Seeing through fog is difficult...

While there are multiple low-level ways to make fog, mist, and smoke go away (the 1st-level Sor/Wiz spell Darsson's cooling breeze [evoc] (45-6), the 2nd-level Sor/Wiz spell dispel fog [abjur] (Shining South 46), the 2nd-level Sor/Wiz spell gust of wind [evoc] (PH 238), for example¹), as you point out, few spells continuously allow the caster, instead, to see through fog sufficiently to make effective ranged attacks through it.² Here are two.

• The 2nd-level Sor/Wiz spell listening lorecall [div] (Spell Compendium 133), for 10 min./level, grants the caster a +4 insight bonus on Listen skill checks. A caster with at least 5 ranks in the skill Listen gains blindsense 30 ft. A caster with at least 12 ranks in the skill Listen gains blindsight 15 ft. An effect like the spell silence negates this blindsense and blindsight.

  Thus, while only a 2nd-level spell, the caster must typically have 9 character levels or Hit Dice to realize the benefit he wants, and, even then, that range probably just won't be enough, and I am unaware means to extend it. Further, the typical wizard doesn't even have the skill Listen as a class skill, and the Listen skill is difficult to put on a wizard's class skill list.³ I don't know what prestige classes are available in the campaign, but having Listen on a prestige class's skill list is a solid criterion if this is how you're choosing to play.

• The 4th-level Sor/Wiz spell sandform [trans] (Sandstorm 119) for 1 min./level grants the caster, among other benefits, blindsight 60 ft., but the caster is otherwise blind, has a speed of 20 ft., and when under stress (like, for example, in combat) makes a Willpower saving throw (DC 19). Failure means, until the stress passes, the caster suffers a −2 penalty on attack rolls, saving throws, ability checks, and skill checks. Casting spells, it should be noted, is unimpeded by the spell sandform. (The sandform ring (Sandstorm 132) (24,000 gp; 0 lbs.) for 10 min./day in as little as 1 round increments on command grants the wearer an effect like the spell sandform if you're interested. Tip: You're not; see below.)

So, technically, the lowest-level wizard spell that allows the caster to continuously ignore fog's total concealment is the 2nd-level spell listening lorecall, but that spell's got serious baggage. Otherwise, the lowest-level wizard spell is the 4th-level spell sandform (but see below).

...And maybe it shouldn't be

The Dungeons and Dragons, Third Edition splatbook Masters of the Wild contains the following spell, which was never updated for Dungeons and Dragons 3.5, and is, therefore, legal in 3.5 campaigns with minor adjustments made by the DM.⁴

• The 0-level Drd spell fire eyes [trans] (Masters of the Wild 88) for 10 min./level grants 1 touched creature the ability to see normally through mundane fire, fog, and smoke, negating any concealment they provide.

So this spell exists, but no greater firesight spell was ever published. Were such a spell to have been published, precedent implies it would be a 2nd-level spell (following the usual pattern of an improved spell being 2 spell levels higher than the unimproved spell), yet the hypothetical spell greater firesight would probably also be a druid spell. Perhaps a friendly druid could develop a greater firesight spell by Researching an Original Spell (DMG 198)—one that allows sight through magical fire, fog, and smoke—and cast that on you before you enter battle?

The spells blindsight, greater blindsight, and improved blindsight

As this answer notes, the 3rd-level Clr spell blindsight [div] (SpC 32), until the publication of the Spell Compendium, appeared on the sorcerer/wizard spell list in Magic of Faerûn (2001) (as a 3rd-level Sor/Wiz spell), Savage Species (2003) (as a 2nd-), Underdark (2003) (as a 2nd-), and Player's Guide to Faerûn (2004) (as a 2nd-). That is, for nearly four years (about half the game's lifespan) sorcerers and wizards could cast the spell blindsight. Then, with the Spell Compendium's release, suddenly and inexplicably, they couldn't. Given such events, a DM, after learning the spell's history, may allow a wizard to develop (or redevelop) this spell using the rules for Researching an Original Spell (DMG 198), probably as a 3rd-level spell as per the Spell Compendium.

