Over The Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Over The Hill Observatory clear sky chart from 2005-11-06 to 2024-11-03.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	101126
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 44%	97794
Transparency	Day	39% 7% 13% 23% 15% 3	100933
	Night	38% 6% 13% 23% 16% 3	97658
Seeing	Day	19% 12% 27% 24% 13% 5%	14964
	Night	17% 10% 28% 31% 13% 1	24572

By Month

Percentage Cloud Cover SE (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	16% 5% 2 5% 4 3 4 4 5% 6% 46%	2741
	11th to 20th	Day	11% 11% 3 8% 5% 4 4 4 5% 6% 39%	2840
	21st to end	Day	15% 9% 3 8% 5% 5% 4 4 5% 6% 38%	3124
February	1st to 10th	Day	12% 8% 3 6% 5% 5% 4 5% 6% 6% 39%	1710
	11th to 20th	Day	16% 8% 2 6% 4 4 3 4 5% 7% 40%	1667
	21st to end	Day	17% 8% 3 7% 4 3 3 4 5% 6% 40%	1599
March	1st to 10th	Day	19% 8% 3 6% 4 4 5% 4 4 6% 37%	3435
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 6% 7% 51%	3480
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 6% 39%	3815
April	1st to 10th	Day	11% 6% 2 5% 6% 5% 4 4 6% 7% 43%	2270
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 5% 48%	2254
	21st to end	Day	17% 6% 2 7% 5% 4 4 4 5% 6% 40%	2300
May	1st to 10th	Day	20% 7% 2 7% 6% 5% 5% 5% 6% 8% 29%	4082
	11th to 20th	Day	22% 7% 3 6% 4 4 5% 4 6% 8% 31%	4130
	21st to end	Day	11% 5% 2 5% 4 4 5% 5% 7% 10% 42%	4823
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 9% 42%	2660
	11th to 20th	Day	10% 7% 1 7% 5% 4 6% 5% 7% 10% 38%	2660
	21st to end	Day	9% 4 2 5% 5% 5% 5% 5% 7% 10% 43%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 6% 8% 11% 42%	4423
	11th to 20th	Day	8% 5% 2 6% 4 6% 6% 6% 9% 11% 38%	4120
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 4 8% 8% 35%	4466
August	1st to 10th	Day	13% 5% 2 8% 6% 5% 6% 5% 8% 10% 32%	4104
	11th to 20th	Day	9% 4 2 6% 4 5% 6% 5% 8% 9% 40%	3953
	21st to end	Day	11% 4 2 5% 4 4 5% 4 6% 8% 48%	4180
September	1st to 10th	Day	13% 6% 3 6% 4 4 5% 4 5% 6% 45%	2280
	11th to 20th	Day	13% 6% 2 5% 3 3 5% 4 5% 8% 47%	2241
	21st to end	Day	22% 7% 2 6% 3 4 4 4 5% 6% 37%	2004
October	1st to 10th	Day	15% 6% 2 6% 3 4 4 4 5% 7% 45%	1900
	11th to 20th	Day	15% 5% 1 5% 3 4 4 3 4 6% 50%	1900
	21st to end	Day	14% 5% 2 4 3 3 4 3 4 6% 51%	2090
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 2 5% 6% 53%	1858
	11th to 20th	Day	14% 6% 1 4 4 3 4 3 5% 6% 50%	1635
	21st to end	Day	15% 4 2 4 4 4 3 4 5% 7% 49%	1520
December	1st to 10th	Day	15% 7% 2 6% 4 4 5% 5% 6% 7% 40%	1520
	11th to 20th	Day	15% 6% 2 5% 3 4 4 3 5% 6% 48%	1910
	21st to end	Day	19% 6% 2 5% 4 4 5% 5% 6% 8% 37%	2772
January	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 49%	4201
	11th to 20th	Night	16% 8% 2 5% 3 3 3 3 5% 6% 47%	4120
	21st to end	Night	17% 9% 3 6% 4 3 4 3 4 5% 43%	4532
February	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 6% 5% 42%	2470
