Cliff's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cliff's Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 32%	75495
	Night	16% 10% 4 7% 6% 3 4 4 5% 6% 35%	74555
Transparency	Day	50% 6% 11% 19% 11% 1	75213
	Night	50% 6% 11% 20% 12% 2	74358
Seeing	Day	19% 10% 26% 23% 15% 7%	14207
	Night	20% 8% 27% 30% 14% 2	23889

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	19% 16% 8% 11% 8% 4 5% 5% 4 4 17%	1706
	11th to 20th	Day	20% 19% 8% 11% 8% 4 5% 4 3 5% 15%	1710
	21st to end	Day	19% 17% 8% 11% 9% 4 5% 4 4 4 16%	1836
February	1st to 10th	Day	20% 15% 6% 10% 9% 4 4 3 5% 4 20%	1620
	11th to 20th	Day	16% 12% 7% 13% 7% 4 5% 4 4 6% 21%	1719
	21st to end	Day	18% 13% 5% 10% 6% 3 4 3 5% 6% 27%	1480
March	1st to 10th	Day	19% 11% 6% 9% 6% 3 4 4 4 5% 27%	1871
	11th to 20th	Day	18% 13% 5% 8% 6% 3 4 3 5% 8% 27%	1980
	21st to end	Day	24% 11% 5% 7% 6% 3 4 4 4 6% 26%	2178
April	1st to 10th	Day	15% 13% 5% 6% 6% 4 4 4 5% 8% 31%	2037
	11th to 20th	Day	19% 9% 3 6% 6% 3 4 5% 5% 6% 34%	2272
	21st to end	Day	21% 8% 4 6% 6% 3 4 5% 4 6% 32%	2340
May	1st to 10th	Day	19% 9% 3 6% 5% 4 5% 6% 5% 7% 31%	2340
	11th to 20th	Day	20% 8% 3 7% 6% 4 4 5% 5% 7% 32%	2301
	21st to end	Day	9% 5% 3 6% 6% 4 6% 7% 7% 9% 38%	2531
June	1st to 10th	Day	13% 5% 2 7% 6% 4 5% 6% 6% 9% 38%	2340
	11th to 20th	Day	13% 6% 2 5% 6% 4 5% 5% 6% 10% 37%	2449
	21st to end	Day	12% 4 2 5% 6% 4 6% 6% 7% 10% 39%	2520
July	1st to 10th	Day	11% 6% 2 5% 7% 3 6% 5% 7% 9% 39%	2520
	11th to 20th	Day	11% 5% 2 4 4 3 5% 5% 7% 10% 43%	2358
	21st to end	Day	9% 5% 2 4 5% 4 5% 6% 7% 11% 41%	2547
August	1st to 10th	Day	9% 4 2 6% 6% 4 6% 7% 7% 9% 42%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 6% 6% 10% 39%	2303
	21st to end	Day	9% 5% 2 6% 6% 3 5% 5% 7% 9% 42%	2540
September	1st to 10th	Day	13% 5% 2 6% 5% 3 4 4 5% 7% 46%	1997
	11th to 20th	Day	11% 6% 2 5% 5% 4 5% 5% 6% 8% 45%	1980
	21st to end	Day	18% 5% 2 5% 4 3 4 4 4 6% 44%	1980
October	1st to 10th	Day	16% 6% 3 6% 5% 3 4 4 6% 6% 41%	1928
	11th to 20th	Day	17% 8% 2 5% 6% 3 4 6% 6% 7% 38%	1800
	21st to end	Day	22% 10% 4 6% 5% 4 5% 4 5% 6% 30%	1906
November	1st to 10th	Day	12% 8% 3 5% 5% 3 4 4 4 6% 45%	1672
	11th to 20th	Day	18% 12% 5% 6% 5% 3 4 4 5% 5% 34%	1710
	21st to end	Day	20% 14% 5% 8% 5% 3 4 3 3 7% 27%	1710
December	1st to 10th	Day	19% 16% 9% 10% 7% 3 4 4 4 6% 18%	1710
	11th to 20th	Day	18% 15% 7% 11% 5% 3 5% 5% 5% 6% 21%	2153
	21st to end	Day	25% 22% 8% 10% 7% 3 4 3 3 4 10%	3124
January	1st to 10th	Night	19% 15% 8% 10% 7% 4 5% 4 3 4 21%	2459
	11th to 20th	Night	21% 16% 6% 9% 8% 4 5% 3 3 4 20%	2470
	21st to end	Night	19% 18% 7% 9% 7% 3 4 4 4 5% 21%	2664
February	1st to 10th	Night	22% 14% 5% 9% 7% 3 5% 4 5% 4 23%	2340
	11th to 20th	Night	18% 13% 5% 8% 7% 3 5% 5% 5% 5% 26%	2125
	21st to end	Night	21% 14% 6% 9% 6% 3 4 4 3 4 26%	1776
