Harken Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harken Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 9% 4 8% 5% 4 5% 4 6% 7% 37%	131461
	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 38%	130958
Transparency	Day	44% 6% 12% 24% 13% 1	134960
	Night	45% 5% 11% 23% 14% 2	135534
Seeing	Day	16% 13% 30% 25% 12% 4	17799
	Night	17% 11% 31% 28% 11% 1	27796

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	10% 13% 6% 14% 6% 6% 6% 4 5% 6% 24%	4691
	11th to 20th	Day	6% 12% 6% 13% 5% 4 5% 6% 6% 7% 30%	4710
	21st to end	Day	10% 12% 7% 10% 5% 4 5% 3 4 5% 35%	5198
February	1st to 10th	Day	10% 13% 6% 11% 5% 6% 5% 4 5% 7% 30%	1890
	11th to 20th	Day	12% 12% 3 8% 5% 5% 5% 3 5% 6% 36%	2043
	21st to end	Day	11% 15% 5% 11% 5% 5% 4 4 4 6% 30%	1730
March	1st to 10th	Day	13% 8% 3 7% 4 3 4 3 5% 6% 42%	4580
	11th to 20th	Day	10% 7% 3 7% 5% 4 5% 4 5% 7% 42%	4777
	21st to end	Day	20% 13% 4 10% 5% 4 4 3 5% 6% 28%	6099
April	1st to 10th	Day	18% 13% 4 8% 5% 4 5% 4 5% 6% 29%	2520
	11th to 20th	Day	15% 11% 3 7% 6% 5% 4 4 5% 7% 33%	2446
	21st to end	Day	17% 11% 3 6% 5% 4 5% 4 6% 7% 33%	2530
May	1st to 10th	Day	14% 9% 3 7% 4 4 4 4 5% 7% 40%	5880
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 4 6% 8% 40%	6319
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 9% 37%	6935
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 7% 9% 44%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 7% 9% 42%	2800
	21st to end	Day	11% 7% 2 6% 5% 4 5% 5% 6% 10% 40%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 4 7% 9% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 6% 9% 45%	6360
	21st to end	Day	8% 6% 3 7% 4 5% 5% 4 7% 9% 43%	6924
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 40%	4545
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 7% 8% 42%	3919
	21st to end	Day	8% 7% 3 6% 4 4 5% 4 7% 8% 44%	4312
September	1st to 10th	Day	11% 6% 3 6% 4 4 3 4 7% 8% 44%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 5% 8% 49%	2285
	21st to end	Day	12% 7% 3 6% 5% 4 4 3 6% 7% 43%	2200
October	1st to 10th	Day	12% 7% 2 6% 4 3 4 4 5% 6% 47%	2120
	11th to 20th	Day	18% 10% 3 7% 4 5% 4 4 5% 7% 33%	1975
	21st to end	Day	17% 10% 3 7% 4 5% 4 2 5% 5% 37%	1980
November	1st to 10th	Day	11% 10% 3 6% 5% 5% 5% 3 5% 5% 43%	1852
	11th to 20th	Day	11% 12% 5% 10% 5% 5% 5% 4 5% 7% 31%	1890
	21st to end	Day	17% 13% 3 8% 5% 3 3 2 5% 6% 34%	1743
December	1st to 10th	Day	7% 8% 5% 10% 6% 6% 5% 4 5% 8% 35%	2690
	11th to 20th	Day	12% 16% 5% 11% 5% 5% 5% 4 6% 7% 25%	3088
	21st to end	Day	19% 21% 9% 12% 5% 4 3 3 4 4 15%	4070
January	1st to 10th	Night	11% 12% 6% 13% 6% 4 4 3 4 4 32%	8232
	11th to 20th	Night	8% 13% 7% 15% 7% 6% 6% 4 5% 5% 23%	8280
	21st to end	Night	9% 13% 6% 10% 6% 5% 6% 4 6% 6% 30%	8998
February	1st to 10th	Night	11% 14% 5% 9% 6% 4 4 3 5% 6% 33%	2730
	11th to 20th	Night	12% 10% 3 7% 5% 4 5% 4 5% 5% 40%	2462
