Hay House Farm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hay House Farm Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	143367
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	138531
Transparency	Day	39% 4 12% 23% 18% 4	146971
	Night	39% 4 10% 23% 19% 5%	142997
Seeing	Day	19% 11% 27% 25% 14% 5%	17364
	Night	17% 9% 28% 31% 13% 1	27029

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	15% 6% 2 5% 3 4 4 3 4 6% 48%	5240
	11th to 20th	Day	13% 7% 2 4 3 3 3 3 3 4 54%	5280
	21st to end	Day	14% 7% 2 6% 4 4 5% 3 4 5% 46%	5780
February	1st to 10th	Day	13% 6% 3 5% 5% 5% 5% 3 6% 6% 41%	1890
	11th to 20th	Day	21% 7% 2 5% 4 3 3 3 5% 6% 41%	1862
	21st to end	Day	18% 7% 3 6% 5% 4 4 3 5% 6% 38%	1779
March	1st to 10th	Day	17% 6% 2 6% 4 5% 4 4 5% 7% 40%	5045
	11th to 20th	Day	11% 5% 2 6% 3 3 4 3 5% 7% 51%	5090
	21st to end	Day	16% 6% 2 6% 5% 5% 4 3 5% 7% 41%	5586
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 49%	2501
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2494
	21st to end	Day	15% 6% 3 6% 4 4 4 3 6% 6% 43%	2520
May	1st to 10th	Day	14% 6% 2 6% 4 4 5% 4 7% 8% 38%	7609
	11th to 20th	Day	17% 6% 2 5% 4 3 5% 4 5% 8% 39%	7626
	21st to end	Day	21% 5% 2 6% 4 4 4 4 5% 8% 38%	8946
June	1st to 10th	Day	10% 5% 2 5% 4 5% 6% 4 6% 9% 44%	2940
	11th to 20th	Day	11% 5% 2 5% 5% 4 6% 4 7% 11% 40%	2940
	21st to end	Day	8% 4 2 6% 5% 4 5% 5% 7% 10% 44%	2940
July	1st to 10th	Day	9% 4 2 6% 5% 4 5% 4 7% 11% 43%	8041
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 6% 9% 13% 30%	7680
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 10% 37%	8340
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 5% 8% 10% 32%	4364
	11th to 20th	Day	9% 5% 2 7% 5% 6% 5% 5% 8% 10% 39%	4101
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 10% 49%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 6% 8% 45%	2353
	21st to end	Day	22% 7% 2 5% 4 3 5% 4 5% 6% 36%	2110
October	1st to 10th	Day	15% 5% 2 6% 3 3 3 3 5% 6% 50%	2000
	11th to 20th	Day	16% 4 2 4 4 4 4 3 3 5% 52%	2000
	21st to end	Day	13% 5% 2 5% 4 3 3 3 4 6% 53%	2200
November	1st to 10th	Day	12% 4 1 5% 4 4 3 3 4 6% 55%	2057
	11th to 20th	Day	14% 6% 2 4 3 3 3 4 6% 5% 51%	1863
	21st to end	Day	15% 5% 2 5% 3 4 3 4 5% 5% 50%	1680
December	1st to 10th	Day	12% 5% 2 6% 3 5% 4 4 4 6% 49%	2680
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 6% 47%	3070
	21st to end	Day	18% 7% 2 7% 4 3 4 3 5% 7% 40%	4048
January	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 5% 50%	7635
	11th to 20th	Night	14% 6% 2 5% 3 3 2 2 3 4 55%	7680
	21st to end	Night	14% 6% 2 6% 3 3 3 2 4 5% 51%	8401
February	1st to 10th	Night	13% 7% 2 6% 4 5% 5% 4 5% 7% 43%	2730
	11th to 20th	Night	20% 8% 2 5% 3 3 3 3 4 6% 43%	2642
	21st to end	Night	15% 8% 3 5% 5% 5% 3 3 4 5% 45%	2024
March	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 47%	5028