	11th to 20th	Night	17% 7% 2 5% 4 3 5% 3 5% 5% 42%	2399
	21st to end	Night	15% 9% 3 5% 4 4 4 3 5% 7% 42%	1851
March	1st to 10th	Night	19% 7% 2 6% 4 5% 5% 3 6% 5% 38%	3418
	11th to 20th	Night	10% 5% 2 4 4 4 5% 3 5% 7% 52%	3480
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 6% 49%	3817
April	1st to 10th	Night	13% 6% 3 6% 5% 4 4 4 4 6% 43%	1910
	11th to 20th	Night	11% 5% 3 5% 4 3 4 4 5% 8% 48%	1875
	21st to end	Night	16% 4 2 5% 3 5% 4 5% 5% 6% 44%	1895
May	1st to 10th	Night	21% 7% 3 7% 5% 4 4 4 5% 8% 31%	2813
	11th to 20th	Night	25% 8% 2 6% 4 4 4 3 4 6% 33%	2761
	21st to end	Night	12% 4 2 5% 4 4 4 4 6% 9% 45%	2732
June	1st to 10th	Night	11% 5% 2 6% 3 4 4 4 7% 8% 45%	1520
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 4 8% 7% 43%	1520
	21st to end	Night	9% 3 2 6% 5% 5% 6% 5% 7% 9% 44%	1520
July	1st to 10th	Night	7% 4 2 5% 5% 6% 6% 5% 8% 12% 40%	2520
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 11% 39%	2838
	21st to end	Night	9% 7% 2 7% 5% 6% 5% 5% 8% 9% 37%	3087
August	1st to 10th	Night	16% 5% 3 6% 5% 6% 5% 5% 7% 10% 33%	2825
	11th to 20th	Night	9% 7% 2 7% 6% 6% 5% 4 6% 9% 40%	2844
	21st to end	Night	13% 4 1 4 4 3 4 3 6% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 6% 4 3 4 4 5% 8% 45%	1900
	11th to 20th	Night	13% 5% 2 6% 3 3 4 3 5% 7% 49%	1935
	21st to end	Night	23% 6% 2 5% 4 3 3 3 4 5% 42%	2175
October	1st to 10th	Night	18% 6% 2 6% 3 4 4 3 4 5% 47%	2280
	11th to 20th	Night	14% 4 2 5% 4 3 3 3 5% 6% 52%	2280
	21st to end	Night	14% 5% 2 4 3 4 3 3 4 6% 52%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	2262
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 4 6% 50%	2541
	21st to end	Night	14% 5% 2 4 4 4 4 4 5% 7% 48%	2660
December	1st to 10th	Night	15% 7% 2 6% 5% 5% 3 4 5% 6% 42%	2660
	11th to 20th	Night	15% 6% 2 5% 4 4 4 3 5% 7% 45%	3285
	21st to end	Night	16% 6% 2 5% 4 4 5% 4 6% 7% 39%	4884

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	37% 7% 9% 29% 14% 4	2742
	11th to 20th	Day	45% 5% 11% 17% 16% 7%	2840
	21st to end	Day	47% 4 11% 21% 14% 2	3124
February	1st to 10th	Day	43% 6% 12% 24% 13% 3	1710
	11th to 20th	Day	42% 5% 10% 22% 17% 3	1676
	21st to end	Day	44% 5% 13% 17% 16% 4	1599
March	1st to 10th	Day	46% 6% 11% 20% 14% 4	3435
	11th to 20th	Day	31% 6% 9% 26% 23% 6%	3480
	21st to end	Day	45% 6% 12% 19% 15% 4	3815
April	1st to 10th	Day	38% 6% 13% 18% 18% 7%	2270
	11th to 20th	Day	36% 6% 11% 21% 20% 6%	2254
	21st to end	Day	43% 6% 11% 21% 14% 4	2300
May	1st to 10th	Day	50% 8% 12% 19% 10% 2	4120
	11th to 20th	Day	47% 9% 12% 20% 10% 1	4134
	21st to end	Day	33% 9% 17% 20% 18% 3	4837
June	1st to 10th	Day	37% 8% 15% 23% 16% 1	2660
	11th to 20th	Day	38% 9% 15% 22% 13% 2	2660
	21st to end	Day	34% 9% 16% 24% 15% 2	2660
July	1st to 10th	Day	32% 12% 20% 21% 11% 5%	4353
	11th to 20th	Day	35% 9% 17% 28% 10% 1	4051
	21st to end	Day	44% 8% 10% 23% 15% 1	4324