	21st to end	Night	16% 11% 4 7% 4 4 4 3 5% 6% 35%	2076
March	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 4 5% 43%	5493
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 6% 9% 46%	5431
	21st to end	Night	18% 13% 5% 9% 5% 5% 4 3 4 5% 30%	5072
April	1st to 10th	Night	18% 10% 3 7% 4 5% 4 4 5% 6% 35%	2100
	11th to 20th	Night	19% 10% 3 6% 5% 4 3 3 5% 5% 37%	2035
	21st to end	Night	17% 9% 3 6% 4 4 3 4 6% 7% 38%	1870
May	1st to 10th	Night	16% 8% 3 7% 4 4 4 3 6% 7% 39%	4000
	11th to 20th	Night	10% 6% 2 8% 5% 5% 5% 4 7% 7% 43%	3596
	21st to end	Night	15% 9% 2 6% 4 4 4 3 5% 7% 40%	3942
June	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 47%	1600
	11th to 20th	Night	12% 5% 2 6% 5% 4 5% 5% 6% 8% 44%	1600
	21st to end	Night	9% 7% 2 7% 4 5% 4 3 6% 7% 47%	1600
July	1st to 10th	Night	10% 7% 2 7% 4 4 4 3 5% 8% 44%	3618
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 3 6% 8% 53%	3617
	21st to end	Night	6% 5% 2 5% 4 5% 5% 4 7% 9% 49%	3947
August	1st to 10th	Night	9% 5% 2 5% 3 4 5% 3 7% 8% 49%	2612
	11th to 20th	Night	10% 6% 2 5% 4 4 4 3 6% 8% 47%	3127
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 9% 51%	3586
September	1st to 10th	Night	8% 5% 3 5% 4 3 4 3 5% 7% 53%	2000
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 4 6% 54%	2095
	21st to end	Night	12% 6% 3 6% 4 5% 4 3 5% 7% 47%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 4 3 4 5% 5% 49%	2276
	11th to 20th	Night	17% 8% 2 6% 3 4 4 3 4 7% 42%	2425
	21st to end	Night	18% 8% 3 7% 4 4 3 3 6% 7% 37%	2860
November	1st to 10th	Night	13% 7% 3 6% 3 4 4 3 5% 6% 45%	2666
	11th to 20th	Night	13% 10% 4 7% 6% 5% 5% 3 5% 7% 35%	2730
	21st to end	Night	15% 12% 3 7% 5% 5% 4 3 5% 6% 35%	2873
December	1st to 10th	Night	9% 10% 3 7% 5% 4 5% 4 5% 7% 41%	4720
	11th to 20th	Night	15% 16% 5% 9% 6% 5% 5% 3 4 4 27%	5339
	21st to end	Night	19% 20% 5% 11% 6% 5% 3 3 4 5% 19%	7150

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	60% 5% 14% 15% 6%	4692
	11th to 20th	Day	51% 5% 9% 19% 15% 1	4710
	21st to end	Day	51% 4 9% 23% 12%	5198
February	1st to 10th	Day	54% 5% 11% 19% 10% 1	1890
	11th to 20th	Day	49% 5% 9% 24% 11% 2	2053
	21st to end	Day	55% 5% 10% 18% 10% 2	1730
March	1st to 10th	Day	42% 4 11% 26% 12% 5%	4580
	11th to 20th	Day	41% 5% 14% 26% 13% 2	4777
	21st to end	Day	59% 4 11% 15% 9% 2	6101
April	1st to 10th	Day	55% 6% 11% 18% 10%	2520
	11th to 20th	Day	50% 5% 11% 20% 12% 2	2446
	21st to end	Day	49% 6% 13% 19% 12% 1	2530
May	1st to 10th	Day	43% 5% 11% 25% 15% 1	5915
	11th to 20th	Day	40% 7% 12% 26% 15% 1	6323
	21st to end	Day	40% 7% 11% 27% 13% 2	6949
June	1st to 10th	Day	35% 7% 13% 27% 16% 2	2800
	11th to 20th	Day	37% 6% 13% 25% 17% 1	2800
	21st to end	Day	38% 7% 14% 23% 15% 2	2800
July	1st to 10th	Day	39% 7% 13% 29% 10% 1	6291
	11th to 20th	Day	32% 8% 15% 30% 15% 1	6285
	21st to end	Day	35% 8% 16% 22% 17% 2	6728