	11th to 20th	Night	11% 4 2 4 3 3 4 3 6% 8% 53%	5090
	21st to end	Night	12% 6% 2 6% 4 3 4 3 5% 7% 47%	5588
April	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 8% 45%	2117
	11th to 20th	Night	11% 5% 2 6% 4 3 5% 3 6% 9% 47%	2075
	21st to end	Night	13% 5% 2 6% 4 5% 3 4 6% 7% 46%	2100
May	1st to 10th	Night	16% 7% 2 7% 4 4 5% 4 6% 8% 35%	5300
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 6% 7% 38%	5234
	21st to end	Night	22% 8% 3 6% 4 4 4 3 5% 7% 34%	5190
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 5% 7% 9% 44%	1680
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 4 7% 10% 41%	1680
	21st to end	Night	8% 5% 2 6% 4 5% 6% 4 8% 10% 43%	1680
July	1st to 10th	Night	12% 6% 2 7% 5% 4 5% 5% 6% 9% 39%	4911
	11th to 20th	Night	15% 7% 3 9% 6% 5% 6% 5% 8% 9% 28%	5277
	21st to end	Night	11% 6% 2 8% 6% 5% 6% 5% 7% 8% 35%	5736
August	1st to 10th	Night	17% 6% 2 8% 5% 5% 6% 4 6% 8% 33%	3005
	11th to 20th	Night	11% 7% 3 8% 6% 5% 5% 4 6% 8% 36%	3001
	21st to end	Night	10% 6% 2 7% 5% 5% 4 4 5% 9% 43%	3586
September	1st to 10th	Night	14% 5% 2 6% 3 5% 4 4 5% 7% 46%	2000
	11th to 20th	Night	12% 6% 2 6% 4 3 4 4 5% 7% 46%	2043
	21st to end	Night	23% 7% 3 6% 4 3 4 3 4 6% 38%	2289
October	1st to 10th	Night	18% 5% 2 5% 4 3 3 3 5% 6% 47%	2400
	11th to 20th	Night	15% 5% 2 6% 3 4 3 3 5% 5% 50%	2400
	21st to end	Night	14% 5% 2 4 4 3 4 2 4 6% 52%	2640
November	1st to 10th	Night	12% 5% 2 6% 4 3 3 3 4 5% 53%	2461
	11th to 20th	Night	14% 5% 2 6% 4 3 3 3 5% 6% 49%	2753
	21st to end	Night	12% 4 1 5% 4 4 4 4 5% 6% 50%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 54%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 43%	5345
	21st to end	Night	17% 6% 2 5% 3 3 4 4 6% 7% 42%	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	39% 2 7% 23% 20% 8%	5241
	11th to 20th	Day	35% 2 4 19% 25% 15%	5280
	21st to end	Day	42% 2 6% 28% 18% 4	5780
February	1st to 10th	Day	42% 4 9% 26% 16% 3	1890
	11th to 20th	Day	45% 3 7% 21% 20% 4	1871
	21st to end	Day	47% 3 10% 18% 17% 5%	1779
March	1st to 10th	Day	44% 3 8% 19% 17% 10%	5045
	11th to 20th	Day	33% 3 7% 26% 22% 9%	5090
	21st to end	Day	45% 3 10% 18% 17% 7%	5586
April	1st to 10th	Day	35% 4 10% 23% 20% 8%	2501
	11th to 20th	Day	35% 3 11% 22% 22% 6%	2494
	21st to end	Day	41% 4 10% 25% 15% 4	2520
May	1st to 10th	Day	44% 4 13% 23% 15% 1	7644
	11th to 20th	Day	43% 5% 11% 24% 16% 1	7630
	21st to end	Day	45% 4 11% 18% 18% 3	8961
June	1st to 10th	Day	35% 6% 16% 26% 16% 2	2940
	11th to 20th	Day	37% 6% 16% 23% 16% 2	2940
	21st to end	Day	34% 7% 16% 26% 16% 2	2940
July	1st to 10th	Day	36% 5% 20% 24% 11% 3	7974
	11th to 20th	Day	44% 6% 19% 24% 7% 1	7611
	21st to end	Day	40% 5% 14% 22% 18%	8156
August	1st to 10th	Day	45% 5% 18% 25% 6% 1	4351
	11th to 20th	Day	39% 5% 16% 25% 14% 1	4140
