Finch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Finch Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	7% 5% 2 7% 6% 6% 6% 5% 8% 10% 38%	100970
	Night	7% 6% 2 7% 5% 5% 6% 5% 7% 10% 41%	98615
Transparency	Day	39% 8% 18% 20% 12% 3	104745
	Night	37% 8% 19% 21% 13% 3	102978
Seeing	Day	7% 4 16% 27% 26% 19%	15825
	Night	8% 3 16% 33% 29% 11%	25907

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	9% 9% 3 7% 4 4 5% 4 6% 8% 41%	2836
	11th to 20th	Day	15% 8% 3 8% 6% 5% 4 4 6% 9% 32%	2840
	21st to end	Day	7% 6% 3 8% 6% 5% 6% 4 6% 7% 42%	3382
February	1st to 10th	Day	7% 6% 2 7% 4 4 6% 4 7% 9% 44%	1900
	11th to 20th	Day	6% 4 1 3 3 3 4 4 7% 9% 57%	1848
	21st to end	Day	5% 3 1 5% 4 4 5% 4 6% 10% 52%	1712
March	1st to 10th	Day	4 4 2 7% 6% 7% 6% 6% 8% 11% 38%	3435
	11th to 20th	Day	3 3 1 4 4 3 5% 4 7% 12% 53%	3480
	21st to end	Day	9% 4 2 4 4 3 4 3 5% 8% 54%	3815
April	1st to 10th	Day	4 3 2 4 3 4 5% 4 7% 11% 52%	2090
	11th to 20th	Day	5% 4 1 5% 4 6% 6% 4 6% 10% 49%	2066
	21st to end	Day	5% 4 2 5% 4 5% 4 5% 8% 9% 50%	2266
May	1st to 10th	Day	2 2 1 3 3 3 4 3 6% 10% 64%	3821
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 7% 11% 47%	4081
	21st to end	Day	5% 4 1 6% 4 4 4 4 7% 9% 51%	4476
June	1st to 10th	Day	12% 9% 3 8% 8% 6% 8% 5% 9% 10% 21%	2470
	11th to 20th	Day	9% 7% 3 9% 7% 8% 8% 6% 10% 10% 24%	2470
	21st to end	Day	8% 7% 3 9% 8% 8% 9% 7% 11% 9% 21%	2470
July	1st to 10th	Day	7% 6% 3 9% 8% 8% 8% 7% 9% 12% 24%	4120
	11th to 20th	Day	5% 5% 3 10% 9% 8% 10% 7% 9% 12% 23%	4120
	21st to end	Day	6% 5% 2 8% 7% 7% 9% 7% 10% 13% 27%	4466
August	1st to 10th	Day	5% 6% 4 11% 8% 7% 8% 6% 9% 11% 25%	4104
	11th to 20th	Day	5% 4 2 8% 7% 8% 9% 8% 9% 13% 27%	3561
	21st to end	Day	7% 4 3 9% 7% 7% 8% 7% 10% 14% 24%	3828
September	1st to 10th	Day	6% 6% 3 10% 9% 8% 9% 7% 10% 11% 22%	2090
	11th to 20th	Day	8% 6% 3 10% 8% 8% 8% 6% 10% 9% 22%	2090
	21st to end	Day	11% 9% 3 9% 8% 8% 8% 6% 7% 9% 23%	2090
October	1st to 10th	Day	8% 6% 2 9% 8% 6% 7% 5% 8% 10% 30%	2090
	11th to 20th	Day	6% 5% 3 8% 6% 5% 7% 5% 8% 10% 36%	1905
	21st to end	Day	8% 6% 3 5% 4 5% 5% 5% 7% 9% 44%	2090
November	1st to 10th	Day	7% 4 2 6% 5% 4 5% 4 5% 9% 49%	1957
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 8% 43%	2000
	21st to end	Day	5% 5% 2 6% 5% 6% 4 4 7% 9% 49%	2000
December	1st to 10th	Day	9% 5% 2 5% 5% 5% 4 3 6% 9% 48%	1724
	11th to 20th	Day	7% 7% 2 5% 4 5% 5% 4 8% 10% 44%	2153
	21st to end	Day	10% 6% 2 8% 5% 6% 6% 5% 7% 10% 34%	3124
January	1st to 10th	Night	7% 7% 3 7% 6% 5% 5% 4 8% 11% 38%	4109
	11th to 20th	Night	14% 11% 4 9% 6% 5% 5% 4 6% 7% 29%	4120
	21st to end	Night	6% 6% 3 8% 5% 5% 5% 4 7% 8% 41%	4271
February	1st to 10th	Night	7% 7% 2 5% 4 4 4 4 7% 7% 48%	2280
	11th to 20th	Night	7% 5% 2 4 4 3 4 4 6% 7% 56%	2218
	21st to end	Night	4 3 1 5% 3 3 4 3 6% 9% 60%	1739
