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 2025-04-11.

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 4 4 6% 8% 43%	113442
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	114845
Transparency	Day	39% 4 11% 23% 18% 5%	117090
	Night	38% 4 10% 23% 19% 5%	119336
Seeing	Day	19% 10% 27% 25% 14% 5%	15544
	Night	17% 9% 28% 31% 14% 1	25372

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% 5% 2 4 3 3 4 3 4 6% 51%	4038
	11th to 20th	Day	14% 7% 2 4 4 3 4 3 4 4 51%	4060
	21st to end	Day	15% 7% 2 6% 4 4 4 4 4 5% 45%	4466
February	1st to 10th	Day	14% 6% 3 5% 5% 5% 5% 3 7% 6% 40%	1800
	11th to 20th	Day	20% 7% 2 5% 4 3 3 3 5% 6% 42%	1771
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 5% 6% 39%	1697
March	1st to 10th	Day	17% 6% 2 6% 4 5% 4 4 5% 7% 40%	4935
	11th to 20th	Day	10% 5% 2 6% 3 3 4 3 5% 7% 51%	4980
	21st to end	Day	16% 6% 2 6% 5% 5% 4 3 5% 7% 40%	5465
April	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2382
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 48%	2254
	21st to end	Day	16% 6% 3 6% 4 4 4 3 5% 6% 42%	2280
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 7% 7% 32%	4075
	11th to 20th	Day	21% 8% 3 6% 4 4 5% 4 6% 8% 32%	4081
	21st to end	Day	10% 4 2 6% 4 4 5% 5% 7% 10% 44%	4779
June	1st to 10th	Day	10% 5% 2 4 5% 5% 5% 4 7% 9% 43%	2660
	11th to 20th	Day	11% 5% 2 5% 5% 4 7% 4 6% 10% 41%	2660
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 9% 43%	2660
July	1st to 10th	Day	7% 4 2 5% 5% 4 7% 5% 8% 11% 42%	4313
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 12% 36%	4120
	21st to end	Day	12% 7% 2 7% 6% 5% 6% 5% 8% 9% 32%	4466
August	1st to 10th	Day	16% 6% 2 6% 5% 6% 5% 5% 8% 10% 32%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 6% 5% 5% 8% 9% 39%	3923
	21st to end	Day	9% 4 1 5% 4 4 5% 4 6% 10% 48%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 8% 45%	2236
	21st to end	Day	22% 7% 2 5% 4 3 5% 4 4 6% 37%	2005
October	1st to 10th	Day	15% 5% 2 6% 3 3 3 2 5% 6% 49%	1900
	11th to 20th	Day	16% 4 2 4 4 4 4 3 4 5% 53%	1900
	21st to end	Day	13% 5% 2 5% 4 3 3 3 4 6% 53%	2090
November	1st to 10th	Day	12% 4 1 5% 4 4 3 3 4 6% 54%	1957
	11th to 20th	Day	14% 6% 2 4 3 3 3 4 5% 5% 50%	1775
	21st to end	Day	15% 5% 2 5% 3 4 3 4 5% 6% 50%	1600
December	1st to 10th	Day	11% 5% 2 6% 3 5% 4 4 4 6% 50%	2600
	11th to 20th	Day	19% 7% 2 5% 2 3 3 3 4 6% 46%	2990
	21st to end	Day	18% 7% 2 6% 4 3 4 3 5% 7% 40%	3960
January	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 54%	5889
	11th to 20th	Night	14% 6% 2 4 3 3 2 2 3 4 55%	5900
	21st to end	Night	15% 7% 3 5% 3 3 3 2 4 4 50%	6490
February	1st to 10th	Night	14% 7% 2 6% 4 5% 5% 4 5% 7% 42%	2600
	11th to 20th	Night	20% 8% 2 5% 3 3 3 3 4 6% 43%	2513
	21st to end	Night	15% 8% 3 5% 5% 5% 3 3 4 6% 45%	1930
March	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 47%	4918