March	1st to 10th	Night	18% 10% 5% 8% 7% 3 4 4 4 5% 32%	2088
	11th to 20th	Night	16% 10% 4 5% 5% 3 4 4 5% 7% 38%	1980
	21st to end	Night	19% 9% 4 8% 6% 3 4 3 5% 6% 33%	2178
April	1st to 10th	Night	16% 9% 3 6% 5% 3 5% 4 5% 7% 36%	1939
	11th to 20th	Night	17% 7% 4 6% 5% 3 5% 4 6% 7% 37%	1635
	21st to end	Night	17% 8% 2 6% 6% 4 4 4 6% 8% 36%	1620
May	1st to 10th	Night	19% 9% 3 5% 7% 4 4 5% 5% 6% 32%	1620
	11th to 20th	Night	20% 6% 4 6% 5% 2 4 4 5% 7% 38%	1592
	21st to end	Night	8% 4 2 6% 6% 3 5% 7% 7% 10% 43%	1744
June	1st to 10th	Night	9% 7% 3 6% 5% 3 5% 5% 5% 7% 46%	1620
	11th to 20th	Night	10% 5% 2 4 5% 3 5% 5% 7% 10% 44%	1504
	21st to end	Night	10% 4 2 5% 6% 3 5% 5% 6% 11% 44%	1440
July	1st to 10th	Night	10% 4 2 5% 5% 3 5% 5% 6% 9% 46%	1440
	11th to 20th	Night	8% 4 2 5% 5% 3 4 5% 6% 10% 49%	1596
	21st to end	Night	9% 5% 3 5% 5% 3 5% 5% 5% 8% 47%	1771
August	1st to 10th	Night	6% 3 3 5% 5% 3 5% 7% 8% 11% 44%	1611
	11th to 20th	Night	9% 5% 3 5% 6% 3 4 5% 6% 8% 47%	1598
	21st to end	Night	7% 3 2 4 5% 4 4 5% 6% 8% 52%	1812
September	1st to 10th	Night	12% 6% 2 5% 4 2 4 4 5% 7% 50%	1944
	11th to 20th	Night	11% 4 2 4 3 2 3 3 4 7% 55%	1980
	21st to end	Night	14% 5% 2 5% 5% 3 4 4 5% 6% 47%	1980
October	1st to 10th	Night	11% 5% 2 5% 5% 3 4 5% 5% 7% 48%	2031
	11th to 20th	Night	19% 8% 3 5% 6% 3 4 4 4 6% 39%	2160
	21st to end	Night	24% 8% 2 5% 5% 3 4 3 4 5% 35%	2448
November	1st to 10th	Night	15% 6% 3 6% 4 3 3 4 5% 6% 45%	2406
	11th to 20th	Night	17% 12% 5% 6% 5% 3 3 3 5% 7% 34%	2470
	21st to end	Night	20% 12% 5% 7% 5% 3 4 4 5% 6% 28%	2470
December	1st to 10th	Night	20% 15% 8% 9% 7% 3 3 4 4 6% 22%	2470
	11th to 20th	Night	19% 14% 7% 9% 6% 3 4 4 4 5% 24%	3042
	21st to end	Night	26% 20% 8% 8% 6% 3 4 4 4 5% 14%	4532

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	71% 5% 7% 12% 5%	1707
	11th to 20th	Day	73% 5% 11% 9% 3	1710
	21st to end	Day	72% 4 9% 11% 4 1	1836
February	1st to 10th	Day	68% 5% 9% 12% 6%	1620
	11th to 20th	Day	65% 5% 8% 14% 7% 1	1729
	21st to end	Day	60% 4 12% 15% 7% 2	1480
March	1st to 10th	Day	59% 4 12% 18% 6%	1871
	11th to 20th	Day	57% 5% 11% 17% 9% 2	1980
	21st to end	Day	61% 4 10% 19% 6% 1	2178
April	1st to 10th	Day	52% 6% 12% 18% 11% 1	2037
	11th to 20th	Day	49% 6% 12% 17% 13% 2	2272
	21st to end	Day	52% 6% 10% 20% 11% 2	2340
May	1st to 10th	Day	51% 7% 13% 18% 8% 1	2340
	11th to 20th	Day	52% 7% 12% 17% 11% 1	2305
	21st to end	Day	39% 10% 15% 24% 11% 1	2531
June	1st to 10th	Day	43% 7% 11% 24% 14% 1	2340
	11th to 20th	Day	42% 8% 14% 22% 13% 1	2449
	21st to end	Day	38% 9% 15% 21% 14% 3	2520
July	1st to 10th	Day	40% 8% 12% 22% 16% 2	2451
	11th to 20th	Day	34% 9% 14% 25% 16% 2	2288
	21st to end	Day	34% 9% 15% 26% 13% 3	2405
August	1st to 10th	Day	35% 9% 14% 26% 14% 1	2314
	11th to 20th	Day	38% 10% 13% 26% 11% 2	2303