	21st to end	Day	32% 4 17% 29% 17% 1	4312
September	1st to 10th	Day	37% 5% 12% 19% 21% 5%	2400
	11th to 20th	Day	37% 4 7% 28% 20% 4	2353
	21st to end	Day	48% 5% 10% 18% 16% 3	2110
October	1st to 10th	Day	35% 2 7% 19% 31% 5%	3260
	11th to 20th	Day	27% 2 6% 23% 30% 11%	3240
	21st to end	Day	22% 2 9% 30% 24% 13%	3538
November	1st to 10th	Day	33% 3 8% 24% 27% 5%	2057
	11th to 20th	Day	34% 4 9% 24% 22% 7%	1863
	21st to end	Day	35% 4 9% 25% 23% 4	1680
December	1st to 10th	Day	37% 3 9% 25% 18% 8%	2680
	11th to 20th	Day	41% 2 13% 25% 16% 4	3066
	21st to end	Day	45% 3 12% 24% 16% 1	4048
January	1st to 10th	Night	36% 2 9% 22% 23% 8%	7636
	11th to 20th	Night	35% 2 5% 20% 27% 12%	7680
	21st to end	Night	38% 2 6% 27% 22% 6%	8401
February	1st to 10th	Night	41% 4 10% 25% 18% 3	2730
	11th to 20th	Night	45% 3 7% 20% 19% 7%	2644
	21st to end	Night	42% 3 9% 20% 18% 7%	2024
March	1st to 10th	Night	38% 4 6% 23% 21% 8%	5028
	11th to 20th	Night	30% 3 8% 22% 28% 8%	5090
	21st to end	Night	38% 3 9% 21% 22% 6%	5588
April	1st to 10th	Night	39% 4 10% 19% 22% 6%	2117
	11th to 20th	Night	33% 4 13% 23% 21% 6%	2075
	21st to end	Night	37% 4 12% 26% 16% 5%	2100
May	1st to 10th	Night	47% 3 14% 22% 11% 3	5327
	11th to 20th	Night	43% 5% 13% 26% 13% 1	5235
	21st to end	Night	51% 4 11% 18% 13% 3	5197
June	1st to 10th	Night	35% 7% 13% 26% 18% 1	1680
	11th to 20th	Night	38% 6% 18% 22% 14% 1	1680
	21st to end	Night	33% 7% 16% 23% 18% 3	1680
July	1st to 10th	Night	43% 7% 18% 19% 12% 2	4870
	11th to 20th	Night	52% 7% 16% 18% 6% 1	5226
	21st to end	Night	45% 6% 15% 19% 15%	5606
August	1st to 10th	Night	48% 5% 17% 23% 6% 1	2994
	11th to 20th	Night	44% 5% 14% 24% 12%	3028
	21st to end	Night	39% 4 13% 29% 14% 1	3586
September	1st to 10th	Night	38% 6% 12% 21% 16% 7%	2000
	11th to 20th	Night	37% 5% 10% 25% 21% 3	2043
	21st to end	Night	48% 4 9% 20% 16% 3	2289
October	1st to 10th	Night	42% 3 9% 19% 22% 6%	3920
	11th to 20th	Night	26% 3 7% 22% 30% 11%	3899
	21st to end	Night	22% 2 6% 31% 27% 11%	4252
November	1st to 10th	Night	35% 4 8% 23% 26% 6%	2461
	11th to 20th	Night	37% 3 8% 20% 24% 8%	2753
	21st to end	Night	35% 3 9% 24% 23% 6%	2940
December	1st to 10th	Night	32% 3 9% 28% 24% 4	4720
	11th to 20th	Night	45% 3 10% 19% 17% 6%	5348
	21st to end	Night	41% 3 11% 26% 17% 2	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	23% 24% 29% 19% 5%	327
	11th to 20th	Day	26% 29% 30% 12% 3	330
	21st to end	Day	26% 22% 36% 13% 3	363
February	1st to 10th	Day	24% 19% 41% 11% 5%	330
	11th to 20th	Day	41% 21% 27% 10% 2	349
	21st to end	Day	30% 17% 29% 20% 3	446
March	1st to 10th	Day	23% 16% 36% 19% 6% 1	540
	11th to 20th	Day	23% 18% 35% 17% 6%	540
	21st to end	Day	24% 17% 36% 18% 5% 1	594
April	1st to 10th	Day	23% 18% 32% 18% 8% 1	540
	11th to 20th	Day	20% 14% 32% 22% 9% 3	532
	21st to end	Day	22% 11% 33% 24% 9% 2	540