August	1st to 10th	Day	43% 10% 17% 23% 6%	4091
	11th to 20th	Day	35% 9% 15% 27% 13% 1	3992
	21st to end	Day	33% 8% 15% 29% 14% 1	4180
September	1st to 10th	Day	38% 7% 13% 17% 20% 4	2280
	11th to 20th	Day	35% 6% 10% 27% 19% 3	2241
	21st to end	Day	47% 6% 9% 19% 16% 2	2004
October	1st to 10th	Day	38% 6% 11% 24% 18% 4	1900
	11th to 20th	Day	35% 5% 11% 24% 20% 5%	1900
	21st to end	Day	33% 4 10% 26% 23% 4	2090
November	1st to 10th	Day	32% 3 10% 25% 24% 5%	1858
	11th to 20th	Day	35% 5% 11% 21% 22% 6%	1635
	21st to end	Day	35% 5% 11% 27% 20% 3	1520
December	1st to 10th	Day	42% 6% 13% 25% 12% 3	1520
	11th to 20th	Day	38% 3 11% 31% 12% 5%	1906
	21st to end	Day	42% 7% 15% 25% 10% 1	2772
January	1st to 10th	Night	34% 5% 12% 26% 19% 4	4201
	11th to 20th	Night	38% 4 10% 23% 19% 5%	4120
	21st to end	Night	44% 4 10% 24% 17% 1	4532
February	1st to 10th	Night	41% 6% 13% 23% 15% 2	2470
	11th to 20th	Night	42% 5% 10% 21% 16% 6%	2401
	21st to end	Night	43% 5% 12% 21% 14% 5%	1851
March	1st to 10th	Night	44% 7% 11% 23% 13% 2	3418
	11th to 20th	Night	30% 6% 9% 25% 26% 4	3480
	21st to end	Night	36% 6% 11% 23% 20% 5%	3817
April	1st to 10th	Night	40% 6% 11% 18% 20% 4	1910
	11th to 20th	Night	34% 6% 14% 21% 20% 5%	1875
	21st to end	Night	38% 7% 13% 26% 13% 4	1895
May	1st to 10th	Night	49% 8% 13% 18% 10% 3	2840
	11th to 20th	Night	48% 9% 13% 20% 9% 1	2762
	21st to end	Night	33% 9% 18% 24% 12% 4	2739
June	1st to 10th	Night	33% 9% 14% 27% 15% 2	1520
	11th to 20th	Night	36% 9% 15% 24% 13% 2	1520
	21st to end	Night	33% 9% 16% 22% 18% 3	1520
July	1st to 10th	Night	34% 12% 23% 15% 14% 3	2480
	11th to 20th	Night	35% 11% 20% 22% 11% 2	2785
	21st to end	Night	40% 9% 16% 20% 14% 1	2988
August	1st to 10th	Night	44% 8% 15% 25% 7% 1	2814
	11th to 20th	Night	39% 8% 12% 25% 15% 1	2871
	21st to end	Night	33% 6% 15% 29% 16% 1	3476
September	1st to 10th	Night	37% 7% 13% 20% 18% 5%	1900
	11th to 20th	Night	34% 6% 10% 26% 20% 3	1935
	21st to end	Night	45% 5% 10% 21% 16% 3	2175
October	1st to 10th	Night	40% 5% 11% 23% 18% 3	2280
	11th to 20th	Night	33% 5% 10% 25% 23% 4	2280
	21st to end	Night	33% 4 10% 26% 23% 4	2508
November	1st to 10th	Night	33% 5% 10% 24% 22% 6%	2262
	11th to 20th	Night	36% 5% 7% 24% 22% 7%	2541
	21st to end	Night	34% 5% 12% 24% 20% 5%	2660
December	1st to 10th	Night	42% 5% 12% 24% 13% 3	2660
	11th to 20th	Night	39% 5% 11% 23% 19% 4	3288
	21st to end	Night	40% 8% 16% 22% 13% 1	4884

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	28% 26% 30% 13% 3	243
	11th to 20th	Day	34% 23% 27% 14% 2	270
	21st to end	Day	25% 27% 36% 9% 2 1	297
February	1st to 10th	Day	24% 27% 33% 13% 3	270
	11th to 20th	Day	37% 24% 33% 5%	274
	21st to end	Day	29% 19% 32% 17% 3	382
March	1st to 10th	Day	23% 22% 31% 20% 4	460
	11th to 20th	Day	24% 23% 29% 17% 5% 1	460
	21st to end	Day	25% 19% 35% 16% 4 1	506
April	1st to 10th	Day	22% 25% 27% 18% 7% 1	460
