Obstruction of the field of view

Your observations should be carried out with an unrestricted field of view. If any fraction of the field is covered by clouds you will miss a certain number of meteors. To calculate how many meteors you might have observed had your field been completely clear, we use a factor based on the number of meteors missed as being proportional to the field fraction obscured.

Although we always refer to an unrestricted field of view, due to the properties of the human eye, about 98% of all meteors are witnessed within an almost circular field of around 50° radius. We call this field the effective field of view. Whenever clouds cover your effective field record the time in your notes.

Clearly, it would take a very major effort to record every change in the percentage of cloud cover. Remember that you are outside first and foremost to observe meteors rather than to record cloud obscuration. Instead of recording every change, report the average cloud percentage during the past 10-15 minutes of your watch, until your field is cloudless again or you stop observing. Note the begin of the cloud cover and average the obstruction percentage unless it does not change severely.

At first glance this method seems to be useful for any value of cloud cover. But let us think one step further. Imagine a hole of about 40° radius in the clouds. Observing through this hole about 50% of the area of your effective field is covered. In order to obtain the number of meteors you would have observed in an unrestricted field you have to multiply the number you observed within the partially covered field by a factor of 2. But owing to the construction of the human eye, most meteors are seen in the center of your field of view. In our example you would miss only 10% of the meteors instead of 50%. This example may be an artificial one but the situation is similiar to many other real cases of cloud cover. Please therefore note that the cloud correction is an approximation only, which works primarily with small amounts of cloud cover. Thus, you should take a break as soon as cloud cover exceeds 20%, unless you are monitoring an unusual meteor storm.