Home > Articles > Networking > Network Design & Architecture

This chapter is from the book

The Impact of Weather

So far, the discussions in this chapter have been somewhat theoretical. One of the practical topics of most interest to designers and implementers of FSO systems is the weather.


Rain has a distance-reducing impact on FSO, although its impact is significantly less than that of fog. This is because the radius of raindrops (200–2000 μm) is significantly larger than the wavelength of typical FSO light sources.

Typical rain attenuation values are moderate in nature. For example, for a rainfall of 2.5 cm/hour, a signal attenuation of 6 dB/km can be observed. Therefore, commercially available FSO systems that operate with a 25 dB link margin can penetrate rain relatively unhindered. This is especially the case when systems are deployed in metropolitan areas where building distances are typically much less than 1 km. If, for example, the system is deployed over a distance of 500 m under the same rain conditions, the attenuation is only 3 dB/km. However, when the rain rate increases dramatically to beyond the cloudburst level (> 10 cm/hour), rain attenuation can become an issue in deployments beyond the distance scale of a typical metropolitan area. However, these kind of cloudbursts last for only a short period of time (minutes).

An interesting point to note is that RF wireless technologies that use frequencies above approximately 10 GHz are adversely impacted by rain and little impacted by fog. This is because of the closer match of RF wavelengths to the radius of raindrops, both being larger than the moisture droplets in fog. The lower unlicensed RF frequencies in the 2.4 GHz and 5.8 GHz ranges are relatively unaffected by rain or fog, but incur significant interference risks by nature of the lack of licensing in those frequencies.


Snowflakes are ice crystals that come in a variety of shapes and sizes. In general, however, snow tends to be larger than rain. Whiteout conditions might attenuate the beam, but scattering doesn't tend to be a big problem for FSO systems because the size of snowflakes is large when compared to the operating wavelength. The impact of light snow to blizzard and whiteout conditions falls approximately between light rain to moderate fog, with link attenuation potentials of approximately 3 dB/km to 30 dB/km.


Fog is the most detrimental weather phenomenon to FSO because it is composed of small water droplets with radii about the size of near infrared wavelengths. The particle size distribution varies for different degrees of fog. Weather conditions are typically referred to as fog when visibilities range between 0–2,000 meters. Because foggy conditions are somewhat difficult to describe by physical means, descriptive words such as "dense fog" or "thin fog" are sometimes used to characterize the appearance of fog. When the visibility is more than 2,000 meters, the condition is often referred to as hazy.

Table 3.1 relates visibility and different fog conditions. Scattering is the dominant loss mechanism for fog. Even modest fog conditions can highly attenuate infrared signals over shorter distances. The expected path attenuation in dB/km and its correlation to visibility is shown in the table. The table also clearly illustrates that rain has much less impact on FSO systems' path losses when compared to fog. For example, a medium rainfall results in less attenuation than a thin fog.

Table 3.1 International Visibility Codes for Weather Conditions and Precipitation

Weather Condition


Amount mm/hr


dB Loss/km

Dense fog




0 m, 50 m


Thick fog




200 m


Moderate fog




500 m


Light Fog




770 m 1 km



Thin fog


Heavy rain


1.9 km

2 km





Medium rain


2.8 km

4 km



Light haze


Light rain


5.9 km

10 km







18.1 km

20 km



Very Clear




23 km

50 km



Fog is not well understood, and it is difficult to characterize physically. Although visibility is most commonly used to characterize foggy conditions, other methods such as particle size and density measurements have been undertaken to describe fog conditions in a more quantitative way. The FSO community mainly uses visibility data because these measurements have been taken at major airports over many decades. To some extent, these measurements allow you to characterize different regions and derive statistical availability figures for FSO systems. However, most of the data has been time averaged over years; in general, the temporal resolution of these data points is not very high.

Because microclimate environments such as ponds or rivers can induce foggy conditions, the data taken at airports sometimes is not reliable for nearby environments. However, it has been shown that the visibility at airports provides a good estimate for the minimum expectable availability figure. This is because airports typically are located outside metropolitan boundaries, and the microclimate inside a city typically generates less foggy conditions.

The density distribution of fog particles can also vary with height, which makes the modeling of fog even more complex. The limited amount of information regarding the local impact of fog on the availability of FSO systems is certainly one of the biggest challenges for the FSO industry.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020