Hogsback Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hogsback Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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	14% 10% 4 7% 6% 4 5% 5% 5% 7% 32%	78492
	Night	15% 11% 5% 8% 6% 3 4 4 5% 7% 32%	80967
Transparency	Day	51% 4 11% 21% 11% 1	78199
	Night	52% 4 11% 20% 11% 1	80782
Seeing	Day	19% 9% 28% 24% 14% 6%	14711
	Night	20% 8% 29% 30% 12% 1	24744

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% 19% 6% 10% 7% 4 4 4 4 5% 19%	2527
	11th to 20th	Day	16% 17% 8% 12% 10% 8% 6% 5% 4 5% 11%	2530
	21st to end	Day	27% 20% 7% 10% 6% 3 4 2 2 5% 13%	2911
February	1st to 10th	Day	14% 13% 5% 11% 10% 4 5% 6% 5% 6% 20%	1714
	11th to 20th	Day	14% 12% 6% 13% 9% 4 6% 4 3 5% 25%	1848
	21st to end	Day	15% 15% 6% 11% 7% 3 5% 5% 4 5% 24%	1570
March	1st to 10th	Day	15% 10% 5% 9% 7% 4 6% 5% 6% 7% 28%	1800
	11th to 20th	Day	13% 13% 5% 8% 7% 2 3 4 5% 7% 33%	1975
	21st to end	Day	18% 12% 5% 8% 6% 3 4 3 5% 6% 30%	2178
April	1st to 10th	Day	19% 12% 4 7% 6% 3 4 4 5% 6% 29%	2160
	11th to 20th	Day	16% 9% 3 7% 5% 3 5% 5% 5% 6% 35%	2235
	21st to end	Day	17% 8% 4 7% 7% 3 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	17% 8% 4 7% 6% 4 5% 5% 5% 6% 33%	2340
	11th to 20th	Day	19% 7% 3 7% 4 4 4 5% 5% 7% 36%	2301
	21st to end	Day	13% 5% 3 6% 6% 2 5% 5% 7% 9% 40%	2611
June	1st to 10th	Day	11% 5% 2 6% 5% 3 5% 5% 6% 8% 43%	2520
	11th to 20th	Day	11% 5% 2 6% 6% 3 5% 6% 6% 8% 42%	2520
	21st to end	Day	9% 5% 2 6% 6% 4 6% 6% 6% 10% 40%	2520
July	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 7% 10% 47%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 6% 7% 9% 44%	2520
	21st to end	Day	7% 5% 2 5% 6% 4 6% 7% 6% 10% 43%	2671
August	1st to 10th	Day	8% 5% 3 5% 5% 3 7% 6% 7% 9% 42%	2327
	11th to 20th	Day	10% 6% 2 6% 7% 4 6% 6% 6% 8% 39%	2303
	21st to end	Day	10% 5% 3 5% 5% 3 5% 5% 7% 9% 43%	2481
September	1st to 10th	Day	11% 6% 2 6% 6% 4 6% 5% 6% 8% 40%	2080
	11th to 20th	Day	11% 4 3 5% 5% 3 4 4 7% 8% 46%	1980
	21st to end	Day	13% 6% 3 6% 7% 3 4 5% 5% 8% 39%	1980
October	1st to 10th	Day	13% 6% 3 6% 6% 3 4 5% 5% 8% 40%	1976
	11th to 20th	Day	18% 9% 4 7% 7% 4 5% 4 5% 7% 31%	1790
	21st to end	Day	21% 11% 4 7% 6% 4 4 5% 5% 6% 27%	1782
November	1st to 10th	Day	11% 11% 3 6% 5% 3 4 4 5% 7% 39%	1672
	11th to 20th	Day	14% 14% 4 9% 7% 4 4 5% 4 6% 28%	1710
	21st to end	Day	19% 12% 7% 7% 6% 3 4 4 4 6% 28%	1710
December	1st to 10th	Day	14% 15% 10% 13% 6% 5% 5% 4 4 6% 20%	1689
	11th to 20th	Day	14% 17% 7% 11% 7% 4 5% 4 6% 6% 20%	1918
	21st to end	Day	22% 20% 9% 11% 8% 3 4 4 3 4 11%	2783
January	1st to 10th	Night	16% 16% 8% 11% 8% 3 4 4 4 5% 20%	4428
	11th to 20th	Night	14% 16% 8% 14% 8% 5% 7% 4 4 5% 14%	4440
	21st to end	Night	24% 17% 8% 11% 7% 3 4 4 3 4 14%	4751
February	1st to 10th	Night	15% 16% 6% 9% 9% 4 6% 4 5% 5% 22%	2466
