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 2025-11-18.

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	23% 11% 3 8% 4 4 4 3 5% 6% 28%	131181
	Night	26% 14% 4 8% 4 4 4 3 4 5% 26%	127888
Transparency	Day	55% 7% 13% 16% 8% 1	130891
	Night	59% 6% 11% 16% 7% 1	127755
Seeing	Day	27% 12% 24% 24% 11% 3	16740
	Night	33% 9% 18% 25% 14% 2	24983

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	39% 15% 5% 10% 5% 4 3 2 3 4 9%	3179
	11th to 20th	Day	25% 12% 3 6% 4 4 4 4 5% 7% 27%	3360
	21st to end	Day	39% 10% 3 6% 4 3 2 2 3 4 24%	4372
February	1st to 10th	Day	36% 14% 4 8% 4 5% 4 3 4 5% 14%	1800
	11th to 20th	Day	36% 14% 4 8% 4 4 3 3 3 5% 18%	1755
	21st to end	Day	32% 18% 4 8% 5% 5% 3 3 4 4 13%	1492
March	1st to 10th	Day	25% 17% 4 10% 6% 5% 5% 4 6% 6% 12%	4825
	11th to 20th	Day	28% 14% 4 9% 4 5% 4 3 4 5% 21%	5000
	21st to end	Day	30% 15% 4 10% 6% 6% 5% 4 5% 5% 10%	5522
April	1st to 10th	Day	33% 12% 4 9% 6% 5% 5% 3 5% 6% 13%	2400
	11th to 20th	Day	27% 11% 4 9% 5% 4 4 4 4 6% 21%	2470
	21st to end	Day	25% 12% 4 9% 6% 5% 5% 4 5% 7% 21%	2600
May	1st to 10th	Day	19% 8% 2 7% 4 4 4 3 4 5% 39%	6205
	11th to 20th	Day	17% 13% 4 8% 5% 5% 5% 5% 7% 7% 23%	6277
	21st to end	Day	22% 9% 3 8% 4 4 5% 4 7% 9% 26%	6903
June	1st to 10th	Day	25% 11% 3 8% 4 4 4 3 6% 7% 26%	2940
	11th to 20th	Day	23% 12% 4 9% 5% 5% 5% 4 6% 7% 21%	3000
	21st to end	Day	18% 11% 4 8% 5% 4 4 4 5% 7% 30%	3000
July	1st to 10th	Day	9% 7% 2 5% 3 4 3 3 5% 8% 52%	6780
	11th to 20th	Day	7% 9% 3 5% 3 3 3 3 4 7% 52%	6694
	21st to end	Day	12% 15% 3 7% 4 3 3 3 5% 8% 37%	6874
August	1st to 10th	Day	8% 12% 3 7% 4 4 4 4 5% 10% 39%	4259
	11th to 20th	Day	14% 11% 4 9% 5% 5% 6% 5% 6% 7% 29%	4156
	21st to end	Day	18% 9% 3 6% 4 4 3 3 5% 7% 39%	4554
September	1st to 10th	Day	13% 11% 3 6% 4 3 4 3 4 7% 42%	2400
	11th to 20th	Day	17% 10% 4 8% 4 4 4 3 5% 7% 34%	2338
	21st to end	Day	19% 7% 3 7% 4 5% 4 3 5% 7% 36%	2143
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 4 7% 10% 35%	3280
	11th to 20th	Day	32% 8% 3 6% 4 3 4 3 5% 6% 25%	3261
	21st to end	Day	25% 10% 3 8% 5% 5% 5% 4 6% 7% 21%	3472
November	1st to 10th	Day	36% 14% 3 9% 4 4 4 3 5% 4 14%	1648
	11th to 20th	Day	38% 10% 3 7% 4 3 4 3 4 6% 17%	1656
	21st to end	Day	36% 11% 4 6% 5% 4 3 3 5% 4 20%	1600
December	1st to 10th	Day	24% 9% 3 8% 5% 4 5% 4 4 5% 29%	2620
	11th to 20th	Day	42% 11% 3 9% 5% 5% 4 3 5% 5% 8%	2848
	21st to end	Day	38% 15% 4 9% 5% 4 4 3 4 4 9%	3498
January	1st to 10th	Night	39% 16% 5% 9% 4 4 3 2 4 4 11%	6766
	11th to 20th	Night	24% 11% 3 7% 4 4 4 3 5% 7% 29%	6535
	21st to end	Night	38% 11% 3 5% 3 2 3 2 3 4 27%	6583
February	1st to 10th	Night	32% 15% 3 9% 5% 4 4 3 4 6% 15%	2600
	11th to 20th	Night	31% 16% 4 8% 4 4 3 2 3 4 21%	2532
	21st to end	Night	33% 15% 4 7% 5% 4 3 2 3 5% 17%	2137