	21st to end	Day	36% 9% 14% 21% 16% 5%	2540
September	1st to 10th	Day	38% 6% 12% 22% 17% 4	1997
	11th to 20th	Day	37% 7% 11% 25% 18% 3	1980
	21st to end	Day	41% 6% 10% 22% 20% 1	1980
October	1st to 10th	Day	43% 6% 11% 23% 17% 1	1928
	11th to 20th	Day	43% 6% 11% 23% 14% 2	1800
	21st to end	Day	55% 6% 11% 18% 9% 1	1906
November	1st to 10th	Day	41% 5% 11% 24% 16% 2	1672
	11th to 20th	Day	53% 4 10% 21% 11% 2	1710
	21st to end	Day	59% 4 10% 18% 9% 1	1710
December	1st to 10th	Day	69% 3 10% 13% 4	1710
	11th to 20th	Day	63% 6% 9% 16% 5%	2150
	21st to end	Day	79% 4 7% 6% 3	3124
January	1st to 10th	Night	68% 4 6% 15% 6%	2459
	11th to 20th	Night	69% 4 10% 12% 4	2470
	21st to end	Night	66% 4 9% 14% 5% 1	2664
February	1st to 10th	Night	64% 5% 10% 15% 6%	2340
	11th to 20th	Night	60% 5% 11% 15% 8% 1	2126
	21st to end	Night	62% 5% 8% 15% 9% 1	1776
March	1st to 10th	Night	55% 5% 11% 19% 10% 1	2088
	11th to 20th	Night	46% 5% 11% 24% 11% 3	1980
	21st to end	Night	54% 4 8% 22% 11% 2	2178
April	1st to 10th	Night	47% 6% 13% 18% 14% 2	1939
	11th to 20th	Night	46% 4 11% 18% 18% 3	1635
	21st to end	Night	47% 5% 12% 19% 15% 2	1620
May	1st to 10th	Night	52% 5% 11% 17% 11% 3	1620
	11th to 20th	Night	46% 6% 10% 20% 15% 2	1593
	21st to end	Night	33% 10% 15% 26% 14% 2	1745
June	1st to 10th	Night	37% 7% 13% 23% 18% 2	1620
	11th to 20th	Night	34% 8% 14% 26% 16% 3	1504
	21st to end	Night	33% 9% 14% 24% 16% 4	1440
July	1st to 10th	Night	35% 8% 13% 20% 19% 5%	1400
	11th to 20th	Night	31% 8% 13% 28% 18% 2	1544
	21st to end	Night	33% 7% 14% 27% 15% 4	1672
August	1st to 10th	Night	30% 9% 16% 26% 17% 1	1600
	11th to 20th	Night	34% 9% 14% 25% 17% 1	1598
	21st to end	Night	28% 7% 16% 23% 21% 4	1812
September	1st to 10th	Night	34% 6% 13% 24% 17% 6%	1944
	11th to 20th	Night	30% 4 11% 28% 25% 3	1980
	21st to end	Night	37% 7% 10% 21% 23% 2	1980
October	1st to 10th	Night	35% 6% 11% 26% 21% 2	2031
	11th to 20th	Night	46% 5% 8% 25% 15% 1	2160
	21st to end	Night	52% 4 9% 21% 12% 2	2448
November	1st to 10th	Night	41% 5% 12% 22% 17% 3	2406
	11th to 20th	Night	51% 4 9% 21% 13% 2	2470
	21st to end	Night	57% 4 12% 18% 8% 1	2470
December	1st to 10th	Night	64% 5% 9% 16% 6%	2470
	11th to 20th	Night	61% 5% 11% 17% 5%	3044
	21st to end	Night	74% 5% 9% 10% 3	4532

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	27% 17% 32% 17% 6% 1	270
	11th to 20th	Day	24% 19% 41% 10% 5%	270
	21st to end	Day	30% 17% 29% 17% 6%	282
February	1st to 10th	Day	31% 18% 34% 16%	240
	11th to 20th	Day	26% 21% 37% 13% 3	240
	21st to end	Day	24% 21% 42% 12% 1	198
March	1st to 10th	Day	22% 17% 37% 20% 4	311
	11th to 20th	Day	26% 14% 36% 18% 5% 1	420
	21st to end	Day	31% 11% 31% 21% 6%	462
April	1st to 10th	Day	24% 11% 33% 22% 9% 2	420
	11th to 20th	Day	20% 17% 27% 21% 10% 5%	412
