Joseph Dale Perry Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph Dale Perry Memorial Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	15% 9% 3 7% 5% 4 4 4 6% 7% 37%	101498
	Night	16% 9% 3 7% 4 4 4 3 5% 7% 38%	98243
Transparency	Day	45% 5% 11% 24% 14% 2	105240
	Night	45% 5% 10% 22% 16% 3	102640
Seeing	Day	22% 11% 25% 22% 14% 6%	15057
	Night	21% 9% 23% 29% 16% 2	25145

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	17% 18% 7% 11% 4 3 4 3 5% 6% 23%	2836
	11th to 20th	Day	18% 18% 8% 12% 7% 4 3 2 3 3 21%	2840
	21st to end	Day	24% 18% 4 8% 4 3 3 2 4 6% 24%	3124
February	1st to 10th	Day	21% 14% 4 9% 4 4 3 3 4 5% 30%	1714
	11th to 20th	Day	22% 11% 5% 8% 4 4 3 4 4 5% 30%	1848
	21st to end	Day	22% 14% 4 6% 4 4 3 3 4 5% 31%	1570
March	1st to 10th	Day	23% 15% 3 7% 4 4 4 3 4 5% 28%	3330
	11th to 20th	Day	15% 7% 3 6% 5% 4 3 3 5% 5% 44%	3480
	21st to end	Day	18% 8% 2 6% 4 4 4 4 6% 7% 36%	3815
April	1st to 10th	Day	15% 8% 2 8% 5% 5% 4 3 6% 7% 38%	2090
	11th to 20th	Day	17% 7% 3 7% 3 4 4 3 4 6% 42%	2366
	21st to end	Day	17% 7% 2 7% 5% 3 5% 3 5% 7% 38%	2470
May	1st to 10th	Day	19% 10% 3 8% 6% 5% 4 3 6% 6% 29%	4082
	11th to 20th	Day	19% 9% 3 8% 4 4 5% 4 5% 6% 33%	4081
	21st to end	Day	10% 5% 2 7% 6% 5% 6% 5% 8% 10% 36%	4476
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2470
	11th to 20th	Day	9% 5% 2 7% 4 6% 5% 5% 7% 10% 40%	2470
	21st to end	Day	10% 6% 2 6% 4 5% 6% 4 7% 10% 40%	2470
July	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 7% 9% 42%	4120
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 7% 10% 43%	4120
	21st to end	Day	18% 9% 3 10% 6% 5% 6% 4 6% 8% 26%	4466
August	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 10% 37%	4104
	11th to 20th	Day	11% 6% 2 6% 4 5% 6% 5% 8% 9% 38%	4044
	21st to end	Day	6% 3 2 5% 3 3 4 4 6% 9% 56%	4374
September	1st to 10th	Day	14% 6% 2 4 4 4 4 3 6% 6% 48%	2090
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 5% 8% 46%	2090
	21st to end	Day	19% 7% 2 6% 4 3 3 4 5% 5% 43%	2090
October	1st to 10th	Day	13% 4 2 6% 3 3 3 4 6% 7% 49%	2033
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 4 6% 48%	1900
	21st to end	Day	21% 6% 3 6% 3 3 3 2 3 6% 43%	2090
November	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 5% 4 51%	1786
	11th to 20th	Day	15% 12% 3 6% 4 3 4 4 5% 6% 39%	1800
	21st to end	Day	17% 12% 4 6% 3 4 3 3 5% 5% 39%	1800
December	1st to 10th	Day	24% 13% 4 8% 5% 3 4 3 4 5% 28%	1782
	11th to 20th	Day	19% 14% 4 7% 4 4 3 3 5% 5% 31%	2153
	21st to end	Day	19% 18% 4 9% 5% 5% 5% 4 5% 6% 20%	3124
January	1st to 10th	Night	18% 18% 5% 10% 5% 4 4 3 4 4 24%	4109
	11th to 20th	Night	23% 13% 5% 9% 5% 3 3 2 3 4 29%	4120
	21st to end	Night	25% 14% 4 7% 4 5% 3 3 4 5% 27%	4532
February	1st to 10th	Night	20% 12% 4 8% 5% 5% 4 4 4 6% 29%	2461
	11th to 20th	Night	24% 12% 3 8% 4 3 4 3 5% 5% 29%	2218