March	1st to 10th	Night	25% 16% 3 9% 5% 5% 5% 4 6% 6% 16%	5055
	11th to 20th	Night	25% 16% 3 8% 5% 4 4 3 3 5% 24%	4999
	21st to end	Night	37% 17% 3 9% 5% 4 3 3 4 4 12%	5452
April	1st to 10th	Night	28% 16% 4 9% 6% 5% 4 3 3 5% 17%	2000
	11th to 20th	Night	23% 12% 4 8% 6% 4 3 3 4 6% 25%	1876
	21st to end	Night	25% 13% 4 9% 6% 4 4 3 4 6% 23%	1800
May	1st to 10th	Night	18% 9% 3 6% 4 3 2 2 3 5% 43%	3746
	11th to 20th	Night	19% 14% 5% 10% 6% 5% 5% 4 4 6% 23%	3616
	21st to end	Night	19% 16% 4 8% 4 4 4 3 5% 6% 28%	3972
June	1st to 10th	Night	22% 13% 4 8% 4 3 3 3 3 6% 31%	1455
	11th to 20th	Night	19% 13% 5% 8% 4 5% 4 3 5% 7% 27%	1400
	21st to end	Night	15% 16% 3 7% 4 3 3 2 4 7% 36%	1400
July	1st to 10th	Night	8% 11% 3 4 3 3 3 2 3 5% 58%	3180
	11th to 20th	Night	6% 14% 4 7% 4 3 3 3 4 4 49%	3262
	21st to end	Night	10% 22% 4 6% 3 2 4 2 3 4 40%	3969
August	1st to 10th	Night	8% 23% 6% 9% 5% 3 3 2 4 5% 33%	2897
	11th to 20th	Night	17% 15% 4 7% 3 3 3 2 4 6% 36%	2902
	21st to end	Night	18% 11% 3 6% 3 3 2 3 3 6% 42%	3358
September	1st to 10th	Night	11% 11% 2 5% 3 3 2 3 4 6% 48%	2000
	11th to 20th	Night	18% 11% 4 7% 5% 3 4 3 4 6% 34%	2060
	21st to end	Night	22% 9% 2 7% 4 3 2 3 4 5% 38%	2256
October	1st to 10th	Night	15% 7% 3 7% 4 3 4 4 6% 9% 39%	3920
	11th to 20th	Night	29% 11% 3 7% 4 4 3 3 4 5% 27%	3897
	21st to end	Night	26% 9% 3 8% 5% 5% 5% 4 6% 6% 23%	4333
November	1st to 10th	Night	36% 11% 3 7% 4 5% 4 3 4 4 17%	2870
	11th to 20th	Night	37% 13% 3 7% 4 4 3 3 3 3 19%	2898
	21st to end	Night	31% 11% 4 9% 5% 4 3 3 4 5% 21%	2800
December	1st to 10th	Night	24% 10% 3 7% 5% 4 5% 3 4 5% 31%	4560
	11th to 20th	Night	43% 14% 4 9% 5% 3 4 2 3 3 10%	5344
	21st to end	Night	40% 16% 4 9% 4 3 3 3 3 3 9%	7458

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	78% 6% 5% 7% 3 1	3179
	11th to 20th	Day	57% 5% 10% 21% 5% 1	3382
	21st to end	Day	66% 4 8% 13% 9%	4372
February	1st to 10th	Day	72% 7% 9% 8% 4	1800
	11th to 20th	Day	72% 4 8% 10% 5% 1	1764
	21st to end	Day	73% 6% 8% 8% 5% 1	1492
March	1st to 10th	Day	70% 8% 11% 9% 2	4825
	11th to 20th	Day	64% 7% 9% 16% 4	5000
	21st to end	Day	73% 8% 12% 7% 1	5522
April	1st to 10th	Day	70% 9% 11% 9% 1	2400
	11th to 20th	Day	63% 7% 11% 12% 6% 1	2470
	21st to end	Day	62% 8% 13% 13% 4 1	2600
May	1st to 10th	Day	46% 7% 17% 19% 11%	6205
	11th to 20th	Day	52% 12% 19% 14% 3	6280
	21st to end	Day	53% 11% 19% 14% 3	6904
June	1st to 10th	Day	57% 8% 16% 13% 6%	2940
	11th to 20th	Day	60% 9% 13% 14% 3	3000
	21st to end	Day	50% 10% 17% 15% 6% 2	3000
July	1st to 10th	Day	28% 5% 20% 18% 21% 8%	6706
	11th to 20th	Day	30% 4 12% 25% 27% 1	6613
	21st to end	Day	41% 6% 15% 27% 9% 1	6681
August	1st to 10th	Day	36% 7% 12% 27% 17% 1	4242
	11th to 20th	Day	50% 10% 15% 17% 7% 2	4195
	21st to end	Day	45% 6% 12% 24% 12%	4553
