His Glassworks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the His Glassworks Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	12% 7% 3 6% 6% 4 5% 6% 6% 8% 37%	74929
	Night	11% 7% 3 6% 6% 3 5% 5% 6% 8% 39%	74892
Transparency	Day	42% 9% 12% 18% 15% 3	74652
	Night	39% 9% 13% 19% 16% 3	74688
Seeing	Day	19% 14% 23% 21% 15% 8%	13931
	Night	17% 12% 21% 28% 19% 3	24061

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	14% 7% 2 7% 6% 3 4 5% 6% 7% 38%	1706
	11th to 20th	Day	14% 8% 3 6% 6% 4 5% 5% 5% 7% 37%	1671
	21st to end	Day	12% 9% 3 6% 5% 2 4 5% 5% 7% 43%	1782
February	1st to 10th	Day	15% 7% 4 6% 6% 3 5% 4 6% 7% 37%	1692
	11th to 20th	Day	16% 11% 5% 7% 5% 3 5% 4 4 6% 35%	1748
	21st to end	Day	15% 8% 4 8% 5% 3 5% 5% 5% 6% 37%	1480
March	1st to 10th	Day	14% 7% 3 5% 4 4 4 5% 5% 7% 43%	1934
	11th to 20th	Day	17% 9% 4 8% 5% 4 4 5% 6% 6% 33%	1980
	21st to end	Day	15% 10% 4 6% 6% 3 5% 5% 5% 7% 35%	2178
April	1st to 10th	Day	11% 6% 2 6% 6% 3 6% 5% 5% 8% 42%	1980
	11th to 20th	Day	15% 7% 3 5% 5% 3 4 5% 6% 8% 41%	2150
	21st to end	Day	12% 6% 4 6% 6% 4 4 4 5% 6% 43%	2340
May	1st to 10th	Day	10% 7% 4 7% 6% 3 5% 6% 6% 7% 38%	2340
	11th to 20th	Day	13% 6% 3 7% 7% 4 6% 6% 7% 8% 32%	2301
	21st to end	Day	10% 7% 2 7% 7% 4 6% 6% 7% 11% 34%	2531
June	1st to 10th	Day	10% 7% 2 6% 8% 5% 7% 6% 7% 9% 32%	2340
	11th to 20th	Day	7% 5% 3 7% 8% 5% 7% 7% 8% 9% 33%	2340
	21st to end	Day	8% 6% 3 6% 7% 4 7% 8% 7% 9% 34%	2340
July	1st to 10th	Day	9% 7% 4 7% 9% 5% 8% 7% 7% 10% 26%	2340
	11th to 20th	Day	11% 6% 4 7% 7% 5% 7% 9% 8% 10% 26%	2340
	21st to end	Day	9% 5% 3 7% 9% 4 9% 8% 8% 10% 28%	2547
August	1st to 10th	Day	11% 7% 3 7% 7% 5% 7% 8% 9% 11% 24%	2327
	11th to 20th	Day	11% 7% 3 7% 9% 5% 7% 8% 7% 10% 28%	2303
	21st to end	Day	9% 7% 3 8% 7% 6% 6% 7% 7% 9% 31%	2395
September	1st to 10th	Day	10% 6% 3 6% 8% 5% 7% 6% 7% 8% 35%	1980
	11th to 20th	Day	14% 7% 3 5% 7% 4 6% 6% 6% 8% 34%	1980
	21st to end	Day	12% 6% 3 6% 6% 3 6% 6% 6% 7% 38%	1980
October	1st to 10th	Day	11% 6% 3 6% 4 2 5% 5% 4 7% 47%	1945
	11th to 20th	Day	10% 6% 3 5% 5% 3 3 4 4 6% 52%	1800
	21st to end	Day	13% 6% 2 5% 4 2 3 3 4 6% 52%	1980
November	1st to 10th	Day	9% 6% 3 5% 3 2 3 3 4 6% 55%	1772
	11th to 20th	Day	11% 6% 3 5% 4 3 4 3 5% 7% 49%	1710
	21st to end	Day	14% 7% 3 5% 4 2 4 4 4 7% 46%	1710
December	1st to 10th	Day	16% 9% 3 7% 6% 4 5% 5% 5% 6% 34%	1710
	11th to 20th	Day	13% 8% 4 6% 5% 2 4 6% 5% 8% 40%	2153
	21st to end	Day	13% 9% 4 8% 6% 4 4 4 5% 7% 36%	3124
January	1st to 10th	Night	15% 6% 4 6% 7% 4 5% 5% 6% 7% 35%	2459
	11th to 20th	Night	14% 9% 4 7% 6% 3 5% 4 6% 6% 35%	2417
	21st to end	Night	12% 9% 4 8% 5% 3 4 5% 5% 5% 40%	2574
February	1st to 10th	Night	14% 8% 3 6% 5% 3 6% 5% 6% 7% 38%	2264
	11th to 20th	Night	17% 8% 3 6% 7% 3 4 4 5% 6% 36%	2098
	21st to end	Night	15% 8% 5% 8% 6% 3 4 5% 4 7% 34%	1776