	11th to 20th	Night	13% 13% 5% 9% 9% 4 6% 5% 5% 6% 26%	2218
	21st to end	Night	18% 11% 5% 8% 7% 3 4 5% 5% 7% 28%	1884
March	1st to 10th	Night	15% 11% 4 8% 6% 3 4 4 5% 6% 34%	2167
	11th to 20th	Night	13% 10% 4 5% 5% 3 5% 5% 4 7% 39%	1985
	21st to end	Night	20% 8% 5% 7% 6% 3 4 5% 4 6% 33%	2178
April	1st to 10th	Night	14% 11% 3 7% 6% 3 4 5% 6% 8% 35%	1815
	11th to 20th	Night	18% 9% 3 6% 5% 3 4 4 5% 6% 37%	1674
	21st to end	Night	17% 8% 3 6% 5% 3 4 5% 5% 8% 37%	1620
May	1st to 10th	Night	17% 8% 4 7% 5% 3 5% 5% 5% 8% 33%	1620
	11th to 20th	Night	20% 6% 2 3 5% 3 4 4 5% 7% 40%	1592
	21st to end	Night	10% 6% 3 5% 5% 3 4 6% 6% 11% 42%	1659
June	1st to 10th	Night	11% 5% 3 4 5% 3 4 4 6% 8% 46%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 4 5% 5% 9% 45%	1440
	21st to end	Night	9% 4 2 5% 4 3 5% 4 6% 11% 47%	1440
July	1st to 10th	Night	7% 4 2 4 4 3 4 5% 6% 9% 52%	1440
	11th to 20th	Night	8% 5% 2 4 3 2 5% 5% 6% 9% 51%	1440
	21st to end	Night	6% 4 2 4 4 3 4 5% 6% 10% 51%	1643
August	1st to 10th	Night	7% 3 3 5% 4 3 4 5% 7% 10% 48%	1611
	11th to 20th	Night	8% 3 2 4 5% 4 5% 5% 6% 9% 49%	1598
	21st to end	Night	9% 6% 2 5% 4 2 4 5% 5% 7% 51%	1872
September	1st to 10th	Night	9% 6% 2 5% 4 3 4 6% 5% 8% 47%	1862
	11th to 20th	Night	11% 4 3 4 4 3 4 3 5% 6% 54%	1980
	21st to end	Night	13% 6% 3 4 6% 3 5% 4 6% 7% 43%	1980
October	1st to 10th	Night	15% 7% 3 6% 4 2 4 4 4 6% 44%	1984
	11th to 20th	Night	18% 11% 4 7% 6% 3 4 4 5% 5% 32%	2168
	21st to end	Night	21% 10% 3 6% 5% 3 4 4 5% 6% 31%	2574
November	1st to 10th	Night	14% 9% 3 7% 5% 3 4 3 4 6% 41%	2406
	11th to 20th	Night	15% 13% 4 8% 5% 3 5% 6% 5% 7% 29%	2470
	21st to end	Night	18% 14% 5% 7% 5% 3 4 4 5% 6% 28%	2470
December	1st to 10th	Night	17% 15% 8% 11% 6% 4 5% 4 5% 6% 18%	2489
	11th to 20th	Night	14% 15% 7% 13% 7% 4 5% 4 4 5% 21%	3279
	21st to end	Night	26% 22% 6% 10% 6% 3 4 3 4 5% 12%	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	68% 4 11% 14% 4	2528
	11th to 20th	Day	75% 5% 10% 7% 2	2530
	21st to end	Day	77% 2 9% 8% 3	2911
February	1st to 10th	Day	64% 5% 8% 15% 7% 1	1714
	11th to 20th	Day	63% 3 7% 16% 9% 1	1858
	21st to end	Day	62% 3 10% 15% 7% 2	1570
March	1st to 10th	Day	55% 4 12% 19% 9% 1	1800
	11th to 20th	Day	51% 4 10% 19% 14% 2	1975
	21st to end	Day	57% 3 10% 20% 9% 1	2178
April	1st to 10th	Day	56% 4 9% 19% 11%	2160
	11th to 20th	Day	49% 4 11% 19% 15% 2	2235
	21st to end	Day	51% 4 12% 17% 14% 2	2340
May	1st to 10th	Day	51% 5% 11% 22% 10% 2	2340
	11th to 20th	Day	48% 4 11% 20% 14% 2	2305
	21st to end	Day	40% 5% 14% 27% 12% 1	2611
June	1st to 10th	Day	38% 4 12% 28% 16% 2	2520
	11th to 20th	Day	38% 6% 12% 25% 18% 2	2520
	21st to end	Day	39% 6% 14% 23% 16% 2	2520
July	1st to 10th	Day	32% 5% 12% 29% 18% 4	2451
	11th to 20th	Day	35% 5% 12% 27% 19% 2	2445
	21st to end	Day	35% 6% 12% 26% 17% 3	2523