March	1st to 10th	Night	5% 6% 3 8% 6% 6% 6% 4 7% 7% 42%	3418
	11th to 20th	Night	4 4 1 5% 3 4 5% 4 7% 10% 54%	3480
	21st to end	Night	7% 5% 2 6% 4 4 4 4 5% 7% 53%	3817
April	1st to 10th	Night	4 3 2 4 4 3 3 4 6% 9% 57%	2090
	11th to 20th	Night	4 3 2 5% 4 5% 4 4 7% 9% 53%	2063
	21st to end	Night	5% 4 2 5% 4 4 5% 4 7% 9% 53%	1912
May	1st to 10th	Night	4 4 2 6% 4 4 5% 4 6% 7% 53%	3100
	11th to 20th	Night	5% 3 1 4 3 4 4 4 8% 9% 55%	2812
	21st to end	Night	4 4 2 4 4 4 4 4 6% 8% 57%	3079
June	1st to 10th	Night	11% 8% 3 7% 6% 5% 7% 6% 8% 10% 28%	1710
	11th to 20th	Night	8% 6% 2 7% 6% 5% 6% 6% 8% 12% 34%	1710
	21st to end	Night	5% 5% 3 9% 7% 7% 7% 5% 8% 15% 28%	1710
July	1st to 10th	Night	6% 6% 3 9% 8% 7% 7% 6% 9% 11% 28%	2840
	11th to 20th	Night	6% 6% 3 9% 8% 8% 7% 6% 10% 12% 25%	2840
	21st to end	Night	6% 6% 3 9% 8% 7% 8% 7% 10% 12% 25%	3087
August	1st to 10th	Night	5% 6% 3 9% 6% 6% 8% 6% 10% 11% 29%	2825
	11th to 20th	Night	7% 4 3 9% 7% 8% 7% 6% 10% 12% 27%	3216
	21st to end	Night	5% 5% 2 7% 6% 6% 9% 7% 10% 15% 26%	3828
September	1st to 10th	Night	5% 5% 2 7% 7% 7% 8% 8% 10% 13% 28%	2090
	11th to 20th	Night	7% 6% 3 10% 6% 6% 6% 5% 9% 12% 30%	2090
	21st to end	Night	10% 8% 3 9% 7% 6% 6% 6% 9% 10% 26%	2090
October	1st to 10th	Night	8% 8% 3 8% 5% 5% 7% 5% 6% 9% 36%	2090
	11th to 20th	Night	7% 7% 2 7% 5% 4 5% 4 6% 10% 44%	2273
	21st to end	Night	7% 6% 2 7% 4 5% 5% 3 7% 9% 44%	2508
November	1st to 10th	Night	7% 5% 3 5% 5% 4 5% 3 6% 8% 50%	2341
	11th to 20th	Night	7% 6% 2 6% 4 4 4 4 6% 9% 49%	2400
	21st to end	Night	7% 5% 2 6% 5% 4 5% 4 6% 8% 49%	2400
December	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 6% 8% 53%	2485
	11th to 20th	Night	10% 7% 2 5% 4 3 4 3 6% 8% 49%	3042
	21st to end	Night	9% 7% 3 7% 5% 4 6% 4 7% 9% 37%	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	40% 5% 11% 23% 16% 5%	2837
	11th to 20th	Day	49% 4 12% 18% 13% 4	2840
	21st to end	Day	40% 4 9% 23% 19% 5%	3382
February	1st to 10th	Day	34% 6% 10% 23% 20% 7%	1900
	11th to 20th	Day	22% 5% 10% 27% 24% 11%	1858
	21st to end	Day	26% 6% 12% 21% 26% 10%	1712
March	1st to 10th	Day	37% 4 16% 23% 16% 4	3435
	11th to 20th	Day	24% 4 22% 29% 16% 4	3480
	21st to end	Day	29% 4 9% 30% 19% 9%	3815
April	1st to 10th	Day	25% 6% 18% 29% 17% 6%	2090
	11th to 20th	Day	30% 7% 16% 25% 17% 5%	2066
	21st to end	Day	29% 5% 12% 23% 26% 5%	2266
May	1st to 10th	Day	16% 6% 12% 37% 25% 4	3856
	11th to 20th	Day	30% 5% 20% 34% 10% 1	4085
	21st to end	Day	28% 8% 14% 34% 16% 1	4489
June	1st to 10th	Day	55% 9% 19% 14% 3	2470
	11th to 20th	Day	50% 11% 22% 14% 2	2470
	21st to end	Day	52% 14% 23% 10% 1	2470
July	1st to 10th	Day	49% 11% 28% 12%	4055
	11th to 20th	Day	50% 11% 32% 7%	4051
	21st to end	Day	44% 13% 33% 11%	4324
August	1st to 10th	Day	49% 11% 29% 10% 2	4091