March	1st to 10th	Night	12% 8% 4 6% 4 3 5% 5% 6% 6% 41%	2026
	11th to 20th	Night	14% 8% 4 7% 6% 3 5% 6% 5% 8% 35%	1980
	21st to end	Night	13% 9% 3 6% 5% 3 5% 5% 5% 8% 37%	2178
April	1st to 10th	Night	12% 7% 3 6% 4 3 4 4 6% 7% 45%	1980
	11th to 20th	Night	10% 8% 3 5% 5% 3 4 4 5% 7% 45%	1770
	21st to end	Night	11% 7% 3 4 5% 2 4 4 4 8% 46%	1620
May	1st to 10th	Night	9% 8% 4 6% 5% 3 4 6% 7% 9% 40%	1620
	11th to 20th	Night	10% 6% 2 8% 6% 3 6% 7% 6% 8% 37%	1592
	21st to end	Night	8% 7% 2 6% 5% 3 6% 6% 7% 11% 38%	1744
June	1st to 10th	Night	8% 6% 2 5% 7% 4 6% 7% 7% 13% 35%	1620
	11th to 20th	Night	6% 5% 3 5% 4 3 5% 6% 9% 12% 42%	1620
	21st to end	Night	7% 4 2 6% 6% 4 6% 5% 8% 12% 41%	1620
July	1st to 10th	Night	8% 7% 4 6% 7% 4 6% 5% 7% 10% 36%	1620
	11th to 20th	Night	8% 5% 3 5% 7% 4 6% 7% 10% 12% 33%	1620
	21st to end	Night	6% 5% 3 8% 6% 5% 7% 8% 9% 11% 33%	1771
August	1st to 10th	Night	8% 5% 4 6% 6% 3 6% 6% 7% 13% 36%	1611
	11th to 20th	Night	9% 6% 3 8% 7% 5% 5% 6% 7% 9% 35%	1598
	21st to end	Night	8% 7% 3 6% 6% 4 7% 6% 7% 10% 35%	1939
September	1st to 10th	Night	10% 5% 2 6% 7% 5% 5% 6% 5% 9% 40%	1980
	11th to 20th	Night	12% 7% 3 8% 6% 3 5% 6% 5% 8% 38%	1980
	21st to end	Night	9% 7% 3 6% 6% 3 4 4 5% 7% 47%	1980
October	1st to 10th	Night	14% 6% 2 5% 5% 2 4 4 5% 6% 48%	2012
	11th to 20th	Night	10% 8% 3 6% 4 3 4 4 6% 7% 47%	2160
	21st to end	Night	10% 8% 2 5% 3 2 4 4 5% 6% 51%	2376
November	1st to 10th	Night	11% 6% 2 4 4 3 4 6% 5% 6% 49%	2303
	11th to 20th	Night	11% 7% 3 5% 5% 3 4 4 5% 6% 47%	2470
	21st to end	Night	12% 6% 3 7% 4 3 4 5% 5% 6% 44%	2470
December	1st to 10th	Night	17% 9% 4 8% 6% 3 4 5% 5% 6% 33%	2470
	11th to 20th	Night	13% 11% 4 9% 4 3 4 4 5% 6% 38%	3042
	21st to end	Night	13% 10% 4 8% 7% 4 5% 6% 7% 7% 30%	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	39% 6% 13% 21% 16% 5%	1707
	11th to 20th	Day	42% 6% 13% 17% 18% 5%	1671
	21st to end	Day	37% 5% 11% 22% 19% 5%	1782
February	1st to 10th	Day	43% 5% 13% 20% 16% 3	1692
	11th to 20th	Day	46% 6% 11% 17% 16% 4	1758
	21st to end	Day	45% 7% 10% 16% 16% 6%	1480
March	1st to 10th	Day	37% 6% 10% 20% 20% 7%	1934
	11th to 20th	Day	47% 7% 12% 16% 14% 4	1980
	21st to end	Day	46% 8% 13% 18% 13% 2	2178
April	1st to 10th	Day	35% 8% 13% 22% 19% 4	1980
	11th to 20th	Day	40% 6% 9% 19% 20% 6%	2150
	21st to end	Day	39% 7% 13% 24% 15% 2	2340
May	1st to 10th	Day	41% 10% 14% 20% 14% 2	2340
	11th to 20th	Day	47% 14% 13% 16% 10% 1	2305
	21st to end	Day	42% 12% 14% 22% 9% 1	2531
June	1st to 10th	Day	45% 13% 17% 17% 7% 1	2340
	11th to 20th	Day	41% 13% 17% 16% 11% 2	2340
	21st to end	Day	42% 13% 14% 18% 11% 2	2340
July	1st to 10th	Day	48% 14% 15% 13% 8% 1	2275
	11th to 20th	Day	47% 15% 17% 11% 9% 1	2271
	21st to end	Day	45% 15% 17% 15% 7% 1	2405
August	1st to 10th	Day	49% 17% 14% 16% 4	2314
	11th to 20th	Day	48% 15% 15% 15% 6% 1	2303
