This shows how gradually nature
pressure has been betweenand where the lowest pressure existed in March. provides for the northern advance of the doldrum and its followingTrade, for in May we shall find that Southeasterly winds prevail betweenand which could not be the case if there were not a higher barometer there than in the zone to the northwardBoth pressure and temperature have increased this month, which is rare; perhaps it is a kind of heaping up of the air before the increased activity of theTrade sets in, causing freer egress to the upper current. Perhaps the great amount of Easterly wind experienced in higher northern latitudes at this season may be related to this heaping up of air in the doldrums; for before Easterly winds can blow in our islands, the barometer must be higher to the northward than to the southward of them, and any check to the upper current which supplies air to the area of high pressure in about would tend to produce such a state of thingThe plain wind arrows show that the strongest Northeasterly wind is still in the northwestern corner of the square ; that theTrade has increased in force betweenand, especially on the western side of the square; also that betweenand the prevailing wind is still very northerly.The dotted wind arrows show that theTrade prevails up to, and that it is weaker and more easterly than the Isotherms of Air and Sea.The isotherms of air and sea are remarkably similar in their shape and positio The air still remains one degree colder than the sea. From the Equator to there has been a slight decease in the temperature of the air, whilst that of the sea has increase The sun having been a month in the northern hemisphere, we may expect the air of theTrade to be cooler, as it would feel the change sooner than the water. The temperature of both air and sea is remarkably uniform fromto, but there is a gradual decrease of aboutFahr. to each degree of latitude between this andsouthern part of the square during the year, appear in its southeastern corner in April.Current ArrowThe plain current arrows show that Westerly currents are the most prevalent throughout the square, whilst their relative lengths indicate that they are strongest in th.