Observe and Measur

Observe and measure

Why aerosols?

Aerosols are small particles, smaller than 10 micrometers, which are suspended in the atmosphere for period of time. Interacting with solar radiation (sun- light), they play an important but not so well understood role on the climate change and may impact on air quality. We measure aerosols in AOT (aerosol optical thickness) in our measurements in the GLOBE Program.

The photos below demonstrate how a  high concentration of aerosols in the air (in this case air pollution) can affect visibility.


How to observe clouds and sky conditions?

Cluds form in the presence of aerosols in the air. Aerosols on their own and through the formation of clouds influence Earth’s Climate. For example, when large quantities of volcanic ashes are projected in the atmosphere they can have a measurable cooling effect on the Earth’s climate. 

Learn how to observe clouds and sky conditions (sky color, visibility …) through GLOBE e-training:

How to measure aerosols (AOT)?

If we want to assess how much aerosols are in the air, we measure the aerosol optical thickness (AOT) of the atmosphere. AOT shows how much sun light is scattered or absorbed by particles suspended in the air.

The instrument that is used to measure AOT is called a sun photometer. By using a sunphotometer students can actually measure the amount of sunlight reaching the ground when clouds do not cover the sun.

Students also observe the sky conditions near the sun, perform the Cloud Protocol, record Barometric Pressure (optional) and Relative Humidity, and measure current air temperature.

For instructions visit the GLOBE e-training:


Validation of satellite data by using ground data collected by students - Watch the video!

Satellite instruments such as the OMI on the Aura satellite, MODIS on the Terra and Aqua satellites, or instruments on board of the CALIPSO satellite, are used to monitor and investigate aerosols in the atmosphere. Satellite data always need validation ('check') with observations from the ground. This is called ground truthing. It is done with the help of professional instruments at special measurement sites and GLOBE students can be very useful in this too.


Conduct your own research

Examples of research questions to start your aerosol investigations:

1. How does AOT vary during a week, month and season in your area?

Take sunphotometer measurements at the same time every day and consecutive clear sky days throughout the measurement period.

2. How does AOT vary in a single day? In spring? In fall?

Take multiple sunphotometer measurements in a single day (if possible at moments evenly spread over the day). 

3. How does AOT vary throughout the Netherlands, Europe, and elsewhere?

Compare observations done at the same (local) times but different locations.

4. What do you ‘see', and what does a satellite ‘see'?

Compare your observations with AOT from for example the MODIS satellite instrument. Read here how to view and download MODIS AOT data with the NASA's Giovanni tool.

Examples of research questions to start your cloud investigations:

1. Do cloud patterns/parameters change during the year?

Explore “All Cloud Types” or “Cloud Cover” Layers.

2. Does the amount of cloud cover affect the local temperature?

Add air temperature protocols.

3. How do the clouds you see relate to nearby mountains, lakes, large rivers, bays, or the ocean?

Add maps or satellite imagery.

4. How do our cloud observations compare with satellite images of clouds?

Explore NASA or NOAA resources. If there’s a satellite measurement made at the same time over your location, you’ll receive satellite imagery that you can use to explore this question.