	11th to 20th	Day	43% 13% 30% 14% 1	3597
	21st to end	Day	47% 12% 33% 7% 1	3828
September	1st to 10th	Day	51% 13% 25% 11% 1	2090
	11th to 20th	Day	51% 13% 20% 12% 3	2090
	21st to end	Day	54% 11% 16% 12% 5% 1	2090
October	1st to 10th	Day	51% 8% 17% 14% 8% 2	3480
	11th to 20th	Day	37% 7% 18% 22% 14% 2	3145
	21st to end	Day	33% 6% 9% 26% 21% 5%	3428
November	1st to 10th	Day	32% 6% 11% 19% 20% 11%	1957
	11th to 20th	Day	37% 6% 9% 22% 19% 8%	2000
	21st to end	Day	31% 5% 12% 22% 23% 7%	2000
December	1st to 10th	Day	33% 5% 12% 25% 20% 4	1724
	11th to 20th	Day	33% 7% 11% 21% 18% 10%	2150
	21st to end	Day	41% 6% 15% 20% 15% 3	3124
January	1st to 10th	Night	40% 4 13% 23% 17% 4	4109
	11th to 20th	Night	54% 4 12% 17% 10% 3	4120
	21st to end	Night	38% 5% 11% 21% 20% 5%	4271
February	1st to 10th	Night	33% 4 13% 23% 20% 7%	2280
	11th to 20th	Night	27% 4 11% 25% 23% 11%	2219
	21st to end	Night	21% 4 12% 24% 29% 9%	1739
March	1st to 10th	Night	39% 5% 13% 24% 15% 4	3418
	11th to 20th	Night	25% 4 22% 29% 16% 3	3480
	21st to end	Night	29% 6% 12% 25% 23% 7%	3817
April	1st to 10th	Night	23% 6% 17% 34% 15% 5%	2090
	11th to 20th	Night	26% 5% 17% 26% 21% 5%	2063
	21st to end	Night	27% 6% 14% 27% 23% 4	1912
May	1st to 10th	Night	25% 7% 17% 33% 15% 3	3130
	11th to 20th	Night	23% 6% 24% 36% 11%	2813
	21st to end	Night	25% 6% 14% 36% 18% 1	3087
June	1st to 10th	Night	48% 9% 21% 19% 3	1710
	11th to 20th	Night	40% 11% 29% 18% 2	1710
	21st to end	Night	44% 13% 29% 11% 3	1710
July	1st to 10th	Night	47% 12% 30% 11%	2795
	11th to 20th	Night	48% 14% 31% 6%	2786
	21st to end	Night	47% 13% 28% 12%	2988
August	1st to 10th	Night	43% 12% 30% 11% 3	2814
	11th to 20th	Night	46% 13% 32% 8%	3245
	21st to end	Night	44% 16% 34% 5% 1	3828
September	1st to 10th	Night	41% 16% 31% 11% 1	2090
	11th to 20th	Night	44% 13% 27% 13% 3	2090
	21st to end	Night	48% 12% 21% 14% 5%	2090
October	1st to 10th	Night	56% 6% 18% 12% 6% 2	3480
	11th to 20th	Night	39% 7% 17% 20% 16% 2	3772
	21st to end	Night	35% 5% 11% 28% 18% 4	4120
November	1st to 10th	Night	30% 7% 12% 21% 21% 9%	2341
	11th to 20th	Night	31% 6% 10% 25% 21% 7%	2400
	21st to end	Night	32% 5% 11% 22% 23% 6%	2400
December	1st to 10th	Night	29% 6% 14% 29% 18% 4	2485
	11th to 20th	Night	32% 5% 9% 25% 19% 10%	3044
	21st to end	Night	41% 6% 18% 18% 14% 3	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	8% 10% 39% 30% 13%	270
	11th to 20th	Day	12% 11% 34% 28% 12% 2	270
	21st to end	Day	12% 11% 41% 27% 8% 1	297
February	1st to 10th	Day	6% 10% 31% 35% 15% 3	270
	11th to 20th	Day	4 8% 31% 40% 12% 5%	270
	21st to end	Day	4 7% 25% 37% 26% 2	367
March	1st to 10th	Day	2 9% 32% 38% 18% 2	460
	11th to 20th	Day	6% 8% 27% 34% 21% 3	460
	21st to end	Day	4 5% 25% 38% 26% 3	506
April	1st to 10th	Day	4 8% 31% 35% 18% 5%	460