August	1st to 10th	Day	37% 7% 15% 28% 12% 1	4530
	11th to 20th	Day	36% 8% 13% 31% 11% 1	3957
	21st to end	Day	35% 6% 13% 26% 19% 1	4311
September	1st to 10th	Day	37% 7% 11% 28% 17% 2	2400
	11th to 20th	Day	32% 5% 12% 33% 17% 1	2285
	21st to end	Day	39% 6% 11% 30% 14%	2200
October	1st to 10th	Day	24% 3 6% 40% 25% 1	3458
	11th to 20th	Day	36% 4 11% 27% 21% 1	3191
	21st to end	Day	41% 4 10% 28% 14% 2	3180
November	1st to 10th	Day	42% 4 13% 24% 15% 1	1852
	11th to 20th	Day	52% 5% 13% 20% 10% 1	1890
	21st to end	Day	52% 4 13% 24% 7%	1743
December	1st to 10th	Day	46% 5% 19% 23% 6% 1	2690
	11th to 20th	Day	56% 6% 11% 17% 8% 1	3085
	21st to end	Day	73% 4 6% 10% 7%	4070
January	1st to 10th	Night	55% 4 9% 21% 10%	8233
	11th to 20th	Night	61% 4 9% 11% 14% 1	8280
	21st to end	Night	53% 5% 11% 20% 11%	8998
February	1st to 10th	Night	52% 4 11% 19% 12% 2	2730
	11th to 20th	Night	45% 6% 10% 23% 15% 2	2463
	21st to end	Night	49% 4 12% 21% 12% 3	2076
March	1st to 10th	Night	44% 4 13% 23% 12% 4	5493
	11th to 20th	Night	35% 4 13% 29% 14% 4	5431
	21st to end	Night	57% 5% 9% 16% 12% 1	5072
April	1st to 10th	Night	48% 6% 9% 20% 16% 1	2100
	11th to 20th	Night	50% 4 7% 20% 17% 3	2035
	21st to end	Night	45% 6% 12% 21% 15% 2	1870
May	1st to 10th	Night	43% 5% 12% 19% 18% 2	4027
	11th to 20th	Night	39% 6% 14% 22% 16% 3	3597
	21st to end	Night	43% 6% 10% 22% 13% 5%	3950
June	1st to 10th	Night	34% 6% 13% 28% 18% 2	1600
	11th to 20th	Night	36% 7% 10% 23% 21% 2	1600
	21st to end	Night	36% 5% 15% 23% 18% 2	1600
July	1st to 10th	Night	38% 6% 13% 31% 11% 3	3580
	11th to 20th	Night	28% 6% 15% 31% 18% 2	3572
	21st to end	Night	29% 8% 15% 23% 23% 2	3832
August	1st to 10th	Night	32% 6% 15% 26% 19% 1	2603
	11th to 20th	Night	33% 6% 14% 32% 15% 1	3155
	21st to end	Night	29% 6% 13% 28% 21% 2	3586
September	1st to 10th	Night	29% 6% 12% 28% 23% 2	2000
	11th to 20th	Night	31% 5% 10% 33% 19% 2	2095
	21st to end	Night	37% 4 9% 32% 16% 1	2200
October	1st to 10th	Night	26% 4 6% 38% 24% 1	3718
	11th to 20th	Night	35% 4 12% 25% 21% 2	3948
	21st to end	Night	42% 5% 9% 26% 17% 1	4610
November	1st to 10th	Night	41% 5% 9% 26% 17% 2	2666
	11th to 20th	Night	48% 5% 13% 21% 11% 2	2730
	21st to end	Night	50% 5% 14% 21% 10% 1	2873
December	1st to 10th	Night	42% 5% 16% 25% 8% 3	4720
	11th to 20th	Night	60% 5% 9% 15% 10% 1	5341
	21st to end	Night	68% 5% 8% 14% 6%	7150

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	16% 20% 35% 23% 5% 1	330
	11th to 20th	Day	20% 21% 38% 18% 2 1	330
	21st to end	Day	22% 17% 39% 17% 3	363
February	1st to 10th	Day	15% 22% 41% 15% 6% 1	330
	11th to 20th	Day	21% 25% 36% 16% 3	330
	21st to end	Day	21% 27% 39% 11% 1	273
March	1st to 10th	Day	17% 28% 40% 13% 2	330
	11th to 20th	Day	26% 21% 31% 16% 5% 1	349
	21st to end	Day	27% 16% 30% 20% 6% 1	594
April	1st to 10th	Day	26% 16% 35% 18% 4	540
	11th to 20th	Day	20% 24% 27% 19% 9% 2	516
