 9% 3 6% 4 4 3 3 5% 6% 39%	4414
September	1st to 10th	Day	14% 10% 3 6% 4 3 4 3 4 7% 44%	2280
	11th to 20th	Day	17% 10% 3 8% 4 4 4 3 5% 7% 33%	2221
	21st to end	Day	19% 7% 3 7% 4 5% 5% 3 5% 7% 36%	2036
October	1st to 10th	Day	18% 7% 2 6% 3 4 4 4 7% 10% 35%	3180
	11th to 20th	Day	33% 8% 3 6% 4 3 4 3 5% 6% 25%	3161
	21st to end	Day	25% 10% 3 8% 5% 5% 5% 4 6% 7% 21%	3365
November	1st to 10th	Day	36% 13% 3 8% 4 4 4 3 5% 4 14%	1544
	11th to 20th	Day	37% 10% 3 7% 4 3 4 3 4 6% 18%	1520
	21st to end	Day	36% 11% 3 6% 5% 4 3 3 5% 4 20%	1520
December	1st to 10th	Day	30% 13% 3 8% 5% 3 4 4 4 5% 22%	1520
	11th to 20th	Day	40% 11% 3 7% 4 4 4 3 6% 6% 11%	1804
	21st to end	Day	36% 16% 5% 8% 4 4 4 4 4 4 11%	2442
January	1st to 10th	Night	39% 16% 5% 9% 5% 4 3 2 3 3 10%	4726
	11th to 20th	Night	34% 12% 3 7% 5% 4 3 3 4 5% 20%	4565
	21st to end	Night	50% 12% 3 6% 3 3 3 2 3 3 11%	4610
February	1st to 10th	Night	34% 15% 3 9% 4 4 4 3 4 6% 15%	2470
	11th to 20th	Night	31% 17% 4 8% 5% 4 3 2 3 4 21%	2402
	21st to end	Night	32% 16% 4 7% 5% 4 3 3 3 5% 17%	2034
March	1st to 10th	Night	27% 17% 3 8% 5% 5% 5% 3 5% 6% 16%	3506
	11th to 20th	Night	22% 15% 3 6% 4 3 3 3 3 6% 32%	3489
	21st to end	Night	32% 19% 4 9% 5% 3 3 2 3 3 16%	3804
April	1st to 10th	Night	28% 16% 4 9% 6% 5% 4 3 3 5% 17%	1900
	11th to 20th	Night	24% 12% 4 8% 6% 4 3 3 4 6% 24%	1781
	21st to end	Night	26% 12% 4 9% 6% 4 4 3 3 6% 22%	1710
May	1st to 10th	Night	24% 11% 3 5% 3 2 2 2 3 4 40%	2618
	11th to 20th	Night	15% 12% 4 8% 5% 4 4 4 4 7% 32%	2516
	21st to end	Night	21% 19% 4 9% 4 4 3 2 3 4 26%	2762
June	1st to 10th	Night	24% 13% 4 8% 4 3 3 3 3 7% 29%	1382
	11th to 20th	Night	19% 13% 5% 8% 4 5% 4 3 5% 7% 26%	1330
	21st to end	Night	15% 16% 3 6% 4 3 3 2 4 7% 36%	1330
July	1st to 10th	Night	8% 12% 4 4 3 3 3 2 3 4 55%	2220
	11th to 20th	Night	7% 15% 4 7% 4 3 3 2 4 5% 46%	2274
	21st to end	Night	13% 15% 3 6% 3 2 3 2 3 5% 45%	2762
August	1st to 10th	Night	8% 23% 6% 9% 5% 3 3 2 4 5% 33%	2808
	11th to 20th	Night	17% 15% 4 7% 3 3 3 2 4 6% 36%	2812
	21st to end	Night	18% 10% 3 6% 3 3 2 3 3 6% 42%	3256
September	1st to 10th	Night	11% 10% 2 5% 3 3 2 3 4 6% 50%	1900
	11th to 20th	Night	18% 11% 4 7% 5% 3 4 3 4 6% 34%	1957
	21st to end	Night	21% 9% 2 7% 4 3 3 3 4 6% 38%	2143
October	1st to 10th	Night	14% 7% 3 7% 4 3 4 4 6% 9% 39%	3800
	11th to 20th	Night	30% 11% 3 7% 4 4 3 3 4 5% 26%	3777
