NASA's Aqua and Terra satellites fly around the world every day capturing images of weather, ice and land changes. Over the last three days these satellites have provided visible and infrared（红外线的） imagery of the ash plume（灰羽） from the Eyjafjallajökull（艾雅法拉火山） volcano in Iceland. Eyjafjallajökull is pronounced similar to "EYE-a-fyat-la-yu-goot," and it is still spewing（呕吐，喷涌） ash into the atmosphere. Volcanic eruptions（火山喷发） are important sources of gases, such as sulphur dioxide (SO2) and volcanic ash (aerosols气溶胶，喷雾器) in the atmosphere.
The Moderate Resolution Imaging Spectroradiometer（分光辐射度计） , also known as MODIS, is an instrument that flies aboard both NASA's Aqua and Terra satellites. MODIS captures daily visible and infrared earth imagery and has provided daily images of the volcanic plume. NASA's MODIS instrument and the Atmospheric Infrared Sounder (AIRS) instrument, both of which fly on Aqua, contain sulfur dioxide (SO2) absorption（吸收，全神贯注） channels to enhance volcanic ash detection. These applications have significantly improved upon existing satellite-based multi-spectral techniques in identifying and tracking ash clouds and estimating their height.
On Saturday, April 17 at 13:20 UTC (9:20 a.m. EDT), Aqua captured a visible image of the ash plume so clearly that in the satellite image a viewer could see the billowing cloud（浪云，波状云） spewing from the volcano and blowing almost due south before turning east over the Atlantic Ocean.
On Sunday, April 18 at 12:05 UTC (8:05 a.m. EDT), NASA's Terra satellite flew over the volcano and captured an image of the brown ash cloud mostly obscured by higher clouds. The brown plume was partly visible underneath the high clouds.
By Monday morning, April 19 at 12:50 UTC (8:50 a.m. EDT) the high clouds had cleared, and the brown line of spewed volcanic ash was visible once again blowing south, then turning east toward the United Kingdom.
The ash cloud basically consists of fine particles of pulverized（粉状的） rock. Volcanic ash is a rare but potentially catastrophic（灾难的，悲惨的） hazard to（对……有危险） aviation（航空，飞行术） . Encounters with volcanic ash while in flight can result in engine failure from particulate（微粒，微尘） ingestion（摄取，咽下） and viewing obstruction of the cockpit（驾驶舱） widescreen from etching by the acidic（酸的， 经典英文文章，酸性的） aerosols. Volcanic Ash Advisory Centers were established to monitor the air space in areas prone to eruptions and to issue volcanic ash warnings.