	21st to end	Day	25% 12% 33% 20% 8% 2	420
May	1st to 10th	Day	26% 9% 30% 23% 9% 4	423
	11th to 20th	Day	24% 7% 25% 26% 14% 5%	447
	21st to end	Day	12% 6% 24% 24% 22% 11%	485
June	1st to 10th	Day	12% 5% 21% 33% 22% 6%	450
	11th to 20th	Day	14% 6% 24% 27% 22% 6%	559
	21st to end	Day	13% 6% 28% 27% 20% 6%	630
July	1st to 10th	Day	12% 4 15% 28% 23% 19%	621
	11th to 20th	Day	12% 3 18% 27% 25% 16%	461
	21st to end	Day	12% 3 17% 23% 29% 16%	488
August	1st to 10th	Day	11% 2 14% 25% 32% 17%	446
	11th to 20th	Day	14% 3 15% 29% 27% 12%	439
	21st to end	Day	11% 3 17% 23% 30% 16%	495
September	1st to 10th	Day	14% 4 17% 26% 27% 13%	450
	11th to 20th	Day	12% 2 15% 23% 30% 18%	450
	21st to end	Day	17% 9% 19% 26% 17% 11%	450
October	1st to 10th	Day	13% 8% 23% 28% 18% 10%	450
	11th to 20th	Day	18% 12% 32% 25% 12% 2	450
	21st to end	Day	26% 14% 32% 21% 5%	421
November	1st to 10th	Day	13% 16% 32% 24% 10% 4	270
	11th to 20th	Day	24% 17% 29% 18% 12%	270
	21st to end	Day	29% 15% 36% 18% 3	270
December	1st to 10th	Day	23% 18% 36% 15% 6% 2	270
	11th to 20th	Day	23% 17% 39% 17% 4	270
	21st to end	Day	34% 15% 31% 16% 3	297
January	1st to 10th	Night	24% 14% 37% 22% 4	760
	11th to 20th	Night	29% 14% 36% 17% 4 1	760
	21st to end	Night	25% 14% 33% 24% 4	820
February	1st to 10th	Night	28% 10% 37% 23% 2	720
	11th to 20th	Night	26% 12% 35% 23% 4	703
	21st to end	Night	30% 10% 39% 17% 3	592
March	1st to 10th	Night	23% 12% 34% 24% 7%	720
	11th to 20th	Night	21% 12% 34% 26% 6% 1	720
	21st to end	Night	24% 10% 32% 27% 7%	792
April	1st to 10th	Night	21% 10% 37% 27% 5% 1	679
	11th to 20th	Night	20% 10% 36% 24% 10% 1	533
	21st to end	Night	20% 9% 36% 25% 8% 1	540
May	1st to 10th	Night	22% 8% 29% 31% 9% 1	540
	11th to 20th	Night	23% 5% 25% 34% 13% 1	531
	21st to end	Night	10% 3 22% 39% 24% 2	584
June	1st to 10th	Night	9% 4 23% 43% 20% 1	540
	11th to 20th	Night	11% 4 18% 44% 21% 1	540
	21st to end	Night	10% 3 21% 41% 24% 2	540
July	1st to 10th	Night	11% 2 14% 38% 32% 4	525
	11th to 20th	Night	8% 4 14% 40% 30% 5%	522
	21st to end	Night	8% 3 12% 42% 32% 2	591
August	1st to 10th	Night	7% 2 9% 46% 31% 4	537
	11th to 20th	Night	9% 1 12% 41% 31% 5%	534
	21st to end	Night	8% 2 13% 37% 35% 5%	594
September	1st to 10th	Night	12% 4 15% 35% 29% 5%	684
	11th to 20th	Night	12% 3 13% 34% 32% 5%	720
	21st to end	Night	15% 5% 21% 33% 22% 3	720
October	1st to 10th	Night	13% 5% 24% 35% 20% 4	720
	11th to 20th	Night	21% 9% 29% 29% 11% 1	720
	21st to end	Night	27% 10% 28% 26% 8%	792
November	1st to 10th	Night	19% 7% 31% 30% 13% 1	740
	11th to 20th	Night	22% 14% 30% 25% 8% 1	760
	21st to end	Night	25% 11% 31% 27% 6% 1	760
December	1st to 10th	Night	27% 12% 34% 23% 5%	760
	11th to 20th	Night	26% 10% 36% 23% 4	760
	21st to end	Night	31% 12% 32% 22% 3	836

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