One of the characteristics of the summer hemi- sphere of Mars is that the polar area is warmer than the equatorial band (EB), contrary to the case of the Earth. This is because the Martian atmosphere is so thin that the polar region is more radiated and heated than the band region located along the equator: In fact if the latitude of the sub-Solar point is 20degsN, then the Arctic region to the north of 70degsN has no night, and the radiation must easily reach the Arctic obliquely but constantly to make the polar area warmer than the remote EB because of the thin atmosphere.
The air mass heated will ascend at the Arctic and will be sent as the upper air-current to the EB region. At the same time the north polar cap (npc) has been melted enough to send much of water vapour to the ascending air-current, and the water vapour will be brought from north to south to the EB through the upper atmosphere. The moving air-mass however receives the Coriolis force and the easterly will thus be yielded in the NH. The easterly however ceases near at the EB because the Coriolis force vanishes at the equator.
The heated moist air-mass at the middle latitudes will be often absorbed in the night area and gradually cooled and then made to descend. However this is delayed by the presence of the Coriolis force and the heat will be retained and accumulated more in the high and middle latitudes than the EB. This causes a relatively lower temperature at the EB, and the water vapour will be saturated near the EB and eventually a whitish mist or haze will appear along the EB. This may be identified the so-called equatorial band cloud (EBC), but the matter is not so thick as to be called the cloud, and so we shall consider that it is more appropriate to call here it the "equatorial band mist (ebm)".
We further note that the summer hemisphere is the warmest in the middle latitudes at noon (or just after it), and the lower wind blows towards the noon line, and hence in the morning a lower westerly prevails which blows from the dawn area containing the cooled water vapour or the matter made of frost in the night. The water vapour will appear as a lower mist or haze in the morning and will vanish towards the noon. In the evening, the water vapour in the air will gradually saturated but remain inside because of the lower easterly which blows towards the noon line. Notable is that in the morning area the upper easterly and lower westerly might make sometimes a front.
The easterly will cease as the summer comes to the end. The EB begins to be warmed up more than the Arctic in autumn, and the relatively cooled air will disappear from the EB and so the ebm will clear up. In the upper atmosphere of the NH, the air-mass will soon change to go from the EB to the Arctic which will be a westerly. During the time when the easterly changes to the westerly the wind will stop at the upper atmosphere.
The above is qualitatively and macroscopically stated, and the local topography and the presence of the local dust storms will give rise to several complicated factors.
In the following we shall see how we observed the phenomena in the 1996/97 apparition. The present writer (Mn) observed already at 051degsLs 13 Dec 1996 at LCM=036degsW an overall mist lying along the EB from the evening Tymiamata through Chryse to the morning Tharsis, though the belt was not uniform but thin and thick. The latitude of the sub-Solar point was already 19degsN at that time, and we considered that the moist air mass had already reached the EB. On 14 Dec at LCM=045degsW the ebm was also apparent. On 16 Dec, it was also seen but after LCM=016degsW and until then it was not so evident to the naked eyes, implying a topographical effect. On 15 Dec, the apparent diameter was 7.2". On 26 Dec (056degsLs) at LCM=232degsW-- LCM= 262degsW, the ebm was clearly visible at the both sides of Syrtis Mj connecting with the evening and morning haze. Syrtis Mj was green-bluish and so was considered to be covered by a white mist. On 29 Dec (058degsLs) at LCM=272degsW, the white belt over Libya was evident, and Syrtis Mj was also green-bluish, that is, misted. The sub-Solar point at 21degsN. As stated above, on 16 Jan 1997, the sub-Solar point reached 23degsN, and hence the mist over the EB should always be apparent. On 17 Jan (065degsLs) at LCM= 028degsW, we clearly saw it.
Gianni QUARRA and his colleagues from Firenze use a high quality blue filter for their CCD camera and produced each time interesting B images. The mist over the EB on 17 Jan at LCM=219degsW is not so strong but the B image made on 31 Jan (072degsLs) at LCM=035degsW (cf CMO #184 p1991) clearly shows the mist and henceforce it becomes very evident on their CCD images including the one on 9 Feb (076degsLs) at LCM=300degsW (cf CMO #186 p2021).
Next stage must have come around mid-Feb: As reported in CMO #186 p2019, a white band made of mist from the morning Tharsis to the eastward made HIKI (Hk) amazed on 20 Feb (081degsLs) at LCM=035degsW & LCM=046degsW. Hk also observed 22 Feb at LCM=021degsW and so on. The present writer also observed it on 20 Feb at LCM=041degsW and so on. Tharsis is such a specific area that it is difficult to discuss the wind tendency as well as the distribution of temperature and moisture, but the white mist activity of the area in the morning since 080degsLs must have been enhanced.
Since then, the observations of the so-called EBC augmented. HIGA (Hg)'s
Video images frequently show the ebm from mid-Feb to March. TROIANI (DTr)
observed EBC on 11 Mar at LCM=075degsW, and SIEGEL (ESg) gave a fresh
observation of the EBC on 06 Apr (102degsLs) and also on 18 May at LCM=
095degsW (cf CMO #189 p2070, #192 p2109). The mist observed by ISHADOH (Id)
on 17 Apr at LCM=258degsW (#190 p2082) is nothing but a trail of the ebm
here defined. The B images of MIYAZAKI (My) on 12 Apr also show the morning
mist from the morning limb to Syrtis Mj (reported in this issue) and the
work by AKUTSU (Ak) on 12 and 13 Apr also conveys the ebm on the B images
though some leakage of the longer waves is present. The activity observed by
Hg and others near 20 Apr (107degsLs), as reported in #197 p2179 must be a
local burst of the ebm.
HST's images taken on 30 Dec (058degsLs), 10 Mar (089degsLs), 30 Mar
(097degsLs), 17 Apr (105degsLs), 17 May (119degsLs), 27 June (140degsLs) and
so on all depict the ebm. A B image on 30 Mar was cited at page 2102 of CMO
#191 where the mist is partly thick and complicated.
Here we will show another B image (together with R image) taken on 30 Mar at LCM=285degsW where a typical ebm crossing Syrtis Mj is seen. We also cite the R and B images on 17 May and on 27 June. These make combinations with those already cited in #193 p2137. On 27 June, the sub-Solar point retreated to 16degsN, but the evening mist is still present towards Syrtis Mj.
We also cite the images taken successively on 9 July, 10 July and 11 July. On 11 July the season was 146degsLs and the latitude of the sub-Solar point was still 14degsN. Notable is the morning cloud/haze which received every- day changes at the rather high latitude. It is considered that the higher easterly and lower westerly sometimes make fronts inside the region still at the season. Note however that the ebm is weaker.
Finally we cite HST images on 12 September 1997 (180degsLs) which are important:
Note that the B image does not show the ebm any longer. The time was exactly at the autumnal equinox, and at that time the EB was intrinsically warmer than before.