	21st to end	Day	46% 14% 14% 16% 7% 3	2395
September	1st to 10th	Day	43% 11% 13% 17% 13% 3	1980
	11th to 20th	Day	46% 10% 11% 17% 14% 2	1980
	21st to end	Day	43% 9% 12% 17% 15% 4	1980
October	1st to 10th	Day	35% 8% 11% 17% 23% 5%	1945
	11th to 20th	Day	31% 7% 11% 23% 21% 8%	1800
	21st to end	Day	33% 5% 8% 22% 23% 9%	1980
November	1st to 10th	Day	30% 4 10% 20% 27% 8%	1772
	11th to 20th	Day	34% 5% 8% 21% 25% 7%	1710
	21st to end	Day	36% 6% 8% 23% 22% 5%	1710
December	1st to 10th	Day	48% 7% 8% 18% 15% 5%	1710
	11th to 20th	Day	35% 6% 11% 20% 22% 6%	2150
	21st to end	Day	41% 7% 10% 24% 17% 2	3124
January	1st to 10th	Night	41% 7% 15% 17% 17% 4	2459
	11th to 20th	Night	44% 6% 13% 16% 17% 4	2417
	21st to end	Night	41% 5% 10% 20% 20% 3	2574
February	1st to 10th	Night	40% 6% 13% 20% 17% 3	2264
	11th to 20th	Night	44% 5% 12% 19% 15% 5%	2099
	21st to end	Night	46% 7% 11% 15% 16% 5%	1776
March	1st to 10th	Night	41% 6% 10% 15% 21% 7%	2026
	11th to 20th	Night	43% 8% 14% 18% 13% 3	1980
	21st to end	Night	44% 8% 12% 22% 13% 2	2178
April	1st to 10th	Night	34% 7% 15% 22% 19% 3	1980
	11th to 20th	Night	35% 7% 11% 23% 19% 5%	1770
	21st to end	Night	35% 8% 13% 25% 16% 2	1620
May	1st to 10th	Night	37% 11% 16% 24% 11% 2	1620
	11th to 20th	Night	42% 13% 14% 20% 10% 1	1593
	21st to end	Night	37% 12% 18% 24% 8% 1	1745
June	1st to 10th	Night	38% 15% 20% 20% 7% 1	1620
	11th to 20th	Night	32% 14% 24% 21% 9% 1	1620
	21st to end	Night	33% 14% 19% 20% 12% 2	1620
July	1st to 10th	Night	41% 15% 18% 17% 9% 1	1575
	11th to 20th	Night	38% 19% 20% 14% 7% 2	1566
	21st to end	Night	37% 17% 21% 18% 7% 1	1672
August	1st to 10th	Night	37% 20% 20% 17% 7%	1600
	11th to 20th	Night	41% 16% 17% 15% 10% 1	1598
	21st to end	Night	41% 14% 17% 20% 6% 1	1939
September	1st to 10th	Night	40% 12% 14% 20% 12% 3	1980
	11th to 20th	Night	45% 10% 13% 16% 16% 1	1980
	21st to end	Night	37% 10% 11% 22% 18% 3	1980
October	1st to 10th	Night	37% 8% 12% 18% 22% 3	2012
	11th to 20th	Night	35% 9% 10% 22% 18% 6%	2160
	21st to end	Night	31% 6% 8% 22% 26% 7%	2376
November	1st to 10th	Night	34% 8% 9% 20% 22% 8%	2303
	11th to 20th	Night	34% 5% 10% 19% 24% 9%	2470
	21st to end	Night	37% 6% 8% 23% 21% 6%	2470
December	1st to 10th	Night	46% 7% 13% 16% 14% 4	2470
	11th to 20th	Night	38% 8% 10% 17% 22% 4	3044
	21st to end	Night	44% 7% 12% 19% 16% 2	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	31% 29% 30% 7% 2	270
	11th to 20th	Day	34% 25% 25% 15% 2	257
	21st to end	Day	27% 33% 29% 8% 3	264
February	1st to 10th	Day	32% 26% 29% 11% 2	240
	11th to 20th	Day	44% 30% 19% 6%	240
	21st to end	Day	40% 30% 21% 8% 1	198
March	1st to 10th	Day	25% 22% 31% 17% 5%	374
	11th to 20th	Day	29% 25% 25% 17% 5% 1	420
	21st to end	Day	27% 23% 25% 18% 6%	462
April	1st to 10th	Day	24% 20% 25% 21% 9% 1	420
	11th to 20th	Day	22% 20% 24% 20% 11% 3	412