	21st to end	Night	26% 9% 4 8% 5% 5% 5% 3 6% 6% 23%	4198
November	1st to 10th	Night	36% 11% 3 7% 4 5% 4 3 4 4 18%	2694
	11th to 20th	Night	35% 12% 3 7% 4 4 3 3 3 4 20%	2660
	21st to end	Night	31% 11% 3 8% 5% 4 3 3 4 5% 22%	2660
December	1st to 10th	Night	30% 12% 3 7% 5% 4 4 3 3 4 24%	2660
	11th to 20th	Night	35% 14% 4 9% 5% 4 4 3 4 5% 14%	3388
	21st to end	Night	38% 17% 4 9% 4 3 3 3 3 3 11%	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	79% 7% 6% 5% 3 1	2219
	11th to 20th	Day	70% 4 9% 11% 3 2	2352
	21st to end	Day	81% 5% 5% 6% 3	3050
February	1st to 10th	Day	72% 7% 9% 8% 3	1710
	11th to 20th	Day	71% 4 9% 10% 4 1	1674
	21st to end	Day	72% 6% 8% 7% 5% 1	1419
March	1st to 10th	Day	67% 9% 13% 9% 2	3354
	11th to 20th	Day	56% 7% 11% 20% 5% 1	3490
	21st to end	Day	70% 8% 12% 9% 1	3848
April	1st to 10th	Day	70% 9% 10% 9% 1	2280
	11th to 20th	Day	65% 7% 11% 10% 7% 1	2345
	21st to end	Day	63% 8% 12% 12% 4 1	2470
May	1st to 10th	Day	48% 9% 17% 17% 9%	4344
	11th to 20th	Day	44% 13% 22% 16% 4	4380
	21st to end	Day	61% 13% 15% 8% 2	4814
June	1st to 10th	Day	59% 8% 16% 11% 6%	2793
	11th to 20th	Day	61% 10% 14% 13% 3	2850
	21st to end	Day	50% 10% 17% 15% 6% 2	2850
July	1st to 10th	Day	30% 5% 8% 17% 27% 12%	4666
	11th to 20th	Day	33% 5% 10% 23% 27% 1	4601
	21st to end	Day	41% 7% 11% 28% 13% 2	4639
August	1st to 10th	Day	35% 7% 12% 27% 18% 1	4111
	11th to 20th	Day	50% 10% 15% 17% 7% 2	4065
	21st to end	Day	45% 6% 12% 24% 12%	4413
September	1st to 10th	Day	42% 6% 13% 21% 13% 5%	2280
	11th to 20th	Day	49% 9% 11% 16% 13% 2	2221
	21st to end	Day	47% 8% 17% 15% 10% 3	2036
October	1st to 10th	Day	43% 7% 16% 28% 6%	3180
	11th to 20th	Day	60% 5% 12% 17% 6%	3161
	21st to end	Day	61% 7% 12% 11% 9% 1	3365
November	1st to 10th	Day	73% 6% 10% 8% 3	1544
	11th to 20th	Day	68% 6% 9% 9% 6% 2	1520
	21st to end	Day	66% 5% 7% 12% 9%	1520
December	1st to 10th	Day	65% 7% 8% 12% 8% 1	1520
	11th to 20th	Day	72% 6% 10% 9% 2	1806
	21st to end	Day	75% 7% 9% 6% 3	2442
January	1st to 10th	Night	79% 6% 6% 5% 3 1	4726
	11th to 20th	Night	64% 7% 9% 15% 4 1	4605
	21st to end	Night	80% 4 5% 6% 4 1	4610
February	1st to 10th	Night	71% 6% 9% 10% 3	2470
	11th to 20th	Night	67% 6% 10% 9% 7% 1	2404
	21st to end	Night	69% 6% 10% 10% 4 1	2034
March	1st to 10th	Night	65% 8% 11% 13% 2	3506
	11th to 20th	Night	53% 7% 12% 21% 6%	3490
