Moscow — Ties between Moscow and Washington are visibly thawing for the first time in four years. But despite this atmospheric change - its latest sign a planned visit by United States Agriculture Secretary John Block - a summit seems not yet on the horizon.
The extent and duration of the thaw could yet hinge on key, unresolved issues of substance, like arms control.
And that issue may get doubly tough if talks stay jammed until the planned December siting of new US missiles in Western Europe. Moscow says, if so far with few specifics, that this move would surely harm overall ties.
In any case, no summit seems yet in the works. Soviet officials sense President Reagan may want a meeting more keenly than Moscow as US elections near. Moscow would like maximum benefit from the US as part of any such parley. When nine US Senate Democrats asked Soviet chief Yuri Andropov about a summit last week, he said he couldn't picture a meeting just to discuss upgrading the Washington-Moscow hotline.
Still, the US and Soviets have begun chipping accumulated ice from their ties recently. Each side, diplomats argue, has good reasons to do so.
Mr. Reagan, in this view, could improve his image both as a reelection candidate and with his allies by restoring correct, if not necessarily chummy, superpower ties. And one issue figuring in the thaw - grain trade - has special importance at a time of trouble for US farmers.
Moscow, meanwhile, is seen as reassessing earlier hopes Mr. Reagan might be forced by domestic or NATO pressure to soften his arms policies. Diplomats assume Moscow also suspects that, with the US economy looking up and the Democrats looking down, he could be in power through 1988 anyway.
The current superpower shift began gradually after Mr. Andropov's accession last year. Each side, promising no major substantive concessions, conveyed a desire in principle to unfreeze the US-Soviet climate to the extent practicable.
Among signs of change since spring:
* Moscow, while rejecting pleas on behalf of prominent dissidents like Andrei Sakharov and Anatoly Shcharansky, did hint at freeing some lesser-known figures by year's end. A group of Pentecostal Christians, including seven who had taken refuge in the US embassy for five years, got exit visas.
* The US agreed to discuss opening a Soviet consulate in New York and a US one in Kiev. The talks had been shelved when the Soviets moved into Afghanistan. The US also said it would now discuss a new accord on scientific and cultural exchanges.
* Both sides eased rival human rights stands as part of a compromise at the Madrid conference.
* Moscow agreed to a new pact on buying US grain, upping the annual minimum 50 percent. This made economic sense. Since Moscow has similar pacts with Canada and Argentina, the US accord assures Moscow of stable and diversified imports. Price and transport aspects, and attendant influence with the US grain lobby, also make American grain a good deal for the Soviets. But still, Moscow could have chosen a far tougher negotiating tack and complicated Mr. Reagan's political position at home.
* On Saturday, Mr. Reagan ordered an end to export controls on pipelayer equipment to the Soviet Union. The tractor-like machines, which place oil and gas pipes into trenches, need no longer be approved by the Commerce Department before shipment.
* Moscow also met US officials on the Reagan hotline proposals, though European sources say the Soviets seemed cooler about discussing better military-to-military communication and upgraded links between superpower embassies and their home capitals.
* This week, Agriculture Secretar9 Block comes to Moscow: the first cabinet-level visit since the Afghan crisis, exceptiNg Vice-Presieft Geor - om trip for the Brezhnev funeral. Mr. Block will sign the grain pact. But diplomats say he expects at least middle-level talks with Soviet officials as well.
The open question is whether such developments herald progress on various, more fundamental issues.
A recent East bloc proposal has somewhat narrowed the gap at 10-year-old talks on conventional forces in Europe. Moscow has also made a new START proposal, though the main dispute there - on Soviet land missiles - seems fundamentally to survive.
On a more immediate front - Euromissiles - informed sources discern virtually no movement yet toward eleventh-hour compromise. One source adds that recent news reports of a Soviet military figure hinting at a concession to visiting US congressmen was ''overstated and misleading.''