September	1st to 10th	Day	43% 6% 14% 20% 12% 5%	2400
	11th to 20th	Day	49% 9% 11% 17% 13% 2	2338
	21st to end	Day	47% 8% 16% 16% 10% 3	2143
October	1st to 10th	Day	43% 7% 16% 28% 6%	3280
	11th to 20th	Day	59% 5% 11% 17% 6%	3261
	21st to end	Day	61% 7% 12% 11% 9% 1	3472
November	1st to 10th	Day	73% 6% 11% 8% 3	1648
	11th to 20th	Day	69% 6% 8% 9% 5% 2	1656
	21st to end	Day	67% 5% 7% 11% 9%	1600
December	1st to 10th	Day	57% 6% 5% 17% 14% 1	2620
	11th to 20th	Day	77% 6% 9% 6% 1	2850
	21st to end	Day	78% 7% 9% 5% 2	3498
January	1st to 10th	Night	78% 5% 7% 7% 3 1	6766
	11th to 20th	Night	54% 6% 10% 23% 6% 1	6575
	21st to end	Night	64% 3 6% 15% 10% 1	6583
February	1st to 10th	Night	70% 6% 9% 10% 4	2600
	11th to 20th	Night	67% 6% 9% 9% 7% 1	2534
	21st to end	Night	70% 5% 10% 10% 4 1	2137
March	1st to 10th	Night	64% 8% 11% 14% 3	5055
	11th to 20th	Night	62% 6% 10% 17% 5%	5000
	21st to end	Night	75% 7% 11% 6% 1	5452
April	1st to 10th	Night	68% 7% 13% 10% 2	2000
	11th to 20th	Night	60% 7% 12% 14% 7% 1	1876
	21st to end	Night	62% 7% 12% 13% 6% 1	1800
May	1st to 10th	Night	44% 5% 15% 27% 9%	3743
	11th to 20th	Night	56% 10% 16% 12% 5%	3616
	21st to end	Night	55% 10% 18% 15% 1	3972
June	1st to 10th	Night	54% 7% 15% 19% 5%	1455
	11th to 20th	Night	52% 10% 17% 19% 2	1400
	21st to end	Night	47% 7% 21% 17% 6% 2	1400
July	1st to 10th	Night	27% 4 18% 25% 17% 9%	3145
	11th to 20th	Night	35% 4 13% 25% 23% 1	3220
	21st to end	Night	42% 4 15% 27% 10% 1	3857
August	1st to 10th	Night	50% 5% 13% 21% 10% 1	2889
	11th to 20th	Night	48% 6% 20% 17% 7% 2	2929
	21st to end	Night	43% 5% 13% 26% 12% 1	3358
September	1st to 10th	Night	36% 7% 14% 25% 14% 4	2000
	11th to 20th	Night	49% 8% 13% 15% 10% 4	2060
	21st to end	Night	49% 5% 10% 20% 13% 3	2256
October	1st to 10th	Night	41% 6% 15% 31% 7% 1	3920
	11th to 20th	Night	60% 4 10% 18% 6%	3897
	21st to end	Night	61% 7% 10% 13% 9% 1	4333
November	1st to 10th	Night	69% 7% 9% 11% 4	2870
	11th to 20th	Night	71% 5% 9% 9% 4 2	2898
	21st to end	Night	66% 5% 9% 10% 9% 1	2800
December	1st to 10th	Night	56% 5% 8% 18% 13% 1	4560
	11th to 20th	Night	80% 5% 6% 7% 2	5341
	21st to end	Night	78% 7% 7% 5% 2	7458

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	48% 15% 26% 10% 2 1	200
	11th to 20th	Day	44% 13% 22% 18% 3 1	200
	21st to end	Day	42% 8% 21% 19% 10% 1	380
February	1st to 10th	Day	40% 15% 20% 19% 5% 1	400
	11th to 20th	Day	43% 13% 25% 14% 4 1	395
	21st to end	Day	38% 15% 25% 15% 6%	336
March	1st to 10th	Day	37% 14% 21% 20% 7% 1	500
	11th to 20th	Day	33% 16% 23% 16% 8% 4	500
	21st to end	Day	39% 17% 19% 17% 7% 1	550
April	1st to 10th	Day	36% 15% 20% 21% 6% 1	500
	11th to 20th	Day	29% 13% 29% 18% 8% 3	492
	21st to end	Day	29% 16% 26% 20% 7% 2	500
May	1st to 10th	Day	21% 16% 23% 28% 10% 3	503
	11th to 20th	Day	31% 13% 24% 19% 9% 4	528