	21st to end	Night	72% 8% 10% 8% 2	3804
April	1st to 10th	Night	69% 7% 12% 9% 2	1900
	11th to 20th	Night	61% 7% 11% 12% 7% 1	1781
	21st to end	Night	63% 7% 11% 12% 6% 1	1710
May	1st to 10th	Night	46% 6% 15% 25% 8%	2618
	11th to 20th	Night	44% 12% 20% 16% 7%	2516
	21st to end	Night	60% 11% 16% 12% 2	2762
June	1st to 10th	Night	56% 7% 14% 18% 5%	1382
	11th to 20th	Night	53% 10% 17% 17% 2	1330
	21st to end	Night	47% 7% 21% 17% 6% 2	1330
July	1st to 10th	Night	30% 5% 10% 21% 20% 13%	2185
	11th to 20th	Night	37% 5% 11% 24% 23% 1	2232
	21st to end	Night	38% 5% 13% 29% 14% 2	2674
August	1st to 10th	Night	50% 5% 13% 21% 10% 1	2800
	11th to 20th	Night	48% 6% 20% 17% 7% 2	2839
	21st to end	Night	43% 5% 13% 26% 12% 1	3256
September	1st to 10th	Night	34% 7% 14% 26% 15% 5%	1900
	11th to 20th	Night	49% 8% 13% 15% 10% 4	1957
	21st to end	Night	49% 6% 10% 20% 13% 3	2143
October	1st to 10th	Night	41% 6% 14% 31% 7% 1	3800
	11th to 20th	Night	61% 4 10% 18% 6%	3777
	21st to end	Night	61% 7% 10% 13% 10% 1	4198
November	1st to 10th	Night	68% 7% 9% 11% 4	2694
	11th to 20th	Night	69% 6% 9% 9% 5% 2	2660
	21st to end	Night	65% 5% 9% 10% 9% 1	2660
December	1st to 10th	Night	64% 7% 9% 11% 8% 1	2660
	11th to 20th	Night	72% 7% 7% 10% 3 1	3385
	21st to end	Night	77% 7% 8% 6% 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	47% 14% 26% 11% 2 1	180
	11th to 20th	Day	48% 13% 20% 14% 3 1	180
	21st to end	Day	44% 9% 19% 19% 8% 1	350
February	1st to 10th	Day	42% 13% 19% 19% 5% 1	370
	11th to 20th	Day	42% 13% 24% 15% 4 1	365
	21st to end	Day	36% 15% 26% 15% 7%	311
March	1st to 10th	Day	37% 15% 22% 20% 6% 1	460
	11th to 20th	Day	32% 15% 23% 17% 9% 4	460
	21st to end	Day	38% 17% 19% 17% 8% 1	506
April	1st to 10th	Day	36% 16% 19% 20% 6% 2	460
	11th to 20th	Day	31% 14% 28% 17% 8% 3	452
	21st to end	Day	31% 16% 26% 19% 6% 2	460
May	1st to 10th	Day	22% 15% 23% 27% 11% 3	463
	11th to 20th	Day	30% 13% 24% 20% 9% 4	488
	21st to end	Day	26% 12% 26% 23% 11% 3	529
June	1st to 10th	Day	26% 12% 26% 23% 10% 3	623
	11th to 20th	Day	23% 16% 32% 19% 8% 1	680
	21st to end	Day	18% 12% 28% 27% 13% 3	680
July	1st to 10th	Day	19% 7% 27% 34% 11% 3	671
	11th to 20th	Day	11% 8% 29% 34% 11% 6%	636
	21st to end	Day	11% 6% 24% 33% 18% 7%	531
August	1st to 10th	Day	16% 5% 21% 33% 18% 6%	487
	11th to 20th	Day	12% 5% 21% 34% 21% 8%	477