	11th to 20th	Night	11% 4 2 4 3 3 4 3 6% 8% 53%	4980
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 6% 47%	5467
April	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 5% 7% 44%	2016
	11th to 20th	Night	11% 5% 2 6% 3 3 5% 3 6% 8% 48%	1882
	21st to end	Night	14% 5% 2 5% 4 4 3 4 6% 7% 45%	1900
May	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 5% 8% 33%	2848
	11th to 20th	Night	23% 7% 2 6% 4 4 5% 4 6% 6% 32%	2812
	21st to end	Night	10% 5% 2 6% 4 5% 5% 5% 6% 10% 43%	2772
June	1st to 10th	Night	10% 5% 2 5% 5% 4 4 5% 8% 9% 43%	1520
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 4 7% 9% 42%	1520
	21st to end	Night	8% 4 2 6% 4 5% 6% 4 8% 10% 43%	1520
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 6% 8% 10% 41%	2641
	11th to 20th	Night	9% 6% 3 8% 5% 5% 5% 5% 8% 10% 36%	2840
	21st to end	Night	11% 8% 2 9% 7% 6% 7% 5% 7% 8% 31%	3087
August	1st to 10th	Night	17% 5% 2 8% 5% 5% 6% 4 6% 8% 33%	2825
	11th to 20th	Night	11% 7% 3 9% 6% 5% 5% 4 6% 8% 36%	2872
	21st to end	Night	10% 6% 2 7% 5% 5% 4 4 5% 9% 43%	3476
September	1st to 10th	Night	14% 5% 2 6% 3 5% 4 4 5% 7% 44%	1900
	11th to 20th	Night	12% 6% 2 6% 4 3 4 4 5% 8% 46%	1941
	21st to end	Night	23% 6% 3 6% 4 3 4 3 4 6% 39%	2174
October	1st to 10th	Night	18% 5% 2 5% 4 3 3 3 5% 6% 46%	2280
	11th to 20th	Night	15% 5% 2 6% 3 4 3 3 5% 5% 50%	2280
	21st to end	Night	13% 5% 2 4 4 3 4 2 4 6% 53%	2508
November	1st to 10th	Night	12% 5% 2 6% 4 3 3 3 4 5% 52%	2341
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 6% 49%	2622
	21st to end	Night	13% 4 1 5% 4 4 4 4 5% 6% 50%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 54%	4580
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 4 5% 42%	5205
	21st to end	Night	17% 6% 2 5% 4 3 4 4 6% 7% 42%	6996

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	36% 3 4 25% 22% 10%	4039
	11th to 20th	Day	37% 3 5% 20% 23% 13%	4060
	21st to end	Day	42% 3 7% 25% 19% 4	4466
February	1st to 10th	Day	43% 4 9% 27% 14% 3	1800
	11th to 20th	Day	45% 3 7% 21% 20% 4	1780
	21st to end	Day	46% 3 10% 18% 18% 5%	1697
March	1st to 10th	Day	44% 3 8% 19% 17% 10%	4935
	11th to 20th	Day	33% 3 7% 26% 22% 9%	4980
	21st to end	Day	45% 3 10% 18% 17% 7%	5465
April	1st to 10th	Day	36% 4 10% 23% 19% 8%	2382
	11th to 20th	Day	36% 3 11% 21% 23% 6%	2254
	21st to end	Day	42% 4 10% 23% 15% 5%	2280
May	1st to 10th	Day	49% 4 13% 19% 12% 2	4113
	11th to 20th	Day	48% 7% 12% 22% 11% 1	4085
	21st to end	Day	34% 5% 16% 22% 21% 2	4793
June	1st to 10th	Day	35% 6% 16% 27% 14% 2	2660
	11th to 20th	Day	38% 6% 15% 23% 16% 2	2660
	21st to end	Day	34% 7% 16% 26% 16% 2	2660
July	1st to 10th	Day	34% 6% 23% 20% 11% 6%	4246
	11th to 20th	Day	38% 5% 20% 28% 8% 1	4051
	21st to end	Day	45% 6% 13% 21% 14% 1	4324
August	1st to 10th	Day	45% 5% 18% 25% 6% 1	4091
	11th to 20th	Day	39% 5% 15% 25% 15% 1	3962