August	1st to 10th	Day	37% 6% 13% 29% 13% 2	2314
	11th to 20th	Day	40% 5% 13% 27% 13% 2	2303
	21st to end	Day	36% 5% 11% 27% 18% 2	2481
September	1st to 10th	Day	41% 5% 14% 24% 14% 2	2080
	11th to 20th	Day	35% 4 11% 33% 16% 1	1980
	21st to end	Day	43% 4 12% 25% 15%	1980
October	1st to 10th	Day	43% 4 11% 26% 14% 2	1976
	11th to 20th	Day	53% 4 11% 20% 11% 1	1790
	21st to end	Day	57% 4 12% 19% 8% 1	1782
November	1st to 10th	Day	44% 5% 12% 23% 15% 1	1672
	11th to 20th	Day	57% 3 12% 20% 7% 1	1710
	21st to end	Day	58% 4 14% 17% 7% 1	1710
December	1st to 10th	Day	67% 4 11% 15% 4	1689
	11th to 20th	Day	64% 4 13% 15% 4	1915
	21st to end	Day	78% 4 8% 6% 4	2783
January	1st to 10th	Night	65% 5% 10% 14% 5% 1	4428
	11th to 20th	Night	73% 4 9% 9% 5%	4440
	21st to end	Night	74% 3 8% 11% 3	4751
February	1st to 10th	Night	65% 4 9% 15% 7% 1	2466
	11th to 20th	Night	59% 4 9% 18% 9% 1	2219
	21st to end	Night	57% 4 13% 15% 9% 2	1884
March	1st to 10th	Night	51% 4 11% 23% 10% 1	2167
	11th to 20th	Night	45% 5% 11% 21% 16% 2	1985
	21st to end	Night	53% 4 8% 22% 11% 2	2178
April	1st to 10th	Night	48% 4 11% 22% 15% 1	1815
	11th to 20th	Night	48% 4 11% 18% 18% 2	1674
	21st to end	Night	46% 4 12% 20% 15% 3	1620
May	1st to 10th	Night	50% 4 11% 23% 10% 2	1620
	11th to 20th	Night	45% 3 10% 20% 19% 3	1593
	21st to end	Night	36% 6% 14% 26% 14% 3	1660
June	1st to 10th	Night	36% 4 9% 31% 18% 3	1440
	11th to 20th	Night	36% 4 15% 25% 18% 3	1440
	21st to end	Night	31% 6% 14% 28% 19% 1	1440
July	1st to 10th	Night	28% 6% 12% 30% 20% 4	1400
	11th to 20th	Night	31% 5% 12% 30% 20% 3	1392
	21st to end	Night	27% 6% 16% 26% 20% 4	1552
August	1st to 10th	Night	30% 5% 14% 33% 16% 1	1600
	11th to 20th	Night	31% 6% 13% 32% 15% 3	1598
	21st to end	Night	32% 6% 11% 27% 22% 2	1872
September	1st to 10th	Night	35% 6% 10% 29% 18% 2	1862
	11th to 20th	Night	31% 4 10% 32% 21% 2	1980
	21st to end	Night	39% 4 12% 28% 16% 1	1980
October	1st to 10th	Night	41% 4 10% 26% 18% 1	1984
	11th to 20th	Night	52% 4 10% 20% 12% 1	2168
	21st to end	Night	54% 4 10% 22% 9% 1	2574
November	1st to 10th	Night	44% 5% 10% 25% 15% 1	2406
	11th to 20th	Night	54% 4 13% 20% 8% 1	2470
	21st to end	Night	57% 3 12% 20% 7% 1	2470
December	1st to 10th	Night	67% 4 10% 13% 6%	2489
	11th to 20th	Night	65% 4 11% 14% 6%	3281
	21st to end	Night	77% 3 10% 7% 3	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	24% 11% 38% 20% 6% 2	270
	11th to 20th	Day	23% 12% 43% 15% 6% 1	270
	21st to end	Day	32% 13% 28% 22% 4	297
February	1st to 10th	Day	25% 19% 34% 17% 3 1	270
	11th to 20th	Day	26% 16% 39% 17% 3	270
	21st to end	Day	26% 24% 34% 14% 3	225
March	1st to 10th	Day	22% 18% 43% 14% 4	240
	11th to 20th	Day	25% 10% 31% 25% 8% 1	415
	21st to end	Day	27% 12% 32% 22% 6% 1	462
April	1st to 10th	Day	25% 10% 33% 22% 9% 1	420
	11th to 20th	Day	27% 15% 29% 16% 8% 5%	412