	21st to end	Day	22% 15% 28% 22% 11% 1	500
May	1st to 10th	Day	21% 12% 34% 24% 7% 2	508
	11th to 20th	Day	20% 13% 31% 25% 9% 3	725
	21st to end	Day	17% 10% 31% 26% 11% 5%	789
June	1st to 10th	Day	9% 8% 34% 30% 14% 5%	730
	11th to 20th	Day	13% 9% 31% 30% 12% 5%	730
	21st to end	Day	13% 8% 32% 31% 12% 4	730
July	1st to 10th	Day	8% 3 24% 31% 24% 9%	721
	11th to 20th	Day	9% 8% 26% 33% 18% 7%	718
	21st to end	Day	7% 4 29% 37% 17% 7%	794
August	1st to 10th	Day	8% 4 23% 36% 20% 10%	721
	11th to 20th	Day	12% 5% 18% 31% 22% 12%	519
	21st to end	Day	9% 8% 21% 31% 24% 8%	583
September	1st to 10th	Day	13% 9% 22% 27% 21% 8%	530
	11th to 20th	Day	11% 3 21% 31% 24% 10%	530
	21st to end	Day	15% 12% 25% 25% 17% 6%	530
October	1st to 10th	Day	16% 10% 27% 27% 15% 5%	450
	11th to 20th	Day	22% 18% 31% 21% 7%	330
	21st to end	Day	26% 15% 36% 18% 4 1	363
November	1st to 10th	Day	17% 19% 35% 19% 8% 1	330
	11th to 20th	Day	20% 20% 37% 17% 6%	330
	21st to end	Day	26% 17% 37% 16% 3	330
December	1st to 10th	Day	15% 23% 41% 15% 6% 1	330
	11th to 20th	Day	19% 22% 36% 17% 6%	330
	21st to end	Day	26% 23% 34% 13% 2 1	363
January	1st to 10th	Night	19% 18% 35% 22% 5%	1050
	11th to 20th	Night	20% 17% 38% 20% 6%	1050
	21st to end	Night	18% 14% 39% 24% 5%	1155
February	1st to 10th	Night	19% 15% 38% 23% 5%	1050
	11th to 20th	Night	17% 19% 34% 25% 4	827
	21st to end	Night	23% 15% 39% 18% 4	692
March	1st to 10th	Night	21% 13% 34% 24% 7% 1	840
	11th to 20th	Night	18% 14% 35% 25% 7%	809
	21st to end	Night	21% 11% 31% 30% 6% 1	693
April	1st to 10th	Night	23% 12% 35% 24% 6%	630
	11th to 20th	Night	23% 14% 30% 25% 7% 1	611
	21st to end	Night	21% 11% 31% 27% 10% 1	600
May	1st to 10th	Night	19% 9% 33% 33% 7% 1	600
	11th to 20th	Night	18% 6% 31% 32% 13%	591
	21st to end	Night	14% 6% 24% 37% 18% 2	650
June	1st to 10th	Night	10% 6% 27% 41% 16% 1	600
	11th to 20th	Night	13% 5% 32% 34% 15% 1	600
	21st to end	Night	11% 6% 26% 40% 17% 1	600
July	1st to 10th	Night	9% 2 17% 40% 28% 4	585
	11th to 20th	Night	7% 4 21% 41% 23% 3	582
	21st to end	Night	5% 4 18% 46% 26% 2	657
August	1st to 10th	Night	7% 2 16% 47% 26% 2	597
	11th to 20th	Night	10% 4 15% 39% 28% 5%	594
	21st to end	Night	9% 3 18% 40% 26% 3	660
September	1st to 10th	Night	9% 4 22% 34% 25% 6%	600
	11th to 20th	Night	10% 5% 22% 29% 29% 4	695
	21st to end	Night	14% 8% 26% 28% 21% 4	800
October	1st to 10th	Night	14% 9% 28% 30% 18% 2	800
	11th to 20th	Night	21% 12% 32% 26% 8% 1	800
	21st to end	Night	23% 10% 31% 25% 10% 1	880
November	1st to 10th	Night	17% 12% 37% 23% 10% 1	820
	11th to 20th	Night	19% 18% 35% 22% 7%	840
	21st to end	Night	21% 16% 34% 22% 5%	983
December	1st to 10th	Night	21% 17% 37% 20% 4	1050
	11th to 20th	Night	23% 15% 35% 21% 5% 1	1050
	21st to end	Night	28% 16% 35% 19% 3	1155

Page generated at 2026-04-14 01:44:24. Version ac07

Copyright © 2026 Attilla Danko.