	21st to end	Day	32% 4 17% 29% 17% 2	4180
September	1st to 10th	Day	38% 5% 12% 19% 21% 5%	2280
	11th to 20th	Day	37% 5% 7% 28% 20% 4	2236
	21st to end	Day	47% 5% 9% 18% 17% 3	2005
October	1st to 10th	Day	35% 2 7% 19% 31% 5%	3160
	11th to 20th	Day	26% 2 6% 23% 31% 11%	3140
	21st to end	Day	22% 2 9% 31% 24% 13%	3428
November	1st to 10th	Day	33% 3 8% 24% 27% 5%	1957
	11th to 20th	Day	34% 4 9% 23% 22% 8%	1775
	21st to end	Day	35% 4 9% 24% 24% 4	1600
December	1st to 10th	Day	37% 3 9% 25% 18% 9%	2600
	11th to 20th	Day	41% 2 13% 24% 16% 4	2986
	21st to end	Day	45% 3 12% 24% 15% 1	3960
January	1st to 10th	Night	33% 3 5% 21% 27% 10%	5889
	11th to 20th	Night	35% 2 5% 22% 26% 10%	5900
	21st to end	Night	40% 2 6% 26% 21% 5%	6490
February	1st to 10th	Night	41% 4 10% 25% 17% 3	2600
	11th to 20th	Night	44% 3 7% 20% 19% 7%	2515
	21st to end	Night	42% 3 9% 20% 18% 7%	1930
March	1st to 10th	Night	38% 4 6% 23% 21% 8%	4918
	11th to 20th	Night	31% 3 8% 22% 28% 8%	4980
	21st to end	Night	39% 3 9% 21% 22% 6%	5467
April	1st to 10th	Night	40% 4 9% 19% 22% 7%	2016
	11th to 20th	Night	33% 5% 13% 22% 22% 6%	1882
	21st to end	Night	37% 4 12% 25% 16% 6%	1900
May	1st to 10th	Night	50% 4 14% 19% 11% 2	2875
	11th to 20th	Night	49% 7% 12% 21% 9% 2	2813
	21st to end	Night	36% 6% 16% 25% 15% 2	2779
June	1st to 10th	Night	34% 7% 14% 28% 16% 1	1520
	11th to 20th	Night	37% 7% 18% 23% 14% 2	1520
	21st to end	Night	33% 8% 16% 23% 18% 3	1520
July	1st to 10th	Night	37% 7% 25% 16% 12% 3	2598
	11th to 20th	Night	41% 7% 21% 23% 7% 1	2786
	21st to end	Night	48% 7% 15% 19% 10%	2988
August	1st to 10th	Night	48% 5% 18% 23% 7% 1	2814
	11th to 20th	Night	44% 5% 14% 24% 13%	2899
	21st to end	Night	39% 4 13% 29% 13% 1	3476
September	1st to 10th	Night	39% 6% 12% 20% 15% 7%	1900
	11th to 20th	Night	37% 5% 10% 25% 21% 3	1941
	21st to end	Night	47% 5% 8% 21% 16% 3	2174
October	1st to 10th	Night	42% 3 9% 19% 21% 6%	3800
	11th to 20th	Night	26% 3 7% 22% 31% 11%	3779
	21st to end	Night	22% 2 7% 32% 27% 11%	4120
November	1st to 10th	Night	35% 4 8% 23% 25% 6%	2341
	11th to 20th	Night	37% 3 8% 20% 24% 8%	2622
	21st to end	Night	35% 3 9% 25% 23% 6%	2800
December	1st to 10th	Night	31% 3 9% 28% 25% 4	4580
	11th to 20th	Night	46% 3 10% 19% 16% 6%	5208
	21st to end	Night	41% 3 11% 26% 17% 1	6996

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% 24% 30% 18% 5%	297
	11th to 20th	Day	25% 28% 31% 13% 3	300
	21st to end	Day	26% 21% 37% 13% 3	330
February	1st to 10th	Day	24% 19% 42% 11% 5%	300
	11th to 20th	Day	41% 22% 26% 10% 2	318
	21st to end	Day	29% 18% 28% 21% 4	414
March	1st to 10th	Day	23% 17% 35% 19% 6%	500
	11th to 20th	Day	22% 18% 35% 17% 7%	500
	21st to end	Day	25% 17% 35% 18% 5% 1	550
April	1st to 10th	Day	24% 19% 32% 17% 8% 1	500
	11th to 20th	Day	19% 14% 30% 22% 10% 4	452
	21st to end	Day	23% 11% 34% 23% 8% 2	460