	21st to end	Night	25% 13% 4 7% 4 4 4 3 4 5% 28%	1884
March	1st to 10th	Night	23% 12% 3 6% 4 4 4 3 4 6% 31%	3523
	11th to 20th	Night	15% 9% 3 5% 4 3 4 4 5% 7% 42%	3480
	21st to end	Night	14% 7% 2 6% 5% 4 6% 4 6% 8% 39%	3817
April	1st to 10th	Night	14% 8% 2 7% 5% 4 5% 3 5% 7% 39%	2090
	11th to 20th	Night	15% 6% 3 5% 5% 3 4 4 6% 7% 41%	1777
	21st to end	Night	16% 8% 3 6% 4 4 5% 5% 5% 7% 39%	1710
May	1st to 10th	Night	23% 8% 3 8% 5% 4 4 4 6% 7% 28%	2813
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 8% 37%	2812
	21st to end	Night	10% 6% 3 6% 5% 5% 4 4 6% 9% 42%	3079
June	1st to 10th	Night	10% 5% 2 5% 5% 5% 4 5% 7% 9% 43%	1710
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 5% 7% 11% 44%	1710
	21st to end	Night	11% 5% 1 5% 4 4 5% 5% 6% 11% 43%	1710
July	1st to 10th	Night	8% 6% 2 6% 4 5% 5% 5% 8% 10% 41%	2840
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 3 5% 10% 48%	2840
	21st to end	Night	14% 9% 4 9% 5% 5% 5% 3 5% 7% 34%	3087
August	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	2825
	11th to 20th	Night	11% 6% 2 7% 4 4 4 5% 6% 8% 43%	2785
	21st to end	Night	5% 4 2 4 3 3 4 4 7% 8% 57%	3244
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 48%	2090
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 3 4 8% 49%	2090
	21st to end	Night	14% 5% 2 5% 5% 5% 4 3 6% 5% 46%	2090
October	1st to 10th	Night	11% 5% 2 6% 3 3 3 3 4 7% 53%	2147
	11th to 20th	Night	15% 6% 2 5% 3 4 4 3 4 5% 47%	2280
	21st to end	Night	19% 7% 3 5% 4 3 3 3 5% 5% 43%	2508
November	1st to 10th	Night	14% 6% 3 4 4 3 3 3 4 5% 50%	2507
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 4 6% 42%	2600
	21st to end	Night	17% 11% 3 6% 4 3 3 3 5% 5% 39%	2600
December	1st to 10th	Night	22% 15% 4 8% 5% 3 3 3 4 4 29%	2581
	11th to 20th	Night	18% 13% 6% 7% 4 4 3 3 4 6% 31%	3042
	21st to end	Night	19% 16% 4 9% 6% 4 5% 3 5% 6% 22%	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	63% 3 10% 16% 8% 1	2837
	11th to 20th	Day	70% 2 7% 16% 5% 1	2840
	21st to end	Day	63% 3 6% 21% 5% 1	3124
February	1st to 10th	Day	59% 4 7% 19% 11% 1	1714
	11th to 20th	Day	57% 4 7% 19% 11% 2	1858
	21st to end	Day	57% 2 8% 19% 12% 3	1570
March	1st to 10th	Day	57% 4 8% 19% 10% 2	3330
	11th to 20th	Day	42% 4 9% 25% 17% 1	3480
	21st to end	Day	46% 5% 9% 25% 13% 1	3815
April	1st to 10th	Day	46% 4 9% 23% 15% 3	2090
	11th to 20th	Day	44% 3 9% 21% 18% 4	2366
	21st to end	Day	46% 5% 12% 23% 12% 3	2470
May	1st to 10th	Day	54% 5% 11% 19% 9% 1	4120
	11th to 20th	Day	50% 5% 11% 22% 11% 1	4085
	21st to end	Day	39% 8% 14% 20% 15% 3	4489
June	1st to 10th	Day	38% 7% 13% 26% 16% 1	2470
	11th to 20th	Day	37% 7% 15% 21% 18% 2	2470
	21st to end	Day	38% 7% 14% 22% 16% 4	2470
July	1st to 10th	Day	37% 7% 16% 25% 13% 2	4055
	11th to 20th	Day	36% 6% 12% 34% 10% 1	4051
	21st to end	Day	56% 6% 9% 17% 11% 1	4324
August	1st to 10th	Day	40% 7% 15% 28% 9% 1	4091
