(Het gebruik van wisselbeelden)
Kees Floor, KNMI, De Bilt, The Netherlands.
EWOC Madrid, July 2003

Introduction
Learning about the weather often is done by comparing and contrasting new concepts with already known ones. Studying similarities and differences between the two leads to new insights and makes it easier to assimilate new knowledge. In fact many meteorologists and forecasters still are using this approach every day in the exercise of their duty.
The easiest way to present images to be compared and contrasted is to display them next to each other. However, for detailed inspection of similarities and differences this approach is usually insufficient. Rollover images are a very useful tool when comparing and contrasting information that is available on digital images. In this case two images are laid on top of each other to enable detailed comparison. One image is displayed when the mouse is on the image, the other when the mouse is in a different position. The rollover images are available on an html-page made with a suitable page-editing tool. Especially satellite imagery, available in different channels or in different enhancements, is a good source of images that can be viewed in the rollover mode. This can also easily be done with nearly real time images, as widely available on the internet.
In this digital paper a number of on-screen examples of rollover images, useful in an educational context, will be shown. In fact most of them deal with satellite images, but first we show there are also other topics that can be explained or illustrated better with the rollover image approach. We will compare mean wind velocities and wind velocities in gusts, show the impact of September 11 2001 on the availability of aircraft data over the United States, compare data plotted from the same information in different resolutions and detect 'blind areas' in radar images.

Mean wind speed and windspeed in gusts
Lots of pupils and members of the general public think about wind as displacing air in a very static way. In fact wind however usually is very variable in speed and direction. For that reason meteorologists measure wind speed over a 10 minute period of time. The Beaufort wind force scale refers to these 10 minute winds. Besides gusts can be measured over a shorter period of time, e.g. 3 seconds. Wind gusts therefore have higher speeds and show more variability. A rollover image can help to visualise the difference, as is shown in figures 1 and 2.

Figure 2 (KNMI).

Figure 2 (KNMI).

Aircraft data In weather forecasting data are used from many sources. Amongst them are the observations from commercial aircraft. Weather centres make available maps showing positions where observations are taken. Aircraft data used for the ECMWF-analysis of 16 September 2001, 06 UTC, are shown in figure 2(mouse off). If we put the mouse on the image, the situation of September 12 2001 appears, shortly after the attack on the World Trade Centre in New York. Easily can be seen how this event affected air traffic in the United States.   Figure 3 (ECMWF).

Looming An object that is below the horizon and that could not normally be observed, on some occasions may be seen on or just above the horizon. This phenomenon is called looming or refered to with the icelandic word hillingar; it is a type of mirage. The image may also be magnified; therefore very distant objects may seem to be closer. For looming to occur, a cold layer of air is required next to the earth surface; above this layer another layer of warmer air should be present. The ray path is shown in figure 4. Figure 4. Although it will take some effort to get two images than can be compared, one taken under looming conditions, the other under normal conditions, it is worth trying, as shown in figure 5 (right; Photographs: Rik Mars).

Routine satellite imagery
Satellite images are widely available real time on the web. The most common types of imagery are images made in the visible or in the infrared. An attentive user of full-globe imagery in both channels might notice that infrared images are available with a white background only, while visible images are provided with a black background (figure 9). When discussing this in the classroom, one may suggest that this is done to make life easier for the forecaster: there is no need to read the annotation to be sure what kind of imagery is looked at.
We know this cannot be the right reason; if this was correct, why are they supplying water vapour images with the same white background as the infrared images? In fact they are not supplying backgrounds at all. In the visible image case, the sensor registers reflected sunlight. As there is no sunlight reflected from space, that area is black on the image. The infrared image shows temperatures: areas with low temperatures are white. The outer space is cold and no infrared radiation is received from that area; therefore the 'supposed background' is white. Things change when an object appears in the black area on the visible image or in the white of the infrared image. An object that sometimes does appear there is the moon, as seen in figure 10.

Figure 9: Full globe satellite images VIS/IR (EUMETSAT/Strasbourg).

Figure 10 (left): Earth and moon (NASA).

Comparing and contrasting visible and infrared imagery is a very interesting thing to do. The images reveal different worlds; things seem to happen earlier in an infrared world than in the visible world. High clouds, easily seen in IR, but usually rather transparent and therefore less significant in VIS-imagery, are more ahead of a frontal system than lower clouds. Sometimes it is hard to believe that the VIS- and IR-images of the rollover images are in the correct position compared to each other, but do the verification: they are! An example of high clouds are condensation trails of airplanes; because they are high up in the atmosphere, the temperature is low and the contrails can be seen easier on IR-imagery than on VIS (figure 11).

Here a couple of not-real-time roll-over images is given.

Figure 11: Spiralling contrail (NASA/Dundee University).   Figure 12 (right). (Met Office, UK).

Figure 13 (EUMETSAT/Dundee University).

Other examples: sunglint example (NOAA/Dundee University). Etna eruption (NOAA/Strasbourg). black fog over The Netherlands (NOAA/Strasbourg) transparant cirrus (NOAA/Strasbourg) showers with shadows (NOAA/Strasbourg)

False colour and enhancements
Some sites present images that have been processed in a non standard way. This might be done by adding colour to infrared images to make them look more like the maps in an atlas. False-colour images also often appear in television broadcasts of the weather. The rollover images of figure15, taken from the websites of Dundee, can be used to discuss preferences of the pupils on one type of presentation or the other.
As said before, infrared images show temperatures of the earth surface or of the cloud tops. Therefore they can be used to construct a cloud-top map; this colourful product is available from the Praha website.

Figure 15 (EUMETSAT/Dundee University).

Figure 16: False color (CMHU, Praha).

Figure 17: Cloud top temperatures (CHMU, Praha) .