	21st to end	Day	20% 17% 27% 20% 12% 5%	420
May	1st to 10th	Day	17% 21% 26% 20% 11% 5%	423
	11th to 20th	Day	17% 11% 24% 25% 15% 8%	447
	21st to end	Day	14% 7% 24% 23% 19% 13%	485
June	1st to 10th	Day	14% 6% 23% 27% 20% 11%	450
	11th to 20th	Day	9% 6% 24% 22% 25% 14%	450
	21st to end	Day	11% 8% 26% 26% 17% 12%	450
July	1st to 10th	Day	9% 4 16% 25% 29% 17%	445
	11th to 20th	Day	8% 4 15% 26% 25% 22%	444
	21st to end	Day	8% 3 12% 27% 30% 19%	488
August	1st to 10th	Day	18% 5% 19% 21% 18% 19%	446
	11th to 20th	Day	10% 3 20% 25% 24% 18%	439
	21st to end	Day	11% 3 17% 25% 28% 15%	495
September	1st to 10th	Day	10% 4 15% 29% 27% 15%	450
	11th to 20th	Day	15% 6% 16% 23% 29% 11%	450
	21st to end	Day	13% 9% 21% 28% 24% 5%	450
October	1st to 10th	Day	17% 10% 20% 26% 19% 7%	450
	11th to 20th	Day	15% 17% 24% 25% 16% 4	450
	21st to end	Day	20% 14% 24% 27% 12% 2	495
November	1st to 10th	Day	19% 14% 26% 28% 10% 3	370
	11th to 20th	Day	24% 20% 31% 19% 5% 1	270
	21st to end	Day	20% 20% 34% 18% 7% 1	270
December	1st to 10th	Day	29% 26% 30% 13% 2	270
	11th to 20th	Day	24% 21% 31% 17% 6% 1	270
	21st to end	Day	27% 23% 31% 14% 4 1	297
January	1st to 10th	Night	26% 20% 25% 23% 6%	760
	11th to 20th	Night	22% 20% 27% 25% 5%	744
	21st to end	Night	21% 21% 30% 26% 3	792
February	1st to 10th	Night	21% 19% 27% 28% 5%	720
	11th to 20th	Night	27% 25% 25% 22% 1	703
	21st to end	Night	24% 24% 28% 22% 3	592
March	1st to 10th	Night	19% 21% 28% 29% 3	720
	11th to 20th	Night	23% 16% 30% 23% 8% 1	720
	21st to end	Night	20% 18% 29% 25% 8%	792
April	1st to 10th	Night	19% 20% 24% 29% 8% 1	720
	11th to 20th	Night	18% 17% 24% 27% 12% 1	623
	21st to end	Night	15% 13% 20% 35% 17% 1	540
May	1st to 10th	Night	16% 10% 23% 31% 18% 1	540
	11th to 20th	Night	15% 7% 21% 31% 24% 2	531
	21st to end	Night	11% 3 16% 27% 35% 8%	584
June	1st to 10th	Night	10% 3 14% 34% 33% 6%	540
	11th to 20th	Night	8% 3 15% 30% 34% 11%	540
	21st to end	Night	6% 3 12% 34% 39% 7%	540
July	1st to 10th	Night	9% 2 10% 26% 46% 7%	525
	11th to 20th	Night	8% 2 9% 23% 45% 13%	522
	21st to end	Night	7% 1 7% 25% 47% 13%	591
August	1st to 10th	Night	10% 1 13% 23% 43% 11%	537
	11th to 20th	Night	11% 3 12% 25% 40% 10%	534
	21st to end	Night	10% 1 9% 28% 42% 10%	643
September	1st to 10th	Night	9% 3 14% 28% 39% 7%	720
	11th to 20th	Night	15% 4 16% 25% 34% 7%	720
	21st to end	Night	14% 6% 17% 32% 28% 4	720
October	1st to 10th	Night	16% 10% 19% 23% 25% 7%	720
	11th to 20th	Night	15% 12% 22% 30% 18% 2	720
	21st to end	Night	16% 14% 19% 34% 15% 2	792
November	1st to 10th	Night	16% 9% 22% 33% 17% 3	740
	11th to 20th	Night	19% 13% 22% 34% 12% 1	760
	21st to end	Night	19% 16% 21% 31% 13% 1	760
December	1st to 10th	Night	25% 18% 21% 29% 7%	760
	11th to 20th	Night	19% 16% 25% 31% 8% 1	760
	21st to end	Night	25% 16% 23% 29% 5%	836

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