	11th to 20th	Day	40% 5% 11% 29% 14% 1	4083
	21st to end	Day	25% 5% 14% 35% 20% 2	4374
September	1st to 10th	Day	36% 5% 10% 23% 20% 6%	2090
	11th to 20th	Day	36% 5% 11% 28% 18% 1	2090
	21st to end	Day	43% 4 10% 23% 19% 2	2090
October	1st to 10th	Day	28% 4 8% 24% 34% 2	3384
	11th to 20th	Day	27% 2 10% 31% 25% 5%	3140
	21st to end	Day	31% 2 10% 30% 22% 5%	3428
November	1st to 10th	Day	35% 4 8% 26% 23% 3	1786
	11th to 20th	Day	45% 3 8% 22% 19% 3	1800
	21st to end	Day	48% 3 9% 23% 15% 1	1800
December	1st to 10th	Day	59% 4 9% 20% 7% 1	1782
	11th to 20th	Day	56% 3 10% 18% 13% 1	2150
	21st to end	Day	64% 5% 9% 17% 4	3124
January	1st to 10th	Night	63% 4 8% 16% 8% 1	4109
	11th to 20th	Night	60% 3 6% 21% 9% 1	4120
	21st to end	Night	62% 3 8% 20% 6% 1	4532
February	1st to 10th	Night	56% 4 9% 19% 11% 1	2461
	11th to 20th	Night	58% 3 8% 18% 12% 1	2219
	21st to end	Night	60% 3 9% 15% 11% 3	1884
March	1st to 10th	Night	55% 4 6% 20% 13% 2	3523
	11th to 20th	Night	41% 5% 8% 28% 16% 1	3480
	21st to end	Night	42% 5% 10% 24% 18% 1	3817
April	1st to 10th	Night	44% 5% 9% 21% 18% 3	2090
	11th to 20th	Night	40% 4 10% 23% 16% 7%	1777
	21st to end	Night	44% 6% 11% 22% 15% 2	1710
May	1st to 10th	Night	55% 6% 10% 17% 10% 2	2840
	11th to 20th	Night	46% 6% 12% 24% 11% 1	2813
	21st to end	Night	37% 7% 13% 23% 16% 3	3087
June	1st to 10th	Night	35% 7% 16% 23% 17% 3	1710
	11th to 20th	Night	33% 8% 17% 19% 20% 4	1710
	21st to end	Night	34% 7% 16% 24% 15% 4	1710
July	1st to 10th	Night	36% 8% 15% 22% 15% 3	2795
	11th to 20th	Night	33% 6% 13% 31% 15% 3	2786
	21st to end	Night	49% 6% 11% 19% 13% 2	2988
August	1st to 10th	Night	41% 7% 13% 27% 11% 1	2814
	11th to 20th	Night	38% 5% 11% 27% 18% 1	2812
	21st to end	Night	23% 5% 13% 32% 24% 3	3244
September	1st to 10th	Night	34% 6% 13% 22% 18% 8%	2090
	11th to 20th	Night	35% 5% 11% 22% 25% 4	2090
	21st to end	Night	39% 5% 10% 23% 22% 2	2090
October	1st to 10th	Night	28% 3 7% 25% 33% 4	3576
	11th to 20th	Night	29% 3 7% 26% 25% 10%	3779
	21st to end	Night	31% 3 6% 26% 26% 8%	4120
November	1st to 10th	Night	37% 3 8% 23% 24% 5%	2507
	11th to 20th	Night	44% 4 7% 20% 20% 5%	2600
	21st to end	Night	47% 3 8% 25% 15% 2	2600
December	1st to 10th	Night	59% 3 9% 19% 9% 1	2581
	11th to 20th	Night	55% 3 8% 23% 10% 1	3044
	21st to end	Night	63% 4 8% 16% 8%	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	29% 22% 32% 11% 4 1	270
	11th to 20th	Day	36% 23% 30% 10% 1	270
	21st to end	Day	33% 23% 32% 10% 2	297
February	1st to 10th	Day	33% 16% 35% 11% 4	270
	11th to 20th	Day	45% 21% 26% 6% 2 1	270
	21st to end	Day	39% 16% 36% 8%	225
March	1st to 10th	Day	33% 17% 30% 16% 5%	389
	11th to 20th	Day	28% 19% 33% 15% 5%	460
	21st to end	Day	30% 18% 31% 15% 5%	506
April	1st to 10th	Day	26% 17% 30% 19% 8% 1	460
	11th to 20th	Day	25% 15% 31% 16% 10% 3	452