May	1st to 10th	Day	22% 15% 32% 20% 9% 2	543
	11th to 20th	Day	20% 11% 30% 24% 11% 4	567
	21st to end	Day	20% 6% 26% 27% 18% 4	617
June	1st to 10th	Day	13% 5% 27% 30% 19% 6%	570
	11th to 20th	Day	14% 6% 26% 29% 18% 7%	570
	21st to end	Day	10% 5% 25% 33% 21% 6%	570
July	1st to 10th	Day	12% 3 15% 35% 25% 10%	565
	11th to 20th	Day	7% 3 14% 35% 28% 12%	564
	21st to end	Day	10% 4 19% 32% 25% 10%	620
August	1st to 10th	Day	14% 4 19% 29% 24% 10%	566
	11th to 20th	Day	12% 3 18% 31% 25% 11%	535
	21st to end	Day	11% 3 17% 31% 23% 14%	583
September	1st to 10th	Day	14% 5% 17% 26% 22% 15%	530
	11th to 20th	Day	16% 3 15% 34% 23% 10%	530
	21st to end	Day	30% 5% 23% 28% 9% 5%	530
October	1st to 10th	Day	18% 5% 22% 29% 18% 8%	530
	11th to 20th	Day	16% 13% 24% 27% 18% 3	530
	21st to end	Day	18% 12% 32% 26% 10% 3	583
November	1st to 10th	Day	16% 9% 31% 29% 12% 3	535
	11th to 20th	Day	18% 13% 34% 24% 8% 3	393
	21st to end	Day	20% 17% 34% 19% 9% 1	220
December	1st to 10th	Day	24% 15% 40% 13% 7%	220
	11th to 20th	Day	24% 14% 32% 20% 7% 2	220
	21st to end	Day	27% 17% 33% 14% 7% 1	242
January	1st to 10th	Night	19% 15% 40% 23% 3	840
	11th to 20th	Night	21% 14% 35% 27% 3	840
	21st to end	Night	19% 15% 42% 23% 2	924
February	1st to 10th	Night	18% 13% 40% 26% 3	840
	11th to 20th	Night	25% 17% 38% 18% 1	823
	21st to end	Night	22% 16% 37% 22% 2	692
March	1st to 10th	Night	18% 16% 37% 26% 3	840
	11th to 20th	Night	22% 15% 36% 22% 5%	840
	21st to end	Night	19% 15% 37% 25% 5%	924
April	1st to 10th	Night	18% 16% 38% 23% 5%	840
	11th to 20th	Night	13% 13% 38% 28% 8%	831
	21st to end	Night	15% 10% 35% 32% 8%	840
May	1st to 10th	Night	19% 8% 32% 30% 11%	654
	11th to 20th	Night	20% 4 26% 34% 15% 1	621
	21st to end	Night	16% 2 20% 37% 22% 2	486
June	1st to 10th	Night	11% 1 20% 43% 24% 1	420
	11th to 20th	Night	11% 5% 16% 44% 23% 1	420
	21st to end	Night	8% 2 20% 43% 24% 2	420
July	1st to 10th	Night	12% 2 14% 39% 28% 4	484
	11th to 20th	Night	8% 1 11% 40% 34% 5%	612
	21st to end	Night	9% 2 13% 43% 29% 4	690
August	1st to 10th	Night	13% 2 10% 40% 31% 4	627
	11th to 20th	Night	15% 4 9% 40% 30% 3	657
	21st to end	Night	12% 3 12% 39% 30% 5%	880
September	1st to 10th	Night	16% 2 13% 35% 29% 5%	800
	11th to 20th	Night	14% 4 15% 35% 29% 4	800
	21st to end	Night	26% 3 18% 32% 20% 1	800
October	1st to 10th	Night	21% 4 19% 30% 23% 4	800
	11th to 20th	Night	17% 9% 25% 33% 16% 1	800
	21st to end	Night	16% 8% 30% 33% 12% 1	880
November	1st to 10th	Night	14% 8% 29% 34% 13% 1	820
	11th to 20th	Night	17% 12% 33% 31% 8%	840
	21st to end	Night	15% 9% 35% 32% 9% 1	840
December	1st to 10th	Night	20% 11% 35% 27% 6%	840
	11th to 20th	Night	19% 10% 35% 31% 5%	840
	21st to end	Night	21% 11% 31% 33% 4	924

Page generated at 2026-08-12 03:37:19. Version ac07

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