	21st to end	Day	23% 10% 31% 22% 12% 2	420
May	1st to 10th	Day	26% 10% 29% 23% 9% 3	423
	11th to 20th	Day	20% 8% 25% 25% 15% 6%	447
	21st to end	Day	16% 7% 21% 27% 21% 9%	565
June	1st to 10th	Day	13% 6% 28% 32% 17% 6%	630
	11th to 20th	Day	15% 5% 27% 29% 19% 6%	630
	21st to end	Day	12% 6% 28% 30% 18% 7%	630
July	1st to 10th	Day	9% 3 19% 29% 26% 14%	621
	11th to 20th	Day	12% 4 24% 28% 21% 11%	618
	21st to end	Day	8% 2 24% 31% 26% 10%	612
August	1st to 10th	Day	9% 2 18% 30% 20% 22%	446
	11th to 20th	Day	15% 2 15% 28% 27% 13%	439
	21st to end	Day	13% 2 19% 29% 24% 13%	495
September	1st to 10th	Day	15% 3 19% 26% 24% 13%	450
	11th to 20th	Day	11% 2 19% 22% 29% 16%	450
	21st to end	Day	17% 9% 20% 24% 20% 10%	450
October	1st to 10th	Day	20% 6% 26% 23% 18% 8%	450
	11th to 20th	Day	22% 14% 29% 26% 8% 1	440
	21st to end	Day	32% 11% 36% 15% 5% 1	297
November	1st to 10th	Day	16% 16% 36% 23% 8% 1	270
	11th to 20th	Day	25% 18% 33% 18% 5% 1	270
	21st to end	Day	26% 15% 39% 13% 6%	270
December	1st to 10th	Day	27% 18% 31% 17% 6% 1	270
	11th to 20th	Day	24% 18% 38% 15% 4 1	270
	21st to end	Day	27% 20% 32% 18% 3	297
January	1st to 10th	Night	23% 13% 35% 22% 6% 1	950
	11th to 20th	Night	25% 12% 35% 22% 4 1	950
	21st to end	Night	20% 10% 35% 29% 5%	964
February	1st to 10th	Night	26% 12% 33% 26% 3	760
	11th to 20th	Night	22% 14% 34% 25% 4	743
	21st to end	Night	25% 14% 38% 20% 3	628
March	1st to 10th	Night	22% 11% 34% 26% 7% 1	723
	11th to 20th	Night	20% 12% 35% 27% 6% 1	720
	21st to end	Night	23% 9% 32% 27% 8%	792
April	1st to 10th	Night	18% 11% 38% 27% 5% 1	555
	11th to 20th	Night	23% 8% 34% 25% 9% 1	533
	21st to end	Night	24% 6% 34% 29% 8%	540
May	1st to 10th	Night	21% 6% 30% 33% 9% 1	540
	11th to 20th	Night	24% 3 28% 33% 11% 1	531
	21st to end	Night	13% 2 23% 39% 21% 2	584
June	1st to 10th	Night	13% 2 23% 45% 16% 1	540
	11th to 20th	Night	14% 2 22% 41% 20%	540
	21st to end	Night	11% 2 21% 41% 22% 2	540
July	1st to 10th	Night	9% 1 15% 41% 31% 3	525
	11th to 20th	Night	11% 2 17% 41% 26% 3	522
	21st to end	Night	7% 1 17% 48% 25% 2	591
August	1st to 10th	Night	8% 1 13% 47% 28% 3	537
	11th to 20th	Night	9% 1 15% 40% 30% 5%	534
	21st to end	Night	12% 3 15% 36% 31% 4	594
September	1st to 10th	Night	13% 2 16% 38% 26% 4	602
	11th to 20th	Night	13% 3 15% 36% 30% 4	720
	21st to end	Night	17% 4 21% 31% 23% 5%	720
October	1st to 10th	Night	18% 5% 23% 34% 17% 3	720
	11th to 20th	Night	24% 10% 28% 28% 9% 1	720
	21st to end	Night	27% 9% 32% 25% 7%	792
November	1st to 10th	Night	20% 9% 30% 29% 11% 1	740
	11th to 20th	Night	23% 13% 34% 23% 7% 1	760
	21st to end	Night	26% 10% 32% 27% 4	760
December	1st to 10th	Night	26% 12% 35% 21% 7%	779
	11th to 20th	Night	22% 13% 37% 24% 4	950
	21st to end	Night	32% 11% 34% 20% 2	1045

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