	21st to end	Day	26% 14% 33% 19% 6% 2	460
May	1st to 10th	Day	26% 15% 31% 17% 8% 4	463
	11th to 20th	Day	21% 11% 26% 24% 14% 5%	487
	21st to end	Day	13% 8% 24% 26% 18% 10%	529
June	1st to 10th	Day	13% 7% 24% 29% 18% 9%	490
	11th to 20th	Day	12% 7% 20% 32% 20% 10%	490
	21st to end	Day	12% 7% 29% 23% 20% 9%	490
July	1st to 10th	Day	13% 4 18% 22% 24% 18%	485
	11th to 20th	Day	10% 4 21% 31% 22% 12%	484
	21st to end	Day	16% 4 16% 29% 24% 11%	532
August	1st to 10th	Day	13% 4 16% 33% 21% 13%	486
	11th to 20th	Day	16% 4 16% 29% 23% 11%	479
	21st to end	Day	10% 2 14% 32% 29% 13%	539
September	1st to 10th	Day	15% 4 14% 28% 28% 12%	490
	11th to 20th	Day	14% 3 13% 31% 27% 11%	490
	21st to end	Day	23% 7% 21% 24% 17% 7%	490
October	1st to 10th	Day	16% 6% 22% 29% 20% 6%	490
	11th to 20th	Day	19% 10% 28% 26% 12% 5%	490
	21st to end	Day	26% 9% 28% 24% 12% 1	539
November	1st to 10th	Day	19% 11% 32% 26% 9% 2	324
	11th to 20th	Day	28% 15% 31% 18% 8% 1	300
	21st to end	Day	29% 15% 32% 17% 6% 1	300
December	1st to 10th	Day	34% 17% 29% 13% 6% 1	294
	11th to 20th	Day	29% 17% 34% 14% 5% 1	270
	21st to end	Day	32% 19% 31% 13% 5%	297
January	1st to 10th	Night	29% 17% 29% 19% 5%	760
	11th to 20th	Night	34% 16% 26% 20% 4	760
	21st to end	Night	28% 16% 31% 22% 3	836
February	1st to 10th	Night	29% 12% 31% 27% 2	760
	11th to 20th	Night	33% 18% 29% 18% 1	743
	21st to end	Night	34% 15% 32% 16% 2	628
March	1st to 10th	Night	26% 14% 31% 25% 5%	760
	11th to 20th	Night	26% 16% 31% 20% 7%	760
	21st to end	Night	23% 13% 32% 25% 7%	836
April	1st to 10th	Night	20% 15% 30% 28% 7%	760
	11th to 20th	Night	19% 11% 30% 28% 12% 1	601
	21st to end	Night	21% 7% 38% 26% 7% 1	570
May	1st to 10th	Night	22% 9% 26% 29% 13% 2	570
	11th to 20th	Night	19% 7% 24% 31% 18% 1	561
	21st to end	Night	12% 3 19% 35% 26% 5%	617
June	1st to 10th	Night	11% 2 18% 38% 26% 3	570
	11th to 20th	Night	9% 2 14% 42% 27% 5%	570
	21st to end	Night	13% 2 18% 37% 28% 3	570
July	1st to 10th	Night	14% 3 11% 34% 31% 7%	555
	11th to 20th	Night	7% 2 13% 33% 39% 6%	552
	21st to end	Night	12% 3 9% 35% 35% 6%	624
August	1st to 10th	Night	13% 2 7% 39% 34% 4	567
	11th to 20th	Night	11% 3 10% 35% 35% 6%	564
	21st to end	Night	12% 2 8% 35% 35% 8%	644
September	1st to 10th	Night	12% 3 10% 33% 34% 7%	760
	11th to 20th	Night	13% 3 15% 28% 37% 5%	760
	21st to end	Night	17% 4 19% 30% 27% 4	760
October	1st to 10th	Night	14% 4 18% 33% 27% 4	760
	11th to 20th	Night	19% 8% 25% 29% 17% 2	760
	21st to end	Night	22% 9% 23% 31% 14% 1	836
November	1st to 10th	Night	16% 8% 25% 33% 16% 2	780
	11th to 20th	Night	22% 13% 26% 29% 10% 1	800
	21st to end	Night	23% 11% 23% 32% 10% 1	800
December	1st to 10th	Night	33% 12% 26% 24% 5%	795
	11th to 20th	Night	25% 11% 30% 27% 6% 1	760
	21st to end	Night	30% 15% 26% 26% 4	836

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