The 4th-level Clr spell greater blindsight [div] (SpC 32) is even more problematic in that it appears only in the Spell Compendium. The 4th-level Sor/Wiz spell improved blindsight [div] (Savage Species 67) does not appear on the Spell Compendium's list of Renamed Spells (5) nor in the Web article "Rules Reference: Feats, Prestige Classes, and Spells." That means legally—technically—the spell improved blindsight has been on the sorcerer/wizard spell list since 2003 and still is. However, the DM may make minor adjustments to the spell because it's unupdated Third Edition material, and, with the existence of the spell greater blindsight, arguing with the DM that spell improved blindsight stay on the sorcerer/wizard spell list will likely end up with the DM throwing dice at you. A very generous (or legalistic) DM may see things differently, though.

A handful of alternatives to spells

I know you didn't ask, but it's worth noting that darn few magic items grant blindsight. This answer mentions the blindfold of true darkness (Arms and Equipment Guide 130) (9,000 gp; 0 lbs.), which is the best way to get blindsight 60 ft. if one doesn't mind being unable to actually see. Also available is magic weapon special ability blindsighted (Underdark 68) (30,000; 0 lbs.) which, when the weapon's wielded, grants the wielder blindsight 60 ft.⁵

More specific to your needs is the costly shoulder-slot wondrous item cloud cloak (Far Corners of the World Web column "The Hidden Realms Above: Magic Items of the Sky") (31,500 gp; 1 lbs.), which provides an endure elements effect, a freedom of movement effect versus fogs, and the ability to ignore any concealment granted by fogs. The endure elements effect is silly, and if you're already ground zeroing the spell solid fog probably you've a freedom of movement effect from another source (perhaps the 3rd-level Sor/Wiz spell heart of water [trans] (Complete Mage 107)?), so you are only getting this for the vision effect. That vision effect, according to the item's creation prerequisites, is likely via (for a wizard) the 6th-level Sor/Wiz spell true seeing [div] (PH 296)... somehow. A generous DM may allow crafting a custom magic item to create that same vision effect using that spell as a prerequisite once you show him this item. It's worth a try.⁶

Notes
¹ Apparently, Shining South is a bit of a misnomer, or, maybe, a bunch of wizards using these spells is what makes these places shiny all the time.
² This answer posits the 1st-level Sor/Wiz spell true strike [div] (PH 296), which permits a lone attack to ignore even total concealment. I'm assuming this is insufficient, but that's a good answer and a solid workaround.
³ Min/Max Boards curates "Alternative Ways to Get Class Skills".
⁴ See the section Why a Revision? (Dungeon Master's Guide 4).
⁵ Both items have as a creation prerequisite the spell see invisibility, yet the spell blindsight actually appears in Underdark. No, I don't understand that either.
⁶ I'd be remiss were I to omit former Dragon magazine publisher Paizo's fogcutting lenses (Fortress of the Stone Giants 50) (8,000 gp; 0 lbs.) with its prerequisites of the spells darkvision and fog cloud (originally published for Dungeons and Dragons 3.5), and from Paizo's D&D 3.5 compatible Pathfinder the even better goz mask (Inner Sea World Guide 298) (8,000 gp; 1 lbs.) with its more difficult prerequisites of the spells control winds and water breathing.

It stops being magical because you (probably) stop concentrating on it.

Note that the spell requires that you concentrate on it for it to be active. If a combat encounter ends and you stop concentrating on it, the weapon stops being magical. You also cannot concentrate on the spell past the 1 hour limit, so the spell will be inactive when you re-summon it for a later fight.

It's possible for a character to continue concentrating on the spell at the end of an encounter. Then, if they begin a new combat encounter before the time is up and they haven't stopped concentrating, they could re-summon their weapon and the spell would still be active.

Extradimensional spaces generally don't freeze time.

Other instances of extradimensional spaces refer to the passage of time as if it progressed normally. For example, the Portable Hole states

A breathing creature within a closed portable hole can survive for up to 10 minutes, after which time it begins to suffocate.

Which does not draw a distinction between the passage of time inside and outside the space. Given that the class features don't address this, I think it's a safe bet that the extradimensional space for pact weapons don't freeze time either.