	11th to 20th	Day	8% 6% 25% 34% 22% 6%	452
	21st to end	Day	6% 5% 22% 33% 28% 7%	460
May	1st to 10th	Day	4 1 17% 36% 29% 13%	463
	11th to 20th	Day	8% 5% 17% 29% 26% 14%	487
	21st to end	Day	6% 2 9% 27% 33% 22%	529
June	1st to 10th	Day	13% 3 8% 22% 32% 23%	490
	11th to 20th	Day	11% 3 8% 20% 24% 35%	490
	21st to end	Day	5% 2 2 14% 26% 51%	490
July	1st to 10th	Day	5% 1 2 13% 27% 52%	485
	11th to 20th	Day	5% 2 1 10% 36% 45%	484
	21st to end	Day	7% 2 3 15% 26% 47%	532
August	1st to 10th	Day	5% 1 3 12% 34% 45%	486
	11th to 20th	Day	5% 1 2 12% 31% 48%	479
	21st to end	Day	9% 2 6% 20% 28% 35%	539
September	1st to 10th	Day	6% 2 4 12% 33% 42%	490
	11th to 20th	Day	7% 2 6% 18% 30% 38%	490
	21st to end	Day	10% 2 7% 15% 36% 30%	490
October	1st to 10th	Day	9% 3 12% 31% 34% 11%	490
	11th to 20th	Day	6% 4 16% 34% 28% 12%	490
	21st to end	Day	10% 4 17% 36% 26% 7%	539
November	1st to 10th	Day	8% 4 20% 32% 29% 7%	495
	11th to 20th	Day	11% 3 24% 32% 24% 6%	500
	21st to end	Day	6% 2 25% 39% 24% 4	500
December	1st to 10th	Day	10% 6% 23% 37% 21% 3	278
	11th to 20th	Day	6% 10% 30% 33% 19% 3	270
	21st to end	Day	7% 9% 25% 37% 19% 4	297
January	1st to 10th	Night	9% 5% 24% 41% 19% 1	760
	11th to 20th	Night	10% 6% 28% 40% 17%	760
	21st to end	Night	9% 6% 32% 41% 12% 1	836
February	1st to 10th	Night	10% 5% 28% 42% 15%	760
	11th to 20th	Night	8% 6% 24% 44% 17% 1	743
	21st to end	Night	4 4 23% 51% 17%	628
March	1st to 10th	Night	5% 5% 29% 45% 16%	760
	11th to 20th	Night	8% 3 26% 44% 18% 1	760
	21st to end	Night	5% 5% 27% 45% 18%	836
April	1st to 10th	Night	5% 4 24% 45% 21% 1	760
	11th to 20th	Night	5% 3 27% 44% 21% 1	751
	21st to end	Night	5% 3 25% 44% 22% 1	760
May	1st to 10th	Night	3 3 17% 46% 28% 3	743
	11th to 20th	Night	9% 1 12% 40% 34% 5%	561
	21st to end	Night	7% 2 5% 35% 44% 7%	617
June	1st to 10th	Night	13% 1 5% 22% 46% 12%	570
	11th to 20th	Night	7% 2 6% 14% 43% 27%	570
	21st to end	Night	5% 2 1 12% 44% 37%	570
July	1st to 10th	Night	4 1 2 7% 46% 40%	555
	11th to 20th	Night	5% 1 9% 43% 42%	552
	21st to end	Night	4 2 3 7% 39% 45%	624
August	1st to 10th	Night	5% 2 3 8% 46% 36%	567
	11th to 20th	Night	7% 2 2 8% 43% 39%	648
	21st to end	Night	11% 2 5% 17% 39% 26%	836
September	1st to 10th	Night	7% 2 4 12% 46% 29%	760
	11th to 20th	Night	10% 2 4 17% 43% 23%	760
	21st to end	Night	13% 2 7% 21% 39% 18%	760
October	1st to 10th	Night	12% 2 11% 34% 32% 9%	760
	11th to 20th	Night	9% 2 11% 38% 33% 7%	760
	21st to end	Night	10% 2 19% 38% 26% 5%	836
November	1st to 10th	Night	9% 3 21% 39% 26% 2	780
	11th to 20th	Night	8% 4 20% 41% 25% 2	800
	21st to end	Night	8% 2 21% 44% 23% 1	800
December	1st to 10th	Night	8% 3 20% 41% 26% 3	768
	11th to 20th	Night	10% 4 20% 38% 26% 2	760
	21st to end	Night	9% 4 22% 40% 24% 2	836

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