	21st to end	Day	17% 9% 23% 30% 17% 4	539
September	1st to 10th	Day	17% 7% 23% 33% 14% 5%	490
	11th to 20th	Day	22% 9% 19% 26% 19% 4	490
	21st to end	Day	21% 9% 23% 29% 15% 3	490
October	1st to 10th	Day	26% 10% 20% 24% 16% 4	490
	11th to 20th	Day	32% 10% 20% 22% 14% 3	490
	21st to end	Day	34% 15% 22% 18% 10% 2	501
November	1st to 10th	Day	43% 11% 24% 15% 5% 2	194
	11th to 20th	Day	41% 8% 21% 23% 6% 1	180
	21st to end	Day	37% 14% 14% 24% 11% 1	180
December	1st to 10th	Day	43% 13% 21% 17% 6%	180
	11th to 20th	Day	44% 13% 20% 17% 6%	180
	21st to end	Day	42% 12% 28% 16% 2 1	198
January	1st to 10th	Night	52% 13% 16% 15% 5%	850
	11th to 20th	Night	45% 8% 17% 23% 7%	810
	21st to end	Night	42% 9% 17% 23% 8%	836
February	1st to 10th	Night	40% 11% 19% 22% 7% 1	760
	11th to 20th	Night	39% 11% 18% 23% 8%	744
	21st to end	Night	39% 12% 19% 21% 8% 1	628
March	1st to 10th	Night	38% 11% 20% 19% 11% 2	760
	11th to 20th	Night	39% 12% 19% 20% 9% 2	760
	21st to end	Night	38% 12% 17% 21% 10% 2	836
April	1st to 10th	Night	35% 10% 19% 22% 13% 1	760
	11th to 20th	Night	30% 8% 22% 25% 12% 2	653
	21st to end	Night	28% 13% 19% 26% 13% 2	570
May	1st to 10th	Night	25% 8% 18% 29% 18% 3	432
	11th to 20th	Night	30% 6% 14% 26% 21% 2	374
	21st to end	Night	25% 10% 19% 25% 18% 2	411
June	1st to 10th	Night	27% 6% 19% 28% 18% 1	380
	11th to 20th	Night	19% 10% 22% 33% 14% 1	380
	21st to end	Night	19% 9% 16% 30% 23% 3	380
July	1st to 10th	Night	14% 4 16% 33% 30% 2	370
	11th to 20th	Night	16% 3 18% 33% 26% 4	368
	21st to end	Night	13% 6% 23% 25% 31% 3	416
August	1st to 10th	Night	21% 3 13% 26% 29% 8%	540
	11th to 20th	Night	14% 3 11% 26% 33% 13%	564
	21st to end	Night	20% 6% 19% 28% 23% 5%	685
September	1st to 10th	Night	16% 4 14% 32% 28% 6%	760
	11th to 20th	Night	24% 5% 13% 26% 26% 7%	760
	21st to end	Night	24% 5% 17% 32% 19% 3	760
October	1st to 10th	Night	23% 5% 14% 32% 24% 3	760
	11th to 20th	Night	33% 5% 12% 28% 19% 3	760
	21st to end	Night	34% 7% 19% 27% 12% 1	836
November	1st to 10th	Night	41% 9% 19% 22% 8%	770
	11th to 20th	Night	41% 11% 20% 24% 4 1	760
	21st to end	Night	35% 10% 22% 24% 8% 1	760
December	1st to 10th	Night	36% 10% 21% 25% 8% 1	760
	11th to 20th	Night	44% 12% 21% 17% 6% 1	793
	21st to end	Night	50% 12% 16% 16% 4	935

Page generated at 2024-11-05 05:15:49. Version ac07

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