	11th to 20th	Day	19% 17% 29% 21% 11% 3	452
	21st to end	Day	26% 13% 33% 17% 10% 1	460
May	1st to 10th	Day	24% 14% 29% 20% 10% 3	463
	11th to 20th	Day	20% 11% 31% 23% 11% 4	487
	21st to end	Day	19% 6% 27% 25% 17% 6%	529
June	1st to 10th	Day	13% 10% 25% 29% 19% 4	490
	11th to 20th	Day	16% 8% 26% 29% 13% 7%	490
	21st to end	Day	10% 7% 28% 33% 17% 5%	490
July	1st to 10th	Day	10% 4 20% 32% 25% 9%	485
	11th to 20th	Day	6% 2 21% 33% 29% 9%	484
	21st to end	Day	11% 4 21% 33% 24% 7%	532
August	1st to 10th	Day	9% 6% 21% 30% 22% 12%	486
	11th to 20th	Day	11% 5% 20% 32% 21% 12%	479
	21st to end	Day	9% 3 19% 33% 23% 13%	539
September	1st to 10th	Day	18% 6% 16% 29% 18% 13%	490
	11th to 20th	Day	16% 4 20% 27% 23% 10%	490
	21st to end	Day	28% 9% 21% 24% 14% 3	490
October	1st to 10th	Day	20% 6% 23% 29% 16% 6%	490
	11th to 20th	Day	17% 14% 26% 26% 16% 2	490
	21st to end	Day	17% 15% 33% 22% 11% 3	539
November	1st to 10th	Day	16% 12% 31% 28% 10% 3	454
	11th to 20th	Day	22% 13% 30% 24% 10% 1	295
	21st to end	Day	20% 19% 33% 23% 5%	180
December	1st to 10th	Day	19% 21% 39% 14% 6% 1	180
	11th to 20th	Day	22% 17% 35% 18% 8%	180
	21st to end	Day	27% 16% 32% 19% 6%	198
January	1st to 10th	Night	18% 18% 39% 22% 4	760
	11th to 20th	Night	20% 14% 33% 29% 4	760
	21st to end	Night	18% 15% 42% 25% 2	836
February	1st to 10th	Night	16% 14% 40% 28% 3	760
	11th to 20th	Night	24% 20% 37% 18% 2	743
	21st to end	Night	22% 15% 40% 20% 3	628
March	1st to 10th	Night	18% 17% 37% 25% 3	760
	11th to 20th	Night	22% 15% 36% 22% 4 1	760
	21st to end	Night	18% 15% 38% 22% 5%	836
April	1st to 10th	Night	17% 19% 35% 24% 5%	760
	11th to 20th	Night	15% 12% 37% 27% 8% 1	751
	21st to end	Night	18% 11% 31% 32% 8%	755
May	1st to 10th	Night	20% 8% 26% 29% 16% 1	570
	11th to 20th	Night	20% 4 28% 32% 14% 1	536
	21st to end	Night	15% 2 19% 40% 21% 1	411
June	1st to 10th	Night	10% 2 22% 44% 21% 1	380
	11th to 20th	Night	13% 4 16% 43% 23% 2	380
	21st to end	Night	8% 2 17% 49% 22% 1	380
July	1st to 10th	Night	12% 4 15% 36% 29% 4	370
	11th to 20th	Night	6% 1 11% 37% 40% 4	551
	21st to end	Night	9% 3 13% 42% 30% 4	624
August	1st to 10th	Night	11% 3 9% 43% 31% 3	567
	11th to 20th	Night	12% 3 13% 39% 30% 3	597
	21st to end	Night	13% 2 14% 38% 29% 4	836
September	1st to 10th	Night	16% 3 14% 35% 28% 4	760
	11th to 20th	Night	14% 4 14% 37% 27% 4	760
	21st to end	Night	26% 4 18% 30% 21% 1	760
October	1st to 10th	Night	20% 6% 20% 31% 20% 4	760
	11th to 20th	Night	15% 8% 27% 34% 14% 1	760
	21st to end	Night	17% 9% 29% 33% 11% 1	836
November	1st to 10th	Night	15% 8% 31% 33% 12% 1	749
	11th to 20th	Night	17% 10% 34% 30% 8% 1	760
	21st to end	Night	15% 11% 35% 29% 9%	760
December	1st to 10th	Night	19% 12% 35% 25% 8%	760
	11th to 20th	Night	17% 10% 38% 31% 4	760
	21st to end	Night	18% 12% 33% 34% 3	836

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