	21st to end	Day	25% 12% 27% 23% 10% 3	573
June	1st to 10th	Day	25% 12% 27% 23% 10% 3	670
	11th to 20th	Day	22% 17% 32% 19% 8% 1	730
	21st to end	Day	18% 11% 29% 27% 12% 3	730
July	1st to 10th	Day	18% 7% 27% 34% 10% 3	721
	11th to 20th	Day	11% 8% 30% 34% 11% 6%	684
	21st to end	Day	11% 6% 24% 34% 18% 7%	575
August	1st to 10th	Day	17% 6% 21% 32% 18% 6%	527
	11th to 20th	Day	13% 5% 21% 33% 20% 7%	517
	21st to end	Day	17% 9% 22% 30% 17% 4	583
September	1st to 10th	Day	17% 7% 23% 33% 15% 5%	530
	11th to 20th	Day	22% 10% 19% 26% 18% 4	530
	21st to end	Day	21% 10% 23% 28% 15% 3	530
October	1st to 10th	Day	25% 10% 21% 24% 16% 4	530
	11th to 20th	Day	32% 11% 20% 21% 13% 2	530
	21st to end	Day	35% 15% 22% 17% 9% 2	542
November	1st to 10th	Day	44% 11% 23% 16% 4 1	220
	11th to 20th	Day	45% 8% 19% 21% 6% 1	214
	21st to end	Day	37% 13% 14% 24% 12% 1	200
December	1st to 10th	Day	41% 12% 23% 19% 6%	200
	11th to 20th	Day	46% 13% 21% 16% 5%	200
	21st to end	Day	45% 13% 26% 14% 2	220
January	1st to 10th	Night	52% 13% 16% 14% 4	900
	11th to 20th	Night	43% 8% 18% 25% 6%	855
	21st to end	Night	41% 8% 18% 24% 9%	880
February	1st to 10th	Night	39% 12% 19% 22% 7% 1	800
	11th to 20th	Night	40% 11% 19% 22% 8%	784
	21st to end	Night	39% 12% 20% 20% 8% 1	660
March	1st to 10th	Night	38% 10% 20% 20% 11% 2	800
	11th to 20th	Night	39% 12% 19% 20% 8% 2	800
	21st to end	Night	40% 11% 17% 20% 10% 2	880
April	1st to 10th	Night	35% 10% 19% 23% 12% 1	800
	11th to 20th	Night	29% 8% 22% 26% 12% 2	688
	21st to end	Night	28% 12% 18% 26% 13% 2	600
May	1st to 10th	Night	24% 7% 19% 30% 18% 3	454
	11th to 20th	Night	31% 7% 15% 26% 20% 2	394
	21st to end	Night	25% 10% 19% 25% 19% 2	433
June	1st to 10th	Night	26% 7% 19% 28% 19% 1	400
	11th to 20th	Night	19% 10% 22% 33% 15% 1	400
	21st to end	Night	19% 8% 16% 31% 24% 3	400
July	1st to 10th	Night	14% 5% 17% 34% 29% 2	390
	11th to 20th	Night	15% 3 19% 34% 25% 4	388
	21st to end	Night	12% 6% 23% 25% 32% 3	438
August	1st to 10th	Night	22% 3 13% 27% 28% 8%	569
	11th to 20th	Night	15% 3 12% 25% 32% 13%	594
	21st to end	Night	19% 5% 18% 28% 24% 5%	721
September	1st to 10th	Night	16% 5% 14% 32% 28% 6%	800
	11th to 20th	Night	24% 5% 14% 26% 26% 7%	800
	21st to end	Night	24% 5% 17% 32% 19% 3	800
October	1st to 10th	Night	23% 5% 14% 32% 24% 3	800
	11th to 20th	Night	32% 6% 13% 28% 18% 3	800
	21st to end	Night	34% 8% 19% 27% 11% 1	880
November	1st to 10th	Night	41% 10% 20% 21% 8%	820
	11th to 20th	Night	43% 11% 19% 23% 4	828
	21st to end	Night	36% 10% 21% 24% 8% 1	800
December	1st to 10th	Night	35% 10% 20% 27% 8% 1	800
	11th to 20th	Night	45% 12% 20% 16% 5% 1	837
	21st to end	Night	50% 13% 17% 15% 4	990

Page generated at 2025-11-14 21:55:56. Version ac07

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