May	1st to 10th	Day	22% 14% 31% 21% 10% 3	463
	11th to 20th	Day	21% 10% 29% 24% 12% 4	487
	21st to end	Day	17% 4 27% 29% 19% 4	529
June	1st to 10th	Day	14% 5% 25% 31% 21% 5%	490
	11th to 20th	Day	13% 5% 26% 30% 18% 8%	490
	21st to end	Day	10% 5% 25% 35% 20% 6%	490
July	1st to 10th	Day	12% 2 16% 35% 26% 10%	485
	11th to 20th	Day	8% 3 14% 36% 28% 11%	484
	21st to end	Day	11% 4 19% 33% 23% 10%	532
August	1st to 10th	Day	13% 3 18% 29% 25% 11%	486
	11th to 20th	Day	12% 2 19% 31% 25% 11%	479
	21st to end	Day	11% 3 16% 31% 24% 15%	539
September	1st to 10th	Day	15% 6% 16% 26% 23% 16%	490
	11th to 20th	Day	16% 3 15% 34% 22% 10%	490
	21st to end	Day	30% 5% 23% 28% 9% 5%	490
October	1st to 10th	Day	19% 5% 22% 29% 17% 7%	490
	11th to 20th	Day	15% 12% 24% 27% 19% 3	490
	21st to end	Day	16% 11% 33% 27% 10% 3	539
November	1st to 10th	Day	17% 8% 28% 30% 13% 4	495
	11th to 20th	Day	18% 12% 33% 25% 9% 3	365
	21st to end	Day	19% 16% 35% 21% 9% 1	200
December	1st to 10th	Day	23% 17% 40% 13% 8% 1	200
	11th to 20th	Day	25% 14% 32% 21% 8% 2	200
	21st to end	Day	28% 17% 31% 15% 8% 1	220
January	1st to 10th	Night	19% 15% 40% 22% 3	800
	11th to 20th	Night	21% 14% 35% 28% 4	800
	21st to end	Night	19% 15% 41% 23% 2	880
February	1st to 10th	Night	18% 13% 39% 26% 3	800
	11th to 20th	Night	25% 17% 37% 18% 2	783
	21st to end	Night	22% 16% 38% 22% 2	660
March	1st to 10th	Night	18% 16% 37% 25% 4	800
	11th to 20th	Night	21% 15% 36% 22% 6% 1	800
	21st to end	Night	19% 15% 36% 25% 5%	880
April	1st to 10th	Night	18% 16% 37% 23% 5%	800
	11th to 20th	Night	14% 12% 37% 28% 9%	754
	21st to end	Night	16% 10% 34% 32% 8%	760
May	1st to 10th	Night	20% 7% 30% 30% 12%	592
	11th to 20th	Night	21% 3 24% 35% 16% 1	561
	21st to end	Night	14% 2 19% 39% 24% 3	439
June	1st to 10th	Night	11% 1 19% 44% 24% 1	380
	11th to 20th	Night	10% 4 14% 45% 25% 2	380
	21st to end	Night	8% 2 20% 46% 22% 1	380
July	1st to 10th	Night	12% 2 15% 39% 28% 4	436
	11th to 20th	Night	7% 1 11% 41% 36% 4	552
	21st to end	Night	9% 2 13% 42% 29% 4	624
August	1st to 10th	Night	13% 2 9% 41% 31% 4	567
	11th to 20th	Night	16% 3 8% 40% 29% 3	612
	21st to end	Night	12% 3 11% 38% 31% 5%	836
September	1st to 10th	Night	16% 2 13% 34% 30% 6%	760
	11th to 20th	Night	14% 4 15% 34% 30% 4	760
	21st to end	Night	25% 3 18% 32% 21% 1	760
October	1st to 10th	Night	21% 4 19% 30% 23% 4	760
	11th to 20th	Night	17% 8% 24% 33% 16% 1	760
	21st to end	Night	16% 8% 29% 34% 12% 1	836
November	1st to 10th	Night	14% 7% 30% 34% 13% 2	780
	11th to 20th	Night	17% 11% 33% 32% 8%	800
	21st to end	Night	15% 9% 34% 32% 9% 1	800
December	1st to 10th	Night	21% 11% 35% 27% 7%	800
	11th to 20th	Night	19% 9% 35% 32% 5%	800
	21st to end	Night	21% 10% 30% 34% 4	880

Page generated at 2025-04-